Technical Paper 309: Recent Progress in the Thawing of Frozen Gravel in Placer Mining
A, Overburden on placer ground, Nome district, Alaska; B, Thawing with steam points on Bonanza Creck, Y ukon Territory Seeaees A
Public-domain full text preserved in the Mountain Man Mining Library. Original source: archive.org.
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Technical Paper 309 DEPARTMENT OF THE INTERIOR
ALBERT B. FALL, SEcRETARY
U.S, BUREAU OF MINES f H. FOSTER BAIN, Dimgcror
Recent Progress In The Thawing Of ' Frozen Gravel In Placer Mining
By
Charles Janin
Washington Government Printing Office
The Bureau of Mines, in carrying out one of the provisions of its organic act—to disseminate information concerning investigations made—prints a limited free edition of each of its publications.
When this edition is exhausted, copies may be obtained at cost price only through the Superintendent of Documents, Government Printing Oflice, Washington, D. C,
The Superintendent of Documents ts not an official of the Bureau of Mines. His is an entirely separate office and he should be adressed:
SUPERINTENDENT OF DocuMENTS, Government Printing Office, Washington, D. C.
The general law under which publications are distributed prohibits the giving of more than one copy of a publication to one person. The price of this publication is 10 cents.
Persons desiring for lecture purposes the use, free of charge, of lantern slides of
the illustrations in this publication, should make request of the Director of the Bureau of Mines, Washington, D. C.
First edition, June 1922,
Digitized ty (OC gle
Introd uctlonic..262 0 uicc goss hesesses Sos stcisy sacdeotabasccbgesscestaess Use of water for stripping Spo star stae Sa ea ans Bale SASS a eee CE ease atS SEE Relations of 'muck' and gravel 2 0-ee cece cece eee eeee Stripping experiments of Yukon Gold Co +--2 Steam thawing of muck and gravel 2 22-2000020-e-e22-2-0-+ Methods of Yukon Gold Co... a3 B3.9.5.a,sriot ule Sngidan aun goswaes ae Methods used in the Cape Noma dinttint.- csv. sds sec ascccssaoosaeies eee Heat required to thaw frozen ground +-2--2-+---++-++- Thawing with water at natural temperature SG sekdahasassessases Experiments of J. H. Miles in Cape Nome GintVitkseatasciuedcnssccwoornes Test by Alaska Mines Corporation 2.-22-022-22202220005- Experiments of E. E. Pearce, in Candle Creek district Method developed by Pearce. 00.0002 eee e eee ee eee eee ence eee Work of Yukon Gold Co., Gold Run district. 222-2-2--+-- Experiments of the North West Corporation (Ltd.), Klondike district Experiments of Fairbanks Gold Mining Co., Fairbanks district Pest: WILD POUNts x55 5am acta welgnig Sale wie alsiei sa sheiaisalnenisina a cee eaten ae
Test with sweaters SA eataeiG sinatouse Arrangement of thawing system 'for 1921 season + 0.00-- Miscellaneous testa. 2:...0 onc pc ost Dee sawed piesaciscipocasataicesee EQuIpMent 22.2 c25.050 52a Pooks s Sag eee TER Diam nk kta obese alae ts Construction and number of sweaters used +
Time required for setting points and thawing ground CONCHISIONES 45 codeec dace asa han acsie cscs catete deed tae pas adage sieleics sees Water supply—Supplementary steam plant Steam thawing of muck and gravel 2--2++ ++--- Need of comprehensive data. .6é 02. 265cc0 cece cccelcsbecsewesceccdouss Temperature of WAteh sa. 2. tase 55 osincb es sennccdessce ers. eteeeseetcess Cost of cold-water thawing 0--.2 sec ec cece eee eee cece eeeeee Need of investigation .ccosic.5 cocci wees pec se ct Macinw eee Vice bbeebeevaces
Priate I.
Tables.
Thawing costs, Yukon Gold Co., 1909-1914 Dredging and thawing costs, Yukon Gold Co. Detail of 1913 thawing costs 2.-2---seeee sees eee eee eee Thawing and dredging costs of individual dredges of Yukon Gold Co.
Illustrations.
A, Overburden on placer ground, Nome district, Alaska; B, Thawing with steam points on Bonanza Creck, Y ukon Territory Seeaees A, Layout of thawing equipment on property of Alaska Mines Co. near Nome, Alaska; B, Ground in front of dredge of Alaska Mines Co., Nome, Alaska, thawed by use of water under natural temperature; C, Cold-water thawing plant in operation on Dominion reek, Y ukon Territory; Canada: cae. csc ccka voce wea On babbsaees Plan and sections of ground in Candle Creek district, Alaska, thawed by the' Pearce: method: 2.25 fs2..c22058eetewsssedee os iccdssnwcaae A, Long single manifold for 3- inch sweaters; B, Double manifold having 16 outlets for g-inch sweaters 22 22..- A, Double manifold for }-inch sweaters; B, Single trench manifold for }-inch sweaters; C, No. 2 dredge on claim No. 1 above Fairbanks! Creeks csevesossesesuiitionsss tofereeosaractadenaatesaese
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Iv Contents.
Page. Ficure 1. Results of thawing with superheated steam, saturated steam, warm water, and cold water sseccssccesceeesees 14 2. Pipe layout for cold-water thawing on Dominion Creek, Yukon, August; TQ2Us 2522 a:04.25 22s clan aa a aateren's wid dein ee eelnaecacedea ces 25 3. Spacing of long and short sweaters 2 2---2 26 4. Diagram of make-up of piping for cold-water thawing on Dominion Creek, Yukon, August, 1921 c-ececesecccees Siss S5. ces 27
Digitized t Gor gle PRINCETON UNI\
Recent Progress In The Thawing Of Frozen Gravel In Placer Mining.
By CHARLES JANIN.
Introduction.
Dredging methods for gold placers and the construction and operation of gold dredges were described in Bulletins 121! and 127? of the Bureau of Mines. The working of frozen gravels, a special problem in placer mining, has received much attention in Alaska and the Yukon Territory, and the methods evolved there are described in this paper.
In placer mining in the far North one of the greatest difficulties encountered is the permanently frozen condition of the ground. In the early days of mining the ground was thawed by the primitive method of building wood fires on the surface, at the bottom of a shaft, or at the face of a drift. Although this method was slow and costly, a more effective one was not discovered until about 1898, when Mr. C. J. Berry* noticed that the steam escaping from the exhaust of a hoisting engine had thawed a hole in the solid ''muck." Mr. Berry picked up the exhaust pipe (a rubber hose), applied it elsewhere, and found the steam thawed the ice and moss so rapidly as to permit the full length of the hose to penetrate the ground within a few minutes. Further experiments followed. At first a rifle barrel and then a short piece of iron pipe, pointed at one end, was used for penetrating the ground, rubber hose being attached to convey the steam. The direct application of steam through points proved so effective that it became the chief method adopted in thawing many millions of cubic yards of gravel for drift, surface mining, and dredging operations.
As a result of experiments, changes were made in the shape of. the steam point, better methods of application were devised, and the best number of points to be used in a unit and the distances at which points should be spaced to get the maximum benefit were determined.
The largest mining operations in frozen gravel were those of the Yukon Gold Co., near Dawson. Detailed figures of operation and
1 Jennings, Hennen, The history and development of gold dredging in Montana, with a chapter on placer-mining methods and costs, by Charles Janin: Bull 121, Bureau of Mines, 1916, 63 pp.
2 Janin, Charles, Gold dredging in the United States: Bull. 127, Bureau of Mines, 1918, 226 pp.
3 Rickard, T. A., Thawing frozen gravel with cold water: Min. and Sci. Press, vol. 120, March 13, 1920, pp. 364-365. 1
2 Thawing Frozen Gravel.
cost, given on page 5, represent the best practice for the methods employed.
Tests with hot-water thawing, the point being supplied with hot water instead of steam, were made at different times. A number of these experiments indicated that ground could be thawed by this method, but in comparison with steam thawing the latter was generally found more effective. At all events, no extensive thawing with hot water was ever done. Comparative tests of hot-water and steam thawing are described elsewhere in this paper. It should be recognized that steam by contact with the frozen ground condenses to water, gradually giving up its heat units and having much the same effect as an application of hot or warm water. In the same manner, water that is applied hot soon loses most of its heat.
Purington' mentions the use of hot water under pressure at a drift mine in the Klondike and says that at that mine a hot-water nozzle thawed more ground than was thawed in the Klondike by the average steam point. Experience in general, however, favored steam.
Payne* performed some experiments for the Yukon Gold Co. and advocated thawing frozen gravel with hot water.
In all, many millions of cubic yards of frozen ground have been mined in the northern fields of Alaska and the Yukon Territory, as well as Siberia, by using various methods to soften or thaw the gravel. As has been said, the most satisfactory method developed was thawing with steam points. This method, however, proved slow and costly under the most efficient management, its cost amounting in general to about 40 per cent of the total cost of mining, and at some mines to perhaps 70 to 80 per cent. The high cost of thawing required that the gravel mined contain a much higher gold content than ground that could be profitably mined where thawing was unnecessary, and prevented the working of the lower-grade ground in the frozen areas.
The working of most of the deeper—40 feet or more—frozen or partly frozen ground seemed economically impossible except where drift mining would be profitable. A number of gravel areas in the far'north have attracted mining men because of the high gold content as compared to dredgable areas of unfrozen ground in other districts. Little attempt to work much of this gravel has been made until recently, however, as no cheap method of thawing was known. A number of experiments have been made from time to time since the beginning of placer mining in the far North with the purpose of de-
Purington, C. W., Methods and costs of gravel and placer mining in Alaska: U. 8. Geol. Survey Bull. 263, 1905, pp. 93.
® Payne, I. M., The development and problem of the Yukon: Trans. Canadian Min. Inst., vol. 16, 1913, p. 237.
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Bureau Of Mines Technical Paper 309 Plate I
A. Overburden On Placer Ground, Nome District, Alaska.
B. Thawing With Steam Points On Bonanza Creek, Yukon Territory.
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Bureau Of Mines Technical Paper 309 Plate I!
A. LAYOUT OF THAWING EQUIPMENT ON PROPERTY OF ALASKA MINES CO., NEAR NOME, ALASKA.
B. GROUND IN FRONT OF DREDGE OF ALASKA MINES CO., NOME, ALASKA, THAWED BY USE OF WATER UNDER NATURAL TEM- PERATURE.
cC. COLD-WATER THAWING; PLANT IN OPERATION ON DOMINION CREEK, YUKON TERRITORY, CANADA; THE NORTH WEST CORPORATION, LIMITED, SUMMER OF 1921.
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Relations Of Muck And Gravel. 8
veloping a more efficient method of thawing the frozen ground, and the files of the Patent Office show drawings of some peculiar machines designed for that purpose.
Use Of Water For Stripping.
Though some experiments were made with cold water—that is, water at natural temperature—instead of steam or hot water, these were mostly to dispose of the overburden of muck. The extensive operations of the Yukon Gold Co. and the Canadian Klondike Co. in the Dawson field, as well as the Pioneer Mining Co. in the Nome district, showed that getting rid of the muck overburden was not difficult where there was a water gradient. The method of treatment was as follows:
A layer of vegetation on top of the muck was burned where possible, or removed by water under hydraulic pressure. In some places a top soil was first loosened by means of strong plows drawn by tractors, and then hydraulicked. After removal of this top layer the muck overburden, about two-thirds ice, mixed with sand and clay, was exposed.
Water led by ditches from surface streams or gathering ditches was turned on this muck, and gradually thawed or cut channels through it and the frozen gravel. This left the muck in blocks and ridges, and further thawing was aided by the sun. The work was facilitated at open-pit workings, if a bank was exposed. Water under hydraulic pressure was then used to thaw completely the overburden and to get rid of it. Under these conditions it was not difficult or costly to dispose of the muck in advance of mining operations. The success of this method led to expectations that the same system could be applied to the frozen gravels. Some claims were made that once the overburden was removed, the underlying gravel and bedrock would readily thaw under the heat of the summer sun. These claims, however, proved fallacious. Numerous articles were written on this subject about 1914. From one by E. E. McCarthy a number of extracts have been taken for this paper.
Relations Of "Muck" And Gravel.
Most of the dredging ground in the far North is covered with an overburden called ''muck,'' which is composed of 25 per cent to 40 per cent organic matter and 60 per cent to 75 per cent ice; its thickness ranges from a few inches to 40 feet, the greater depth being in the gulches. For further information the reader is referred to a paper by Tyrrell.7 Plate I, A, shows muck in the Nome district; Plate I, B, shows steam thawing in the Fairbanks district.
6 McCarthy, E. E., Stripping frozen gravel: Min. Mag. (London), vol. 10, April, 1914, pp. 280-295. 1 Tyrrell, J. B., Frozen muck in the Klondike district: Trans. Roy. Soc. Canada, ser. 3, vol. 11, 1917, pp.
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4 Thawing Frozen Gravel,
Muck is defined by McCarthy as—
A frozen substance having the following physical characteristics: Color, gray to black; composed of organic matter, particles of sand and silt, cemented by ice. In hardness it may be likened to a soft sandstone. The temperature of the frozen muck varies between 19° and 24° F., or 8° to 13° below the freezing point. The average physical characteristics are as follows:
Physical characteristics of muck.
BPaclhiG RAVI: accsaecsdvde us beac iocaceuaehhundietess SUeeeuvecwesewe a BOe Specific heat..-2-.c tw decane eh mapesteodevaecsscasebeessackcaaeenes eee - 192
eight of: L:cubie foot, pounds Ac:.eie%sscadscsssscasscswdocssccsiseee ces 87 Weight of solids in 1 cubic foot, pounds 2222- 222 e eee eee eee eee 49. 33 Volume of solids in 1 cubic foot, per cent 2.0.0... 0202 ee eee eeeeeeee SLO
The figures are taken from a number of experiments made in 1912 by engineers in the employ of the Yukon Gold Co.
The weight of the frozen gravel per cubic foot has been found from the ave ofa number of experiments to be 137.3 pounds, of which 17.4 pounds is ice and 119.9 pounds solid. The temperature varies between 18° and 22° F., averaging 19°. At one place, where it was possible to get a reading 38 feet below the surface, a temperature of 2° F. was recorded. The temperature of the frozen bedrock varies between 8° and 14°, averaging 114° F,
It is true that in almost all cases where there is an overburden of muck, the underlying gravel is frozen; also, where areas of thawed ground have been found, there has been little or no overburden of muck. However, there are many areas free from muck that are frozen from immediately below the seasonal frost line to bedrock. Evidence from thousands of drill holes and holes made with crowbars seems to prove that the transition from the frozen to the thawed condition is gradual and in the form of a curve, and these curves always slope toward a drainage channel. Although it can not be said with certainty, all the data point to the conclusion that, unless the creek has an established drainage channel to bedrock to cary. the water from the melting of the ice in the frozen gravel, thawing will proceed with extreme slowness, and complete thawing will not result until such a channel is established. The layers of clay found in the gravels in some places, and the varying proportion of clay in gravels rendering some more impervious than others, are important factors in the rate of thawing. The Yukon Gold Co, has carefully prospected practically all its properties for frost by means of Keystone drills and steel bars. From the data obtained colored maps have been made showing the outline of the frozen, thawed, and partly thawed areas. 'To the eve the map of each creck looks like a crazy quilt. The only noticeable feature , that the thawed areas are almost invariably connected to a thawed bedrock channel,
An attempt to obtain one of these maps of thawed and frozen areas, which would be of much interest in studying this subject, proved unsuccessful, as the maps, it is understood, were destroyed in a fire at Dawson.
In discussing the early work of the Yukon Gold Co., McCarthy °®
says: STRIPPING EXPERIMENTS OF YUKON GOLD CO.
During the seasons of 1906, 1907, and 1909, the Yukon Gold Co. conducted some' large-scale experiments with the removal of muck overburden. The tests were conducted at three different points on Bonanza Creek, where every convenience was at hand. Two 5-inch giants under 400-foot head were used at one point, and an additional supply of 3,000 miner's inches was available on Bonanza Creek after it had been used in the hydraulic mines. No trouble was experienced in getting rid of the muck in favorable localities, but the solid matter was deposited a short distance from the operation and had to be driven along. The serious drawbacks were that nothing could be done with the areas that were covered with a tailing of sand and gravel on top of the muck, within the limits of reasonable expense. As the cross ditches cut down to a base level, they lost grade, and more and more hand labor had to be employed. Buried roots and stumps had to be cut out by hand at great expense.
8 McUarthy, E. E., Article cited, p. 289, McCarthy, E. E., Article cited, p. 293.
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Steam Thawing Of Muck And Gravel. 5
T have endeavored to show that the stripping alone had its serious difficulties and that it can not be accomplished with the ease and cheapness to be inferred from recent articles on this subject. We now approach the main question: Will the creek gravel and bedrock thaw after the overburden is removed? Is the method practicable and will it reduce the ultimate cost of dredging?
The experiments of the Yukon Gold Co. above described were sufficient to demonstrate to our satisfaction that the method of stripping and so-called actual thawing could not be relied upon for any large-scale operation, The stripping work on claims Nos, 89 and 90 on Bonanza Creek was of no benefit to the dredging operation. Practically all of the ground had to be thawed by steam before the dredges could operate. The ground was stripped and exposed for an average of less than a season before dredging was attempted. On claims Nos. 78 and 79, opposite Trail Gulch, where main-ditch water was used for stripping, the ground was approximately 50 per cent thawed when reached by the dredge a season and a half later. On account of incomplete data, it is not possible to say positively how much of the thaw was due to stripping and how much of the ground was naturally thawed previously. It is my opinion, from the data available, that the proportion of complete thawing due to stripping was small and altogether disproportionate to the expense of doing the work. The same comments apply to the work on Nos. 62 and 63 Bonanza. In this case the stripping was more thorough and the ground was exposed for an average of over three seasons. The dredge reports show that 50 per cent of the ground was frozen and had to be thawed by steam.
Steam Thawing Of Muck And Gravel.
The cost of steam thawing has been from 15 to 20 cents or more per cubic yard, according to the locality, the cost of fuel, the depth of the frozen ground, and the extent of the operations undertaken. The operations of the Yukon Gold Co. in the Dawson district in the years 1909 to 1920, inclusive, during which time over 45,000,000 cubic yards of gravel were dredged and over 26,000,000 hydraulicked, have been the largest operations for mining gravel by these methods in the far North.
Methods Of Yukon Gold Co.
The following description by Perry'? of the method of steam thawing followed by the Yukon Gold Co. is of interest as indicating the best practice known until recently.
Operations in the Yukon district are subject to difficulties which are not met elsewhere, except in Alaska and Siberia. These may be described under two heads: (1) Climatic; (2) character of deposits.
The extreme cold of the northern winter limits the operation to an open season of about six months. Dredging can not be begun until there is sufficient thaw to produce water for flotation and to clear the ponds of the winter ice, which forms to a depth of from 5 to 7 feet. Both the start and the finish of the seasonal operation are hindered by cold. All moving parts, such as the tailings conveyor, must be housed in and warmed by steam atte Also, steam is used to freely keep the ladder and bucket line clear and to keep the gold tables in good condition, The dredging operation is terminated when the creeks freeze to such an extent that water for flotation is no longer available.
The greatest problem in the Yukon as compared to the ordinary dredging Np has come in the handling of frozen ground. The heavy bedrock encountered on the creeks and the necessity of digging it to considerable depth to recover the gold was in itself a serious matter. When this was further complicated by having large areas solidly frozen, the problem of dredging such areas at a profit became one of extreme difficulty. The result of attempts to dredge partly thawed areas and frozen bedrock showed that the frozen condition must be corintaiely changed before dredging
0 Perry, O. B., Development of dredging in the Yukon territory: Trans. Canadian Min, Inst., vol. 18, 1915, pp. 26-44.
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6 Thawing Frozen Gravel,
could be successful. Preparing the frozen ground for dredging by thawing with steam was an entirely new feature of dredge mining.
The improvements that have been made in steam thawing since it was adopted for large-scale dredging operations are too many to attempt to detail in a short paper. They have taken the following g general] lines: First, reduction in fuel; second, increase of efliciency of hose and points; - third, reduction in cost of driving.
The steam point as originally 1 used was a short piece of #-inch pipe, Be down one end to a point with an orifice left for the steam to escape; the other, or head end had a nipple welded to it for steam-hose connection. The point, with steam turned on, was gradually driven into the face of the drift, then allowed to steam until the bed-
rock and gravel were thawed to the height of the drift. From two to six points were used in each breast, spaced from 1 to 2 2 fect apart. This simple operation contained the essential features of the method whereby over 3,000,000 yards of gravel are now being thawed in a season.
In the thawing operation for dredging, the points are driven from the surface down through muck and gravel and into the bedrock from 4 to 6 feet. The points were gradually lengthened in practice, first, to 24 feet, then to 30, and last season were being successiully driven 40 fect to bedrock, The points are made up of triple extra strong hydraulic pipe with drop-forged head and tool-steel tip. he small boilers first used have been replaced by larger sizes until the boiler equipment now comprises eight plants with a total boiler horsepower of over 2,000, The 150-horsepower locomotive type has been adopted as the unit, and the plants usually consist of a pair or, sometimes, three such boilers,
The preliminary work for each season takes the following form: (1) The dredgable area ahead of each boat is barred for frost. From the bar-hole results the naturally thawed and frozen areas are blocked out. The bar tests are checked by drilling where thought necessary. (2) Surveyors " pick up"' the thawed areas which are mapped, and the planimeter measurements are made of the frozen areas. (3) The estimated course of the boat is laid out for the year and its probable position calculated for each month of the season. (4) The total square yards of frozen ground w hich the dredge will dig in a season is measured from the map on which the season's work is laid out; then the necessary amount of wood for thawing is determined and its location is decided.
The thawing operation has been described elsewhere," so that I need only give the sequence of events in a normal operation: (1) Steam-line boxes are strung out by horses, if they are available, otherwise men are used; (2) gooseneck boxes and headers coupled on; (3) bar holes for starters put down, (4) starters used to thaw holes about 8 feet deep; (5) starters pulled and long points placed i in starter holes; (6) points driven down tisuen gravel and into bedrock; (7) points allowed to steam in hoaeeer from 12 to 48 hours, depending on the formation; (8) points pulled, cleaned, and straightened; (9) header mov ed and starters put down and cycle begun again.
Costs vary. Thawing on Hunker Creek, for example, is much more difficult and expensiv e hati thawing on Bonanza. In 191 1 the thawing for dredge No. 4 on Hunker Creck cost $2.18 per square yard as against $1.85 ahead of the No.5 on Bonanza. The depth being practically the same, the cost of thaw ing per cubic yard was much greater in one case than in the other. Purely local conditions govern this cost, such as depth of ground, character of material, ease or difficulty in driving, time of steaming, cost of fuel, etc. Since the gold, practically speaking, is all confined to the bedrock, the cost pe square yard of bedrock exposed by the thawing becomes the controlling 'factor in
etermining what ground can be dredged at a profit and what must be left.
One plant of the Yukon Co., in 1908, consisted of three boilers, two 150 horsepower and one 75 horsepower, fired with wood that cost $8 per cord delivered. The steam gage on the boiler registered a pressure of 150 pounds. By the time the steam reached the gravel the pressure had decreased to 25 pounds per square inch. The main pipe line was 34inches in diameter, wrapped in asbestos packing and encased in a wooden box packed with sawdust. From the main line the steam. was taken several hundred feet in 14-inch branch pipes to where the ground was to be thawed. At intervals of 8 feet along a branch pipe short lengths of hose connected the pipe to the heads of the steam points.
Rickani, T. A., Dredging in the Yukon; Min. and Sci. Press, vol. 97, Sept. 12, 1908, pp. 354-356.
STEAM THAWING OF MUCK AND GRAVEL. vi
This hose was armored, § inch in diameter, and long enough to reach the heads of the points when they were being swung into position.
The steam points consist of to 1 inch pipe, cut into 14 to 20 foot lengths. Both ends of the point are of steel, the lower end having a ,-inch opening to permit the steam to escape and the upper end having a driving head. The steam point is generally started by driving a steel crowbar into the ground until the frozen gravel is struck. Then the bar is withdrawn and the steel point introduced. The head of the steam point is pounded with a 4-pound hammer and given an occasional twist to aid its sinking into the gravel. This is done at intervals and in rotation throughout the entire battery of points. The ground is softened by the admission of the steam and the average rate of thawing is about 2 feet an hour. Should a bowlder be encountered in depth, it is necessary to withdraw the point and drill through the bowlder with a steel bar unless the point is equipped with a steel bit for the purpose of cutting through the bowlder. With a battery of 150 or more points the cost of thawing under the above conditions may run from 12 to 20 cents a cubic yard.
The appended tables show the thawing and dredging cost for six years of the Yukon Gold Co.'s operation. Table 4 shows the variation in working cost for the different dredges in one season. Because of the conditions changing, the dredge that has the lowest cost for one season may show the highest cost for the next. When the costs for he eight dredges are totaled and averaged the results over several seasons are remarkably uniform.
While Table 1 shows a gradual reduction in the thawing cost, it does not reflect the improvement in efficiency, which has been very marked.!?
TaBLE 1.—Thawing cost, Yukon Gold Co., 1909-1914.
Naturally Naturally Thawin Total Frozen thawed thawed Thawing] cost per Year. (square (square (square on st settle ards). yards). equal ao! r cost. s quare y yards). cent). yard. 212, 903 85, 158 28.6 $275, 112 $1. 5571 256, 033 140, 804 35.5 500, 659 1.7727 432, 693 122, 700 22.1 696,034 1. 6166 509, 574 182, 948 26.4 SD4, So4 1, 455 445,624 205, 881 31.6 685,570 1, 546 402,660 184,077 31.4 602, 174 1, 428
TasLe 2.—Dredging and thawing costs, Yukon Gold Co.
Ground Thawing Percentage! Total cost dredged cost per of thawing per cubic
Year. (cubie cubie yard cost of yard yards). (cents). total. (cents).
15, 45 48, 37 31, 94 14.14 45. 57 31.09 17. 62 49,72 35. 43 15. 02 49. 03 30. 64 13. 57 45. 94 29, 53
1914... set ; 12,18 44.09 27.62
12 Perry, O. B., Development of dredging in the Yukon Territory: Trans. Canadian Min. Inst., vol. 1915, pp. 25-44.
8 Thawing Frozen Gravel.
TaBLe 3.—Detail of 1913 thawing costs.
Cost per Percent- Total square age of expendi- yard total tures. (cents). cost.
Fixed Balarieg mn. 5 5.564.055 Sa5905 6 Gra bW59 sons awe abate iaeieeage cies iaeiene $1,614.00
Labor 280,195. 38 65. 2 42.18 Fuel, 3.52.22: 205, 928. 03 66.7 43.17 Shop expense... . - 1, 722. 16 "A 25 Preliminary expense. 36,021. 74 8.1 5. 25 Material and suppli 23, 180. 50 5.2 3,38 Stables..; <22.--<25-5 6, 589. 27 1.5 - 96 Transportation . 198. 00 1 03 Miscellaneous. . 380, 33 ol . 06 Depreciation... 30,731.37 6.9 4.48 POWERS cacacsceemastns Ae LSSOT es CEP RRUS Soe rue Orson oes on aeenaee ee eee B45 eersasecs.s OL
685, 570, 23 154.6 100. 00
Square yards thawed, 443,547. Number of points driven, 63, Cost per point driven, $10.81.
s
TaBLe 4.—Thawing and dredging costs of individual dredges of Yukon Gold Co.
Percent- Thawing| Total
age of cost per cost per Dredge. Creek. area natu-| cubic cubie rally yard yard
thawed. (cents). (cents).
22.1 12.55 31.93 34.6 12. 28 32.55 14.0 18. 36 33.79 7.2 17.95 32.11 44.8 12. 34 27.01 18.6 16. 38 33. 36 33.7 12.97 27.92 58. 6 7.49 21.16
Methods Used In The Cape Nome District.
An article by Arthur Gibson," on thawing frozen ground for different forms of placer mining is quoted below as of particular interest in comparing the cost and the efficiency of the methods known in 1913:
The following efficiency and cost data are collected from actual placer-minin operations in the Cape Nome mining district, Seward Peninsula, Alaska, but wil apply to other localities of like nature by proper adjustment of the cost of fuel, labor, board, and local conditions. One frequently hears that it costs so many cents per cubic yard to thaw frozen ground in a certain locality. These statements, although in many instances true and derived from actual operations, are misleading in that the cost per cubic yard may not only vary with different localities but may even vary considerably in the same locality and within short distances. The principal factors governing the cost of thawing are as follows: (1) Amount of ice contained per cubic yard, (2) cost of fuel, (3) cost of labor, (4) amount of condensation due to long and poorly insulated pipes, (5) amount of condensation due to low temperatures during the winter, (6) method of mining or application, and (7) the scale upon which operations are conducted.
Heat Required To Thaw Frozen Ground.
The temperature of perpetually frozen ground on Seward Peninsula, so far as known, remains nearly constant around 28° above zero Fahrenheit, or 4° below the freezing point, excepting close to the surface, where the temperature is affected by the atmos-
a oer Arthur, Thawing frozen ground for placer mining: Min. and Sci. Press, vol. 108, Jan, 17, 1914, Pp. De
Steam Thawing Of Muck And Gravel. 9
heric heat or cold during the summer and winter seasons, or in the immediate proximity of subterranean water channels or thawed ground. I will assume that the frozen gravel deposit contains 2.850 pounds of solids and 260 pounds of ice per cubic yard, and it is desired to heat this deposit from 28° above to 36°, or to a temperature 4° above the freezing point. How much heat is then required, expressed in B. t. u.'s per cubic yard?
The coefficient of thermal capacity (specific heat) for solids is 0.2, that for ice is 0.5, and that for water is 1. The latent heat of fusion of ice is taken at 142 B. t. u. per pound. Then:
B. t.u. 2,850 pounds of solids raised 8°, from 28° to 36°, 2,850 K 8 X 0.2 4, 560 260 pounds of ice raised 4°, from 28° to 32°, 260 X 4 X 0.5 520 260 pounds of ice at 32° to water at 32°, 260 X 142 2 36,920 260 pounds of water raised 4°, from 32° to 36°, 260 K 4 X 1 1, 040 Total heat required for the ice. 2.20... e eee eee eee eee eee ee 38, 480 Total heat required per cubic yard of ground +--+--+-+-- 43, 040
This plainly illustrates the rapid increase of heat required for only a small increase of ice per cubic yard, and it shows further that under these conditions the ice requires 92.5 times as much heat as the solids. With crude oil of a fuel value of 18,000 B. t. u. per pound, or 142,416 B. t. u. per gallon, or with coal of a fuel value of 12,000 B. t. u. per pound, and assuming the efliciency of boiler and distributing plant at 50 per cent, there will be required in the above example to thaw one cubic yard of frozen ground:
43,040 % 9000. 4-782 pounds of crude oil, or 43,040 3 71,208 0.604 gallon of crude oil, or 43,040
"e000. 7-178 pounds of coal.
The variation in the cost of fuel noted in the accompanying data is due partly to the fact that in general the prices have been raised during the last year, although some unexpired contracts maintain the former and lower prices, and partly to the different freight rates, which are proportional to the various distances between the supply station at Nome and the mines, being lower during the winter than the summer season.
The cost of labor for this pattioulas work, firemen and pointmen, is $3.50 and $4, respectively, during the winter, and $5.50 and $6, respectively, during the summer season. Board, including fuel and cooks' wages, ranges from $1.08 to $1.35 per man per day, according to the number of men employed.
The table presented contains only the actual operating expenses, no charges for management, interest on investment, or depreciation of the outfit being considered or included. Nos. 1 to 5, inclusive, are the figures for underground stope mining where the steam points are used in a horizontal position. No. 6 contains the data with regard to thawing ahead of a dredge where the steam points are used in a vertical
sition.
Pethe fuel used in all of the above cases is California crude oil, averaging 17.5° Baumé, weight 7.912 pounds per gallon, and containing 18,000 B. t. u. per pound, or 142.416 B. t. u. per gallon.
The foregoing data are compiled from the common practice among some of the best miners on Seward Peninsula, Alaska, but which could no doubt be improved under scientific management. Judging from the fuel consumption, it is evident that it requires from 80 to 100 per cent more fuel during the winter than during the summer for underground drift mining. Fuel for thawing ahead of a dredge appears very high. This is principally due in this case to the ground, which contains between 60 and 85 percentice. Labor, duty per man per day for thawing, varies to a great extent, as it depends largely on the composition of the deposit, whether it is sand, light or coarse eres or ground containing large rocks which render the driving of the steam points
ifficult.
Miners frequently overlook some of the most vital factors relative to the most efficient and economical means of thawing, such as ascertaining and gaging the proper distance between the steam points; the correct time of steaming; the proper time for the dirt to lie idle between thawing and excavation to evenly absorb and equalize the heat transmitted. 'The success of the venture depends greatly on the proper adjustment oi these factors. The thawing of deep ground for dredging purposes has so far proved
Princeton Univer
Thawing Frozen Gravel.
more or less unsuccessful, due principally to the steam condensing and losing its
vreatest heating value before reaching the end of the steam point.
is only a matter of application and can be adjusted under a proper system.
This, however,
Comparative data on efficiency and cost of thawing frozen ground with steam for placer mining in Cape Nome mining district, Alaska.
(Joe Wise, mer.).
Operated during Class of placer mining.
New Joe Wise Joe Wise] Eagle & Co. & Co. Mining Otto (Chas. A.|(Chas. A. Co, Olson, Vogel, Vogel, (A.B. operator. mger.). mer.). Brown, mgr.).
Summer. Summer. Summer. Summer.
Underground driving and stoping.
Depth below surface . . 53 ft 81 [t. 81 ft. 45 ft. 50 ft Thickness of pay dirt.. 2.5 ft 2.5 ft. 2.5 ft. 3.5 {t. 2 Thickness of waste. . 1.5 ft. 2 ft. 2 L5ft i Total depth of stope 4 ft. 4.5 ft. 4.5 ft 5ft
Material, dirt and clay Material, gravel... Swelling of loose di
Total boiler horsepower . Steam points, teneth Steam points, number. Steam poiuts working...
Steaming time, hours. 5+ Horsepower per steam point Depye thawed, feet
Le oe Ground thawed per day, cubie yards,
LOOSQ MITE, 25 asics caebae su ansey ed - 128 Ground thawed per day, cubic yards
Girt in place... 0.3.5 .00essweresessreees 100, 94 Duty per point per day cubic yards loose
Gitta. Se c2 oo. Sane ssaieke teas wepecdaiet 3.2 Duty per point per day, cubic yards dirt
in Sik ods ccc aalns pkineace cog 5 DOMES 2. 5235 Duty per foot per hour, cubic yards loose
OVE A ciel ev pattaaccsastcewstalaeas pe¥ 0.051 Duty per foot per hour, cubic yards dirt
AD DIACC vas. oa ytescs Few sceeceesen sy ss 0. 04 Crude oil consumed per day, barrels 4 Crude oil consumed per day, gallons... . 168 Crude oil consumed per point per thaw
or day, gollons. 0..c5. esc ceceewcens 4.2 Crude oil consumed per point per hour,
MANIONS ..ccca aw aseaascassee Se eansans sede 0. 467 Crude oil consumed per cubic yard loose
dirt thawed, gallous 1.3125 Crude oil consumed per cubic yard dirt
in place thawed, gallons. 1. 6644 B. t. u. consumed per cubic yard loose
AINE cS aSuices sta ons qaaalen cat canete .--! 186,921 B. t. u. consumed per cubic yard dirt in
TACOS oo 56 Fo 2dscscom eee RR aca hides ate 237,037
Duty per barrel of oil in cubic yard loose
WUC Geis cos sac ones oven acoadss saysicanae 32 Duty pee barrel of oil in cubic yard dirt
BY DIWGO' 2.5. 0% codenin sie vctals Heeeuawades 25. 235 Number of men employed per day for
CDAWUTR Sac ic cs gt bess ke see ease ease 4 Duty per man per day, cubic yard loose
dirtithawing~ ~.2-sos0c2cxesei bie ssseis 32 Duty per man per day in cubic yard dirt
TMPLAUR: 26-5 coetet tate hpinntsecetes see 25. 235 Cost of crude vil per barrel delivered 2. 97
Horizon- Horizon-| Horizon- Horizontal. tal. tal. tal.
12 12 8 ll 0.777 0.777 1.4 1.08 7 9 7.5 7.5 2 2 hs edivsctes 2.5 205 257 128. 57 327.13
0.02712 0.033994 0.10714 0.092326 Pereree prccecy fog fee: 168 168 4 210
1.867 1.887 3.36] 4.567 0.15558 0, 15558 0.42} 0.4152 0.81951 0.6537 065334 0. 64195 116,711 93,097} 93,046. 91, 424 51.25) 64.25] 64.28] 65. 426
51.25 51.4] 42.85] 109.04 "$3.30 ]°°""$3:30 ""§ 2.90 $2.71
Plein
Con
Plein, mer.).
Summer. Dredging. 9 to 20 ft.
114, 18125
10, 40751 S80
Al Ssis 1, @918
1 220, 936 276, 170
$2.52
1 Assumed values based on excavated dirt swelling 25 2 Number of men employed per day thawing ahead oft
er cent over dirt in place. ie dredge is given as 3.5.
This is thus explained:
The night pointman also acts as oiler on the dredge during the night and his time is thereby divided into one-half for the thawing and one-half for the dredge.
Columns No, 1, 2, and 3 refer to the Linda Association claim on Center Creek; No. 4, bench 1, Center Creek; No. 5, the Tundra Association; and No, 6, the Otter Creek property.
vistizes ty GOOle
Princeton Univ
Thawing With Water At Natural Temperature. 11
Comparative data on efficiency and cost of thawing frozen ground with steam for placer mining in Cape Nome mining district, Alaska—Continued.
Average total cost per day.
New. 7 Joe Wise| Joe Wise| Eagle Plein vr heed We Te" aa BS Mining Otto Dredging (oe Wise, ©) has, A.| (Chas, "AL Co, Olsen, Co. ( mer.) Vogel, Vogel, (A.B, operator. Plein, er. mgr.) mer.) Brown, mgr.) mer.) 1 2 3 4 5 6 Crude or fuel oil. 2-2.2220--006- $11.88 $13. 20 $13. 20 $5. 80 $13.55 $52.79 Labor and board.. none za 20. 50 27, 82 34. 90 20. 45 21.05 25.73 Repairs and renewals 7.50 7.50 7.50 2. 7.50 3. 89 Rent, S-horsapower bollerss <3. 525 sas. 5|oss0sssi0c|esssquneeghtacesancse| sessed ease )eecalcceeee 1.67 Total operating cost 39, 88 48, 52 55. 60 28.75 2. 10 S408 Average cost per cubic yard loose dirt. Crude or fuel oil 22 cents... 9, 28 6. 44 5.13 4.51 4.14 9,31 Labor and board... ..do 16. 01 13, 57 13. 58 15.91 6. 4 4.54 Repairs and renew als BR: Sere 5.86 3. 66 2, 92 Lo 2,29 68 Rent, 50-horsepower boiler NUP pant sas Vesela t ya agieewest leasnnqagaalscesv ears acaap ua des [Messen esas Total operating f cost atipere cubic yard loose dirt .. . ents... 3L.15 23. 67 21.63 22. 36 12. 87 114.82 Total operating cost per cubic
yard dirt in place cents. . SOLS [5c sss eens |sdagescscelestubsaeociassswassae 18,53
1 Assumed ivalind based on excav tod dirt swelling 25 percent over dirt in piss: THAWING WITH WATER AT NATURAL TEMPERATURE.
About 1915 to 1917, different persons started experiments on an entirely new plan for thawing the frozen gravel. Water at the natural summer temperature was applied by drilling holes through the frozen muck and gravel to bedrock and the water was allowed to find its way through the gravel back to the surface. This water was taken from surface ditches, or through pipes under pressure, or pressure was obtained by pumping. The results of these tests were very satisfactory.
During 1917, and thereafter, experiments in cold-water thawing were made in the Dawson field by the Yukon Gold Co. and the Canadian Klondike Co., in Nome by John H. Miles of the Alaska Mines Corporation, on Candle Creek by Edward Pearce, and possibly in other districts by persons at present unknown to the writer. Certainly it seems, as has been true of many discoveries, that a number of men at different places were following similar ideas at the same time. Nearly all the men reported that the results of their experiments were successful: Miles, one of the first to attempt a practical use of the knowledge gained, was the first to apply for a patent for the method he developed, a United States patent,1,339,036, being granted him May 4, 1920, on the method of thawing ground. Following the result of these experiments, operations on a working scale were carried out both in the Dawson field by the Yukon Gold
12 Thawing Frozen Gravel.
Co. and the Canadian Klondike Co. and in Nome by engineers acting for the Alaska Gold Corporation, and through the courtesy of these companies the results of these experiments were made available for this paper.
Although there is still much to be learned regarding practical application of the method, thawing with cold water has passed the experimental stage and is being proved of great value to the gold-mining industry.
The successful application of this method where water is obtainable under pressure without pumping will make large areas of so-called low-grade ground available for dredging, and ground that has previously been considered to be of little or no value will now be of economic importance. Had the knowledge of this method of thawing been available to the Yukon Gold Co. and other companies operating dredges in the far North during the past 10 years, there might have been a considerable saving in costs and a correspondingly additional profit to those companies.
Experiments Of J. H. Miles In Cape Nome District.
As an introduction to the study of cold-water thawing, an abstract of an article by W. S. Weeks" is given. This article, which was prepared by Mr. Weeks at the request of J. H. Miles, describes various tests carried on by Mr. Miles in the Nome district. The result of these tests led to Miles adopting cold-water thawing as the most efficient method of softening the frozen gravel before mining. Several other articles on thawing frozen ground which have recently appeared in the technical press are given below; also information obtained from operating companies, this information never having been published, which should be of value to those interested in mining in the far North because the figures show results from operations and not merely expectations based on small-scale experiments. Excerpts from Mr. Weeks' article follow, with Mr. Miles' observations substantially represented by the material in quotations:
"The equipment consisted of a 12-horsepower boiler and a superheater, made of several coils of 2-inch pipe with brick and sheet-iron housing. The first experiment was with superheated steam. Three points of 4-inch double extra-heavy pipe drawn down to s-inch Spening at one end were used. 'Two of the points were lowered into 6-inch churn-drill holes. The third was put down in the following manner: A 1-inch double extra-heavy pipe drawn down to }-inch opening at one end was used; a clamp was placed around the pipe, and a weight having a hole in the center was slipped over the pipe and attached to the walking beam of a Keystone drill, which raised and dropped the weight, striking the clamp and forcing the pipe. The pipe was rotated slightly at each blow by use of a Stillson wrench. Warm water at 20-foot head was forced through the pipe, thawing and softening the ground at the point. In this manner the pipe was driven 42 feet in 4 hours and 10 minutes.
"Tn the following season a combination rotary percussion drill was developed, consisting of a 2-inch pipe with a serrated steel shoe, using cold water at 80-pound pressure flowing through a pipe. The pipe made 20 revolutions per minute, and an 80-pound
Weeks, W.S., Thawing frozen gravel with cold water: Min. and Sci. Press, vol. 120, March 13, 1920, pp. 367 370.
Thawing With Water At Natural Temperature. 13
ight, which dropped 60 times per minute, helped to force it down. With this
ine it was possible to sink the 2-inch pipe 40 feet in 1 hour and 20 minutes. The pipe used in this machine is left in the ground and used as a thaw pipe, the high-pressure water being replaced with low-pressure water at whatever head is available. This machine proved satisfactory for the soft formation. In like formations of shallow depth it is not necessary to force the pipe down by mechanical means; the pipe will sink of its own weight if rotated slightly byhand.
"The results of the superheated steam thawing were most unsatisfactory. After steaming for a few hours rite ground around the pipe began to sink, forming a pool of hot water, and the longer the thawing proceeded the hotter the water became. It became evident that with so much heat coming to the surface very little thawin was taking place on bedrock. The nature of the ground was such that when thaw it was possible to probe with an iron bar to a depth of about 25 feet. By using this bar it was found that the thawing in these holes was being done in the form of an inverted cone, as was subsequently proved when these thaws were dug out.
'Saturated steam for thawing was next tried, using ordinary steam points, but instead of driving with hammers, as is the common practice on shallower ground, the pipes were dropped into holes previously drilled by a Keystone machine. These tests showed very similar to superheated-steam tests, with the exception of thawing an inverted cone slightly smaller on top and extending to a greater depth.
''Hot water was tried, using three holes. A 1-inch double extra-heavy pipe with §-inch outlet was used. Hot water drawn from the blow-off cock of a boiler was circulated through the pipe at an average temperature of 105°. By probing with an iron bar it was atarnined that this method was far more efficient than the previous steam tests from the fact of the walls of the hole being nearly parallel. But, asin the steam tests, heat units were wasted because the water failed to deliver its heat in the ground but carried it to the surface. This led Miles to think that thawing was a slow process and that it was not a concentrated heat that was necessary but a uniform circulation of water with just a few heat units to give up. It was then determined to try surface water, the temperature at that time being 52° and the temperature of the ground 28° F. Ditch pressure not being available, a steam pump was used with a 2-inch suction and feiss discharge. This pump delivered 20 gallons per minute at an average pressure equal to a 40-foot head. The pump was connected to a 2-inch pipe, which was dropped into a 6-inch Keystone drill hole.
''No results were apparent for 24 hours, when the 'muck' began to slough from the sides of the holes aad patented the circulation of water in the hole. From that time on one could see the progress of thawing. In 36 hours an 8-foot area was thawed, and upon probing with an iron rod it was found that the walls of the thaw were vertical, not the shape of an inverted cone, asin the steam thaws. Atthe end of the eighth day it was decided to stop thawing on this hole and start another for a check, owing to the lateness of the season. The water entering the ground had a temperature of 52°; at the end of 24 hours the outgoing water was at 50°; at the end of 48 hours, 48°; at the end of 72 hours, 45°; at the end of 86 hours, 40°; at the end of 192 hours, 36° F. To obtain maximum efficiency, the amount of water entering the thaw pipe should be governed by the temperature of the outgoing water. Just enough water should be used to keep the temperature of the outgoing water a little above freezing.
"In watching a cold-water thaw in operation, one sees the water boiling up around the outer edge of the hole, notion the center.
"The time consumed in these experiments, which included three superheatedsteam thaws, three saturated-steam thaws, two hot-water, and two cold-water thaws, was from April 20 to October 25, when the weather became too cold for further experiments. Although tests with bars had given a good idea of the amount thawed and the shape of each thaw, it was decided to obtain an exact cross section of each thaw. On December 1, when the surface water had frozen so that it was possible to sink shafts without the necessity of pumping, contracts were let to parties to sink shafts over the thawed test holes in order to ascertain the exact amount of material thawed and the shape of each hole. This work was finished on December 20, 1917.
"The results of these ox pennants will be found in the accompanying chart. (See . Fig. 1.) The cold-water thaw was most advantageous because, K rst, it thaws a larger area on bedrock, and into bedrock itself for a considerable distance, thereby making it possible for dredges to clean the bedrock without encountering the frozen pinnacles and the frozen spots which are invariably leit by the present method of steam thawing Secondly, the heat units needed to thaw are supplied by nature; consequently, ground in the outlying districts, where it has been impossible to procure fuel for steam thawing, can now be thawed at no greater expense than ground near a fuel supply, thus opening for the prospector and operator a large area of the frozen North.
106452°—22—3
14 Thawing Frozen Gravel.
"Thirdly, the actual cost of thawing in ground chosen for the experiments, or ground much deeper, was estimated by Miles to be less than 1 cent per cubic yard.
"Tn the spring of 1918 Miles had occasion to thaw a block of ground 130 feet long by 80 feet wide by 11 feet deep, composed of pure beach sand. It was arranged to compare the difference in time between steam and cold-water thaws in actual operation, so a battery of 12 points was constructed, using steam from a 20-horsepower boiler. The men employed were experienced in steam thawing, working two 12-hour shifts. At the end of the third day an area 20 feet square was partly thawed—that is, was thawel on the surface—but by prodding down with a bar one could feel lumps of frozen ground. Steam thawing was stopped and a 4-inch centrifugal pump used, the discharge pipe having twelve l-inch openings. To each opening a 12-foot length of l-inch garden hose was attached and on the end of each hose a piece of common 1-inch
ipe 12 feetlong. There was Rardly any pressure at the end of the pipe, probably not over 1 pound. The pipes were held up by hand until they had sunk about 1 foot into the sand, then they were left to sink into the sand as they thawed. In 12 minutes the pipe would sink through 1l feet of sand. The hole around the pipe would be about 6 inches in diameter. The pipe was left in the hole 30 minutes longer, then the hole was about 36 inches in diameter. The pipe would then be drawn Gl Mazted in a new hole. It was found best to move the pipe after about 45 minutes, as the water seemed to start circulating under the 11-foot cap of frozen sand and the thawing action on the sides of holes was retarded. With these twelve l-inch pipes two men thawed the remainder of the area and also rethawed the ground partly thawed by steam in four days and nights. The whole area was thoroughly thawed."'
In brief, thaws were ''put in" with superheated steam, saturated steam, hot water, and cold water. In each case the thawing medium was introduced at the bottom of
Cold water.
Warm water. Figure 1.—Results of thawing with superheated steam, saturated steam, warm water, and cold water.
Saturated steam.
Superheated steam.
Thawing With Water At Natural Temperature. 15
a churn-drill hole. When the surface water was frozen, shafts were sunk on the drill holes and the volume of the thaw determined. The chart (Fig. 1) shows the cross section of a thaw of each type.
From the point of view of the heat units involved, the following is of interest:
The Ceaval af the place where the experiments were made weighed about 3,000 pounds per cubic yard, and each cubic yard contained about 300 pounds of ice.'*
early all the heat used in thawing frozen ground is consumed in melting the ice, so for this approximate calculation the small amount of heat used in raising the temperature of the solid material may be neglected. In the steam and hot-water thaws the information taken about the available heat is the amount of oil used. It seems reasonable to assume an efficiency of 50 per cent for the boiler and the delivery system. The experiments in the order in which they appear on the chart (Fig. 1) are as follows:
EXPERIMENT 1.—Superheated steam.
HOUR soi s Hs 2ccaraet hese! mates ohsse tosecee tees th ocksws steaobseenseees 156 Gallons Of OU ss a.5.c,c00 ta 4 das cet ass Gees Oras rac aa darenns Sitaatocueaneees 1, 755 Guble yardsithawell.. 5p siccdetesaensatenadewediecisinaa sea Se. ds-ecine soe sees toch 109
To determine what proportion of the heat supplied in the steam was used in thawing, hourly quantities were taken in order that the figures may be less unwieldly.
Gallonaiof 'oil perhour: 2; s28.252-css sta liss cag cdasaiaraee ea astarteeens 11. 25 Cubic yards thawed per HotBot Sasassadarseana esas SDS Men Se Sete ee RS 0.7 Bt: a1. per gallon of onl 222. sh sc cates Bleeds gS S Meme alee Sale etnies ene 140, 000
lf a boiler efficiency of 50 per cent is assumed, 70,000 B. t. u. will be available for thawing for every gallon of oi) used.
B.t u. necessary to melt 1 pound of 1cessce swscaededavckelas cecsccecceess 144 B. t. u. supplied per hour (11.25% 70,000) 2...2..22222222 2222 cece ee eee 787, 500 B. t. u. necessary to melt ice per hour (0.7 300X144)... ..2 22..2-2-2-- 30, 240 Vegihe : 30, 240 Efficiency of absorption. 787, 500 100 per cent=3.8 per cent.
EXPERIMENT 2.—Saturated steam.
HGUfiicarccecs icccosbeaseasee sees poss mee dae cedves es ueedees seness enseees 98 Gatlous obotl. 22c52 2 towoee sda So te sulecle bors Seabee Sade sone beaelaee Lee 882 Cubie yards thaw Od cos cinnesse435aveaaedaerd taSaadacksSocueasa Se elwlnieaare 83 Gallons'oil per Hour: <2. /seacuse scales hunosesvesulneeg.ces seheuacweseeees Cubic yards thawed per hour B, t. u. supplied per hour (970,000) BL. t. u. necessary to melt ice (0.85300 X 144)... eee e eee ee ee eee eee eee 36, 720 : : 36, 720 Efficiency of absorption 630, 000 100 per cent=5.8 per cent. Experiment 3.—//ot water. FROUPB os oti cli Mise ekrok Belaies pig alore's tiohe oh aretard a's Saino blew ayele loge sale oasaicie 67 Gallons Of ollsct 553s edie s Qed acaned hs tga bade sonia sulecsavaccdensadasass 405 Cubic yards thawed 02..02ecessecce cece eeee cece tec cn nce e nc ceeeeeeee 81 Gallons; of oil Perhours 6 c)ecs52 doce csi geecendssseevoasczagesdeaesdwadeeees 6. 04 Cubic yards thawed, per hots... - cs dees cet sees ch assieews cecescewesacens 1.21 B. t. u. supplied per hour (1.21 300X144)... eee eee eee eee eee eee eee 52, 272 Sree : 52, 272. Efficiency of absorption. 422, 800 X100 per cent=12.3 per cent. IXXPERIMENT 4.—Cold water. Ouray i cicisisedess hescStdataSalen Uasdichdasae hea vegaedereneeeckahhedsabia 192 Water per minute, gallong.-25 22:0. 00252582555 053 vee scdaceebseccs enstecasees 20 Cubic yards PRRWede Metal ach tinea minh Ae ea teul evening inertacuey 511 In this case the available heat units are assumed as follows: Water per. hout-pallons<.25 4s. seltecisca sso noe ccst enacts Saceaese etek shee 1, 200
16 Experiments by others have indicated more joa in a cubic yard of frozen gravel.
16 Thawing Frozen Gravel.
Cubic yards thawed per hour ease Sales ees setosesloees 2. 66
Temperature of water introduced, °F Wededbcesccascacdeh wees 52
B. t. u. per pound of water theoretically available (52 — 32) 20
B. t. u. given up per gallon (20 XK 8.35) 2.2 eee eee ee eee cece cece 167
Total heat available, B. t. u (2.66300 144). 2... 2 ee ee eee ee ee eee eee 144, 912 114, 912
Efficiency of absorption 200, 400 X100 per cent=57.4 per cent.
To summarize: Of the total heat applied in each experiment the following proportion was approximately used in doing the useful work of thawing:
Per cent. Per cent. Superheated steam 4)| Hot Waterss. casee. ci sektes was telnans 12 Saturated steam. 2.. 6 Cold water. 2--2-20- 57
A study of the steam thaw shows that the clay was penetrated only to a slight extent. The description would indicate that most of the heat in the steam was expended in keeping hot a comparatively quiet pool of water. There could have been very little circulation of the hot water, because a 12-horsepower boiler will evaporate only about 1 gallon of water per minute.
he hot-water thaw shows uniform thawing but low efficiency. The efficiency of the heat absorption increases as the area of frozen ground in contact with the thawing medium increases. This is clearly shown in the temperatures of the outgoing water in the cold-water thaw. If the hot-water thaw had been continued as long as the cold-water, then the efficiency doubtless would have been much greater.
The cold water apparently accomplishes all that is desired and at slight expense. eae is no impediment. A vigorous circulation must be maintained, so that the cooled water is quickly removed and replaced by more, which has a few heat units to give up.
Test By Alaska Mines Corporation.
After the experiments by Miles in the Nome district a comprehensive test of cold-water thawing on a working scale was undertaken for the Alaska Mines Corporation in 1919 by the Hammon Engineering Co., of San Francisco. Through the courtesy of one of the principals in the Alaska Mines Corporation the results of the experiments were made available for this paper. Mr. Edwards, chief engineer for the Hammon Co., in summing up the results, says:
Cold-water thawing was given a thorough try out with satisfactory results. The costs when handled on a large scale will be under 4 cents per yard.
Plate II, A, shows the layout of the thawing equipment, and Plate II, B, the thawed ground in front of the dredge. (See p. 3.) The experiments and the results are summarized as follows:
The equipment used comprises one 25-horsepower boiler; one double-acting 4-inch pump, 6 by 10 inch steam end, 4 by 10 inch water end, and 9-inch stroke, supplying water for points giving prosare of 60 pounds. Temperature of water going into points averages 54° and of water returning from bedrock 34°.
Points used were made up of double extra-strong j-inch and 1-inch Pips cut in lengths of 12 feet special-cut threads, special-made couplings, and connected together as they were driven down. Yor the driving of points a slotted anvil, or driving head, weighing 80 pounds, was keyed on pipe at any desired height with grip wedges. This anvil, or driving head, was made from 8-inch shafting, cut in 6 to 8 inch lengths, with handles inserted on opposite sides for turning the point back and forth while working it down; when hard material is encountered, the head is hammered with an 8-pound hammer while the pipe is being turned as in drilling. The anvil, or driving head, can also be made from old bucket pins.
For the driving end cruciform steel was used; it was cut about 6 inches long, with a }-inch hole through the center, sharpened to a chisel-bit edge and welded on end of the pipe. This style bit cut rapidly and with less danger of the point going down at an angle; also it successfully drills through bowlders. Water under pressure flowing through the bit keeps the thawed material from the bit and supplies suflicient water for circulation through the ground.
Thawing With Water At Natural Temperature. 17
It is found that the points must not be driven too fast but that time should be allowed for the ground to thaw and to keep the bit free as it goes down, thus eliminating the danger of plugging the point, which would necessitate polling same and losing the hole. Therefore, from the data on different sets it is found that two men working from point to point in rotation can cut down to bedrock five to six points as quickly as one point worked on continuously; the time for making connections with hose and coupling together of pipe as driven down has taken an average of 10 minutes to each coupling. All this can be shortened with a more Gears ged equipment.
o centrifugal pumps, one 8-inch and one 10-inch, were used to supply water. For shallow thawing common }-inch iron pipe without a bit on end was found to be sufficient as they worked down by hand very readily to depth of 9 and 15 feet. The length of time points were left in ground ranged from seven to eight days. On the third day the ground around the pert would start sinking, and as a rule on the fifth day it would have sunk to a depth of 5 feet, but points were left in seven days before pulling and resetting. On account of the ground having sunk, leaving a lake of water around points, it was difficult to test the ground with a drill to determine the extent of thawing, so this was left until dredge reached it.
As the dredge did not encounter any frost in the deep thawed ground, it was decided that seven days was enough to thaw 45 to 50 feet of ground with points set at 10-foot centers. Points set at 16-foot centers driven to bedrock, and left seven days before pulling, showed that while the ground had sunk around the points, there was an unthawed space of about 2 feet between thawed points. This frozen ground was getting soft, showing that a day or two longer would have thawed the entire block.
ero weather set in and it was necessary to shut down and pull the points. It was thought that points set with 16-foot centers would thaw successfully in 10 to 12 days under ordinary conditions, and estimates of thawing costs given in this report will be based on a spacing of 16 feet. The yardage controlled increases rapidly from the 10-foot centers to the 16-foot centers, being 138 cubic yards at the 10-foot and 363 cubic yards at the 16-foot centers.
In driving points in ground 60 feet deep, heavy wash completely frozen, there was no fete dang difference in driving to bedrock. To determine the minimum cost of thawing is difficult, as the equipment used this year was inadequate, requiring not only the expense of pumping water for points, but also of raising the temperature of the water going to the points, as the water in the pond (supply for this summer) had a temperature of 44°.
From experience it was determined that pressure is necessary for success in driving points. Both 40 and 60 pound pressure were tried and better success found in using 60 pounds. High pressure largely eliminates the danger of plugging the points while driving them and gives greater circulation of water.
By using the Wild Goose ditch it is figured that both the quantity of water required and a pve of 69 pounds is obtainable, thereby avoiding the expense of pumping and of extra equipment. Temperatures, taken at different times, of the water in the ditch have averaged 61° F. and from past records this summer has been cooler than the ae: Therefore, the temperature of the ditch water in a normal summer season will be higher. It is planned to do all the thawing necessary for one season's operation in 90 days of summer.
The following table gives a comparison between the first and last sets of thaw the ah down to bedrock by six men on the first set and by four men on the second set:
Cost data on thawing with points, 1919.
Set A.—10 points at 10-foot centers, 6 pointmen.
Driving time per point, hours 220-eeeeceececeeecceccececeee 2.7 Distance per hour, feebo.254,22..o193.c% 80.8 25 5a Sh SESS So bis whhs s Cu sseoe ses cee 16.1 Area thawed per point, square feet 22.2.2... .222- eee cece eee e eee e eee 86.6 Ground thawed per point, cubic yards. 2-0-02002-2-20-2-- 140 AVeTage. Gepth, ob. sh casas scene ses aes oebastvee eas cy bveasecsedesssacees 43.6 Average water pressure, pounds 2.22-202 ee eeeeee cece eeeee 60 Average temperature of supply water at point, °F ccsscscccc eed saasscacecn 54 Average temperature of return water from point, ° F 34 Pulling time, minutes 2.02.022 cece cece eee eee cece eee eee eens 30
18 Thawing Frozen Gravel.
Set B.—Points at 10-foot centers, 4 pointmen.
Driving time per point, hours 022000ceec cece cece eee ceecees 1.04 Distance per hour, feet 2.22202022e eee ee eee Bes Seses Seek he 41 Area thawed per point, square feet 2.02.0-200 2c cece eee eeee 86.6 Ground thawed per point, cubic yards 2.-.20-- 002 - eee eee eee 140 Average depth; feet, ... 2.525 tea csizeedenos cost oat so dees sceedanccenoced 43.6 Average water pressure, pounds 2.2200200ce cece cece cece eeee 60 Average temperature, supply water at point, ° F 2 54 Average temperature, return water from point, ° F BPE Sc CR UE ices SRE 34 Pulling time; minutesss .o.c50csues scanecs he beeen ste clas Sass Vadasacdaatemas 30
Actual cost, exclusive of piping, of thawing per cubic yard with pumping plant; points set at 10-foot centers; points to be left in 10 days after reaching bedrock.
Rate per Total cost
Item. hour. per hour. ML foramans: 522555338 5edeefeeads bts bictae lee Seh ade cee secaie ccs $0. 67 $0. 67 A POMEMENE 33223465228 oo ois.c055 Bac headed 25 biceoewadeces shore . 54 2.16 VProusta bout. 225s caccseuiicses okt ectesiiesssedscestecesteccantess . 54 .54 lpi . 60 - 60 1 helper - 54 54 1 fireman - 60 - 60 Board... .75 .75 Fuel oil . 80 .80 BleCtric POWeMsc2.5:5..5 o5 ces nos ccc easans Shetcaganedes eapeatc eka. 1.95 1.95 Miscellaneouals. sestesesuc cd yack iase scien sarala SeGeera a teceigeaee -10 -10 Cogtperthour, .2s.22.0002 535 sees edeus sae, obas sceecsae aatee es teehee 8.71 Driving time, 1.04 x $8.71, cost per hour=$9.05 cost per point. Cubic yards thawed per point, 138, or 6.5 cents per cubic yard. Costs with points set at 16-foot centers. Cubic yards: per season. - 5.05 625585 52555565 oc 50k Séaccse coeccaseteeseness 700, 000 Time required to thaw, days 2 2202 eee eee eee ee cence ences 12 Number of holes, 43-foot ground 22. 22-22-02 ee cece ee ee eee eens 1, 986 Cublevyards' per holes 2 v.22 25 ecsiecas screteona ss osesse seat aesesete te 363 Rate per Total cost Item. hour. per hour. WIORGMEN. Saecsce ccc b sce coe hci oe ee CIls EM SEM eee seca bageaevewe $0. 67 $0. 67 ZiGitChMeN ..2ssossee dics csccrcs.oasisiccebestatanwes te baaeieace - 50 1.00 Gpommtmen:: ssestadseteds resets agdnusscsascacsessdietseaeacenes 54 3.24 LPOUstADOUTs asecbscsansksaacaeeeeses eae UNE Ree eos cae ose ees 54 . 54 1 pi NGHOR ss cicewe demsccwiewsnn sila scn wes Valse pels vac ee caustecees - 60 .60 DP RUPP Sau ctuea ty mae stance ccdac she een acdsueus nage ees dea cesess . 54 - 54 Boats: iv. sonsedes sas sjnsans oc beh sae ses dba Sees bends dducaeeebeees - 83 . 83 Miscellaneous: iisc2se0s 52 se.0c ec sme de Saran eaasleasvasdcieSe ani -70 -70 Upkeep opditeh.J3 fei: sic casas qidase Gea ssbsu ruse cy case ebtacs san - 69 . 69 Overhead expense, superintendence, etc 2220---05 ee eee 1.76 Driving time, 1.04, $10.57, cost per hour=$11, cost per point. Cost per cubic yard, 3.03 cents. First cost and depreciation. Estimated cost of material in layout for one dredge + $26, 348.00 Annual charge for interest and depreciation +--+- 5, 888. 00 Annual charge for interest and depreciation, per point 2.96 Annual charge for interest and depreciation, cubic yard - 82 r st - Cost per cubic yard. Pere Tia br: os 5222s se ne nce Fece Seee ane baad Reece es Ba RON eae ROR e aw Me ese eeeeawiowees o 3.03 Matertalosciiciech oeetsae vee nhc cee poecimase cuss areraie<cosssaonee peice . 82 Totalcccsacesajosaseaess Sec eesasasee cose lsat ake Asa aieets 3.85
Google Ps ied
Thawing With Water At Natural Temperature. 19
The operations of the Alaska Mines Corporation during 1920, given herewith, are abstracted from the 1920 report of the superintendent, Mr. Rockwell.
Operations during 1920 continued 100 days, June 15 to September 22, when it was necessary to shut down because of water freezing. During the season 196,000 yards were dredged, 88,800 yards being from ground that had to be thawed and 107,245 yards from ground that did not have to be.
Cost of operations of the Alaska Mines Corporation.
Cost per Total yard, cents. cost. Thawing 88) 807 yardsicise2s 250% odes ave dadcoGd ea sworagaiaciaces 11.5 $10, 195 Power for196,062: yards: 5.25. j.ceccasewaties cuddoeccede cack éswses 15. 6 30, 598 Dredging 196,062 yards 22... 222 c cence ee eee eee eeeee 6.7 13, 186 Overhead. Shae ese aoa eae a se Shue an dion weno eee 5.8 11, 422 Total, 196,062 yards. .<i.ssccaceseccecsececcccsscivecss 33.3 17 65, 402. 94
Mr. Rockwell estimates that the total cost should be reduced 15 cents, of which 5 cents would be on thawing. He also says, 'We have further proved this year that cold-water thawing is a success."'
Experiments Of E. E. Pearce, In Candle Creek District.
A summary of the experiments (see p. 30) carried on in the Candle Creek district by Edward Pearce have been abstracted from a letter written by Mr. Pearce under date of January 20, 1920, as follows:
Three years ago I wintered in the North and did a great deal of experimenting on cold-water thawing, and from my first attempts (in a laboratory with glass tubing) to my fiual operations of last summer, the continuous operating of 100 points, I found the method a complete success. During the entire season of 1919 our dredge did not encounter any frost either in the gravel that had been thawed or in the bedrock, and the thaw penetrated into bedrock as far as necessary for dredging.
Water was pumped from the creek to the reservoir and then brought down in a 12- inch line ander a 50-foot head, the supply being divided into four units of 24 points each. It took five hours to set 25 points, the number set every day, and where there was a large overburden of muck I found it paid to use steam and water jointly. With no help from rain and with no tundra seepage, the cost percubic yard did not exceed 6cents. A year later experimentation in asmall way was carried on in Nome, and the method was pronounced an entire success. From experiments made I can fhaw more ground with 25 points, with less labor and less cost, than I did with 100 points.
This is not because the 100-point plant did not do all and more than was expected from them, but it is carrying the principle of the cold-water thawing further, and utilizing the surface water in conjunction with the point water. It is along that line that cold-water thawing will be still further developed.
Method Developed By Pearce.
The following article by Pearce describes the method of thawing developed by him after many experiments made in the Candle Creek district, and also gives some information on cold-water thawing in
47 Figures for total cost do not check, probably because of cost per yard being taken at nearest tenth; yardage may be estimated. C.J.
8 Vearce, E. E., Cold-water thawing of frozen gravel; Min. and Sci. Press, Feb. 4 , 1922, vol. 124, pp. 14-
20 Thawing Frozen Gravel.
Alaska. Mr. Pearce, as already mentioned, was one of the pioneers in this work.
This is a description of a new process for thawing frozen gravel by means of cold water, showing the methods employed and the results obtained by Iver Johnson and myself on a piece of dredging ground on Candle Creek, Alaska. Mr. Johnson and I were the first to apply the use of the small pipe in cold-water thawing operations. Prior to our use of 100 half-inch cold-water points ahead of our dredge, three holes had been drilled near Nome. Pipes of 24 inches diameter were inserted in these holes, and cold water was used for thawing.
During the summer of 1918 we thawed a shaft 30 feet deep. This shaft was used for cold storage, and the first 25 feet was in ''muck,"'' or frozen vegetal mud. In the same year we used 40 cold-water points, each of j-inch diameter, and dredged the ground so thawed. In 1919 we used 100 half-inch cold-water points, set up in units of 25 points to each unit, and dredged the ground so thawed. It was during these years of experimenting on a working basis that we discovered this new process of thawing. We proved, by dredging it, that the ground had been thawed by the new process; and we found that it not only reduced the cost of thawing, but eliminated all the many elements of uncertainty attendant on thawing by any other method on a large scale. We did enough thawing with cold-water points to know the weaknesses of our system, and we saw that in deep ground it would be only indifferently successful. Some other method would have to be employed.
The following is a description of the new process: Plate III, A, shows a general plan of the ground, A', A', A' being the portion that had been dredged peewee and B', B', B' the ground desired to be dug. This ground is perpetually frozen. It has no muck on the surface, but there is a layer of muck 2 feet above b k.
In order to confine our thawing to the area that we intended to dredge, it was necessary to turn the water of the creek C' over the area B' instead of letting it run over A'. Down the stream at D D a bar of gravel rose 2 feet above the water; and in order to get our seepage to the limits of this bar, which we were to dig, we dug a shallow ditch on each side, as shown at Eand £'. Our thawing area would be then confined within the outside edges of these two ditches. If we had not dug this ditch at F, for instance, all the ground between the shaft F and the side E would have remained frozen, as the seepage to the shaft would all have been from the water side £'. It is therefore important first to block off the area to be dredged, and then see to it that there is surface water at the extreme edges, for no thawing will take place beyond the limits of this surface water.
F is a shaft sunk into the bedrock, F' the suction, and F" the pump discharge. G is a 24-inch rotary pump with a rated discharge of 250 gallons per minute, and H is a 4-horsepower Cushman gasoline engine. The numbers 1 to 46 represent test holes driven at different times to determine the depth that the line of frost has been moved downward. The broken lines are the contours of frost at different dates. The cross sections along these test holes are represented as being taken up in place and laid horizontally on the surface of the ground.
B, Plate III, is a section taken longitudinally above and below the shaft; it shows the contour lines of frost at different dates and also the direction of the seepage. shaft 5 by 5 feet and 9 feet deep is sunk at F'into the bedrock. _It is timbered tightly down to within about 2 feet of the bottom, and is also extended above any surface water, the whole purpose being to permit no water to enter the shaft except from the bottom. Around the sides and at the bottom some coarse gravel is thrown, to keep the fine from filling in. The shaft will fill immediately with water to the water level. The suction of the pump is dropped to within about 1 foot of the bottom of the shaft, and the water from the shaft is exhausted. As fast as this is being done, the creek water K, seeping through the thawed surface area L, follows along the frost line M to seek its own level in the shaft /, thus making a continuous flow of fresh water over the surface of the frost. As this operation is continued, so will the frost line be moved downward until it has reached the level of the bottom of the shaft. The only limits to this system of thawing upstream and sideways, are the outer edges of the surface water. 1t may be advisable, however, in certain instances, to sink two shafts. This can be determined only in each individual case.
In our operations last summer we found on August 4 (the day we started to pump from the shaft) that the seasonal thaw had reached a depth of 34 feet, as represented by the contour line Min B. On August 6 the frost line had been moved downward to contour line N. On August 18 (the day we stopped pumping) the frost line had been moved downward through the entire area to be thawed, passing through and thawing a layer of muck, till it had reached and thawed into the bedrock.
Bureau Of Mines Technical Paper 309 Plate Iii
WG es, S. Sscrese/ wig THE fl Yi PLAN AND SECTIONS OF W1ON TAKEN LONGITUDINALLY ABOVE AND D.
omnes Google —
Princeton Un
Thawing With Water At Natural Temperature. 21
The entire area, 790 feet upstream and 235 feet downstream from the shaft, with an average width of 60 feet, had been completely thawed in a period of 15 days, with an ac pumping time of 80 hours. This ground was dredged during the same summer and no ''horses" of frost were encountered. The ground was thawed well and evenly. The cost per cubic yard to thaw this ground was 1.4 cents per yard, not including any overhead or equipment charges, In many parts of Alaska frozen muck and sod cover the surface of ground that 1s rich enough for dredging. Anyone who has mined in the North knows that, when this condition is encountered, the cost of extracting the gold will be increased materially, no matter what method may be employed. In our new process this condition will increase the cost of thawing, but not to such an extent as when thawing by any other method is adopted.
C, Plate II, represents a cross section of a piece of ground of this nature, having the creek A on one side, then a bare gravel bar K, and then a gravel bar covered with frozen muck and sod G. It is desired to thaw the ground between £ EH' and the shaft F. We do not want to thaw D; if we did we should move the shaft farther to the right; A is the water in the creek, B' the top gravel of the creek, naturally thawed to a depth of 34 feet. H H is frozen gravel, and G' G is frozen muck over the surface of the gravel. A shaft is sunk at F into the bedrock. This shaft will remain dry, and no thawing will take place unless a small thawed area C is created below the layers of muck. When this area is thawed, a seepage will be maintained from the week A through the artificially thawed area C into the shait F. The operations then are the same as they would be were there no muck, as described heretofore. In C, Plate I, cold-water points are used to thaw this area. The points are set in the muck to the top of the gravel only and are spaced 10 feet apart. It may be contended that as water points are used to start the thawing they might as well be used to complete the operation. This contention is wrong, for it is an entirely different matter to set pins to the top of the gravel and to set them in the gravel to a depth of 15 to 40 feet.
'furthermore, there is a tendency in ground of this nature for the frozen collars to extend downward into the gravel, and when thawing with points is completed there is no centainty that no frost 'horses'' are left.
A.—New process of cold-water thawing.
{Equipment used, cost of operations, area thawed, and cost per cubic yard thawed.)
Equipment. One 4-horsepower gasoline engine. 20202 ee eee eee ee ee eee cece ee eeee $135 40 feet of-24-inCh plpe.s.ssaces sceca es oie SAS sae wa aS weiss e Sewidwa se de.4o' sedeeee 40 POGM SENG CllA? ts o25s sie pic vg ia cesar asincaawilais.e eet LinpigeS cose oe Sere wlene Oe 6.355 10 Foot valve and incidentals. . 20-22. eee eee cee cee eee e eens secater OO) Lumber for 5 by 5-foot shait, 9 feet deep 2---2--22.22022eeeee 85 DotalesstaargsasesaiessDigds si tecore} isedeets teageaes ies ynilicssetad 300 Labor.
Digging shaft, five men for 14 days 2.-20--2.2 222 ee ee eee ence eee eee 60 Pumping during digging of shaft, 1 day, 1 man +-- 8 Extraiman, 2 dayesss scocsa it Sat acisesqarcsas bbb egle. odode pen wpa sade Races 16 15 days' pumping from shaft, 1 man 2--- 222-2 e ee eee ee eee ee 120
Distillate weds 106) Gallons ysis osths.d5 Gays davis sincsauee sis nodesauiacwae sions 60 Mibc@haneous ss vost osasa.z hee eseas tyes nnn ceee sarees asec eseaee sh esas eenere 25 DROtah ccs ssh cc cee sReG cue As FATS eARDREA LISS Es Foe S Cabs eaeoas esas 289
Area thawed, 790X235 feet by 609 feet =20,500 cubic yards, at a cost of 1.4 cents per yard.
Total time plant was in operation, shifts + 15 Total time of pumping shait, hours 2 2 2 05- 89 Average mean temperature of creek, °F. 2 2---22+25- 64 Average mean temperature of discharge, °F +-+-+-+--+++++++--- 48
22 Thawing Frozen Gravel.
B.—Old process of cold-water point thawing.
[Equipment used, cost of operations, area thawed, and cost per cubic yard thawed.]
Equipment. ; One 50-horsepower Union gas engine 2. 222... eee eee eee eee Ae re $8, 000 One. Kingsford pump; '8-inch.n ...03 ..cceca ge etic ag aise pee cetseeeeeeceecs 500 800 feet of 12-inch hydraulic pipe LER athorndsing tobi enisaeeciacia gals 1, 000 1200 feet Ol Gites coy wt obi coca demas a uac.a ste cwes tau dude oueeoe Mee baw 500 240 feet of hydraulic hose, 6-inch 22 eee eee e eee ee eee eee 240 100 feet of hydraulic hose, 10-inch 02 0.22 e ee eee ee eee eens 100 4 units,:25 points each 25... nencssy facies classereet cekcesOisecasdewenoeae 150 BOO feet Of ernch Pipe secs asco: 524 seas deweatasa ease sOnssageaesEiOesRee 80 1200 feetGt FineCh' Pipe. secaxy Seocsacicetasahawsloase cit dediacsdesdawseese 100 700 feet of rubber hose. Fem avn ks pao de dae cae bes caatewasa eats 400 Tees, ells; unions, valves, ete s..sa. css c% a0 soe5 sw eee sine badesaensec eecewes 100 Votalostesahs'2 362 cadets nse zenwte gees Rass teead GHeotaeans ee bee Nse le 6, 170 Labor.
One pointnat: SO BHiMB. 52 Sos cec Set kas usu Seed big eek cb ee dees eaecee 300 One helper, SOebifte. Sook gos Fiboos goss aes ead 8 Hasse teac russe ceheesse 240 One engineer, 2b-shilte. s ssa2c as eciaccei oct cnsc sas ie adoh ds eaceewssesoas 250 ' 790
Distillate used ; 500; gallons. .2ewisisiesis p24 do caw ews hese anie dane Wien eee cals ewes 300 Miscellan6gus:..4.6 os cco tes cal oss sseaeae's Sic Jud ad Pwalaigns ggielels' nS -welele's 50 Total. 242.02 asic etdan bei sega Updees bn cd ehes lard oag tee eAe Selendess 1, 140
Area thawed, 450809 feet=12,000 cubic yards, or 94 cents per yard. Comparative statement of Tables A and B. Cold-water New ; point method. process.
Cost of equipment used 2 + Saks hbae sess $6, 170 $300 Cost: Ob lahor..c0.crate5s betes aged. bee ehsescteee sus tags eatin 790 204 (Cost! oP Oils i553 2loeo alike os Fasting a bess aeteecpe ena scdenas eases 300 60 Miscollaneoua ss. co5tcce snes ce ve sparse sticawstiplsdomsticlsletpeteaes's 50 25 Total: cssviameshersnedseerosaaassancssiietansaleweeres 7,310 589 Ground thawed, cubic yards 2..2.0 2 se ce eee eee 12, 000 20, 500 Coat: per cubic yard thawed: : ssc.ic- ee conse soe one 3G 5% sme we aie nie $0. 095 $0.014 Average thaw per shift, cubic yards +.-++-+.+++++-- 400 1, 366 Time of plant in operation, days. 2--2-+ 22--- 30 15
Work Of Yukon Gold Co., Gold Run District.
An abstract from a report on cold-water thawing operations of the Yukon Gold Co., in the Gold Run district, for the year 1920, was obtained through the courtesy of Mr. O. B. Perry. It is understood from some of the engineers of the company that much of the ground where these tests were made in 1920 had been drifted, so that these experiments would not wholly apply to virgin ground. The same thought presents itself about some of the other tests in other fields, complete details being lacking on the amount of thawed and of frozen ground, its proximity to open-working faces, and what effect these factors would have on the results. The Yukon Gold Co. found it necessary or advisable to use steam points to supplement cold water in order to start thawing early in the season and to con-
Thawing With Water At Natural Temperature. 23
tinue work after water thawing stopped because of the low temperature of the water.
Most of the tests in cold-water thawing were made in the months of June, July, and August. When the summer heat raised the temperature of the surface water greater efficiency was obtainable than earlier or later in the season. A large plant would be necessary to thaw enough ground during the summer to supply the dredges for their season's work.
The experiments in cold-water thawing were made by using pumps in tandem; they required considerable power, but the company had electricity available. In other districts the power cost might be much higher.if the water had to be pumped, or might be considerably lower if water under natural head was available.
Steam thawing started at Gold Run on April 12 and continued until June 13. Because of delay in receiving some of the water equipment, cold-water thawing did not start until June 20; it continued until August 24, when it was abandoned because of the temperature of the water being practically at the freezing point. During this period a maximum of 812 points was operated with an average crew of about 48 men. It was possible to handle the larger number of points with a smaller crew because of the longer time required for water thawing the same number of points.
Steam thawing began again on August 21 and continued until the end of the season.
The labor cost for water thawing was higher than had been estimated because of the necessity of using the same water continually; as the water became dirty the moss and roots clogged the points and caused delay. The cost of cold-water thawing compares with steam thawing as follows:
Comparative cost of thawing with steam and with cold water.
Steam Water thawing. thawing.
$23, 292. 38 1,591. 09
94,427, 48 20, 458. 69 Square:yards thawed... 2... .0..sacccdcesnscoessssecaseces spsa ducmeasttatsscase se 61, 882. 00 27, 18.00 Costiper square yard... 5. fseoe pecs cts sevneedga seine es0scs spesaapalanantens danse $1. 527 $1.08
Depth of ground, 25 to 30 feet; men employed, 40 to 45.
In addition to the cost of water thawing as given, there was an extra cost of $19,433 for new equipment, depreciation, and extra freighting; including these would raise the cost to $1.43 per square
24 Thawing Frozen Gravel.
yard, but these charges are not included because they will not occur again. The figures are of much interest, for they show the methods of the company that perhaps has had the most experience in steam thawing, and also show the costs of cold-water thawing as determined by that company.
EXPERIMENTS OF THE NORTH WEST CORPORATION (LTD.), IN KLONDIKE DISTRICT.
In a personal communication to the author, Mr. Frederick P. Burrall, manager for the North West Corporation (Ltd.), and the Canadian Klondike Co., has given the experiments of the former company in detail. Water is supplied from a ditch line; but as there is a shortage of water during the summer, the water after use has to be pumped back to the ditch line; this extra expense would not apply at other seasons of the year. Complete figures of cost are not at present obtainable. Mr. Burrall says:
During 1920, the North West Corporation (Ltd.) tried out this method ahead of a dredge on Upper Dominion Creek, and the results were so satisfactory that this season a full-size plant has been established, and about 300,000 cubic yards have been thawed todate. The deposit here consists of 4 to 6 feet of gravel with an overburden of 10 to 20 feet of muck, glacial silt, and some old tailings. The cold-water points are spaced if squares of 8 feet, with the points down to bedrock. This arrangement produces a very satisfactory thaw of the gravel and bedrock, but is apt to leave the upper part of the overburden in large blocks. To overcome this, short points are placed midway between the longer points, thus (in connection with the long points) making points at intervals of 4 feet. The points are left in the ground four to eight days, according to the water temperatures and the ease with which the material thaws. Considerable difference in the action of the different materials is noted, so no particular time is fixed for the length of the thaw. Ordinary black muck and ice go very quickly, whereas fine sand and glacial silt do not thaw regularly and take twice as long.
The temperature of the ingoing water varies from 40° to 70°, the latter temperature being toward the end of July, which is the period of greatest average warmth. The water coming out of the ground is about 38° to 42° F. Cutting down the velocity of the water makes the temperature of the discharge run lower, but tests have shown that when water is near 40° it is not so efficient in thawing as when it is above that temperature.
The points used are of 4-inch pipe, extra heavy when obtainable, and under ordinary circumstances each point will take nearly one-half miner's inch of water. The
ints are connected with the ''header" by ordinary molded }-inch water hose, Vatened to the pipe with a bit of hay wire. The headers are of 14-inch pipe, 56 feet long with 4-inch nipples welded in at 8-foot intervals. The water hose slips over these nipples and is fastened with wire. Nosmall valvesare used. The headers are connected in the center to a 2-inch connection welded in an 8-inch riveted hydraulic
ipe. On each side of this connection in the 14-inch pipe is a 14-inch gate valve. Thus each 14-inch valve controls four points, there being eight points on each ''fulllength header.'' The connections on the 8-inch flanged hydraulic pipe are placed
8 feet apart, and 10 full headers, equivalent to 80 points, are operated from each 8-inch pipe. Three 8-inch pipes are supplied by a 10-inch main from the water supply. The 8-inch pipes lie on the ground and the 14-inch headers are supported by blocks or wrestling. about 14inches from the ground. (See Plate II, C, p. 3, and Figs. 2and 4, showing arrangement in detail.)
The water is supplied to the 10-inch main from a ditch and has a pressure of about 35 points at the 8-inch pipe. There is a shortage of water during the dry season, and water is pumped back from below the points, 2,000 feet upstream, and then elevated to the ditch. This gives the return water an opportunity to absorb some heat from the air before it again reaches the points.
Thawing With Water At Natural Temperature. 25
At Granville, on Dominion Creek, there is a deposit of 26 feet of gravel with 12 to 14 feet overburden. This overburden has been washed from several million cubic yards of the gravel, and natural thawing has taken place for a depth of 8 to 20 feet.
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Figure 2.—Pipe layout for cold-water thawing on Dominion Creek, Yukon, August, 1921. The North West Corporation, Ltd.
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The ground remaining to be thawed is bedrock and 6 to 18 feet of gravel aboveit. A similar equipment to that described above is used, except that on account of difficulty in getting the points through the gravel, they are driven with light sledges. To protect the pipe, a light head fastened to the pipe with an extra-heavy pipe coupling
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Hardened points are not used on the pipes, as it was found difficult to withdraw the pipes when this was done.
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Figure 3.—Spacing of long and short sweaters: a, sweater; b, fine muck, 12 to 15 feet thick; c, thawed ground, 1 to 3 feet thick; d, pay gravel, 7 feet thick; ' bedrock. rye
Figures of costs and yards thawed per miner's inch are not yet available, as this is
our first active season with this thawing process, and these figures will be made up when we shut down for the winter.
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28 THAWING FROZEN GRAVEL. EXPERIMENTS OF FAIRBANKS GOLD MINING CO., FAIRBANKS DISTRICT.
The following account is from a special field report on the experimental work in the Fairbanks district, the report being made for the Bureau of Mines by K. T. Sparks.
Because of the high cost and incomplete results obtained in thawing frozen ground with steam the Fairbanks Gold Mining Co., which operates two dredges, decided to conduct a series of experiments with cold water during August, 1920.
Test With Points.
The first test made with cold water consisted of connecting a water line under a 30-foot head (13 pounds per square inch) to a manifold, which in turn was connected to the thawing points by means of rubber hose. The points were of the usual steamthawing type and were 26 feet long. The block of ground selected for the test measured ap yroximately 30 by 60 feet and consisted of 15 feet of muck overlying 7 feet ofgravel. The muck had thawed naturally toa depth of about 1 foot. Theremainder, both gravel and muck, was frozen except for a streak of thawed material immediately above the line of contact between the gravel and muck. This thawed streak was to 3 feet thick and was probably the result of the circulation of air in old underground workings, as a large amount of the ground being dredged on Fairbanks Creek has been mined by shaft and drifting methods. Twenty-four points were used in the thaw; they were spaced 5 feet apart, and the water was allowed to run for 36 hours. The temperature of the water at the intake was 58° F. and at the discharge from the points 42° F. At the end of the 36-hour period the ground was thawed about as thoroughly as the usual thaw with steam in this district, but there were numerous blocks (known locally as ''niggers") of unthawed material. These were due, no doubt, to the water rising only as far as the thawed streak, finding a channel through this, and not rising directly to the surface.
Test With Sweater.
The second test was made by using ''sweaters""' made of a piece of j-inch pipe with an open T of the same size threaded on the upper end. The sweater does not taper to a point but is full size to the end of the pipe. Long, 24 feet, and short, 12 feet, sweaters were alternated (Fig. 3). By this arrangement the longsweaters reached to bedrock and the short ones to within 1 to 3 feet of the thawed material. The sweaters were spaced 3 to 4 feet apart. This thaw also continued 36 hours. The frozen ground was completely thawed, no blocks of frozen material being observed.
Another test was made with the same equipment and under the same conditions of water pressure, temperature, etc., but in comparison with a steam thaw on adjoining ground. Results of this test showed that with cold water the ground was thawed uniformly and thoroughly, whereas in the steam-thawed ground there were numerous blocks and ribs of unthawed material. The time allowed for cold water to thaw the ground was 36 hours and the steam 24 hours.
Arrangement Of Thawing System For 1921 Season,
A 2-mile ditch has been constructed on Fairbanks Creek from claim No. 8 above to claim No. 1 above, which gives a head of 125 feet. This ditch supplies a penstock, from which a 10-inch steel spiral-riveted pipe leads to the claim (No. 1 above) where the water is used. Six-inch distributer pipes carry the water to convenient. places for connection to the header pipes feeding he sweaters. This water can also be used for hydraulicking when there is any to be done.
MISCELLANEOUS TESTS. To determine the most efficient diameter of sweaters, a test was made as follows:
A 3-inch and a j-inch sweater were set 6 feet apart in frozen muck. Water under a 125-foot head (54 pounds per square inch) was used and allowed to run 24 hours. The
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Thawing With Water At Natural Temperature. 29
ia acon of the intake water for both points was 60° F.; the temperature of the
discharge water from the 3-inch sweater was 40° F. and from the }-inch sweater 43° F.
Probing with a barshowed that the j-inch sweater thawed an area of approximately
2,124 square inches, whereas the }inch sweater thawed about 1,810 square inches.
bits seemed to indicate that under these conditions the smaller sweater is the more client.
A test was also made, during the first week of June, 1921, to observe the results of low-temperature water. A }-inch sweater was set in gravel (not muck) and the water used under a 125-foot head. The temperature of the intake water was 37° F. and of the discharge water 32° F., or practically freezing. The test was run 24 hours, when the point had thawed a cylinder having a radius of 25 inches.
Another test was made with a #-inch sweater in comparatively coarse gravel. Water under a 125-foot head was used and allowed to run 24 hours. The temperature of the water at the intake was 60° I'. and at the discharge 44° F. At the end of 24 hours a cylinder having a radius of 36 inches was completely thawed.
Equipment.
The present (1921) equipment consists of:
Three double-header pipes or manifolds, 1}-inch diameter each, having 16 outlets for j-inch sweaters.
One long single manifold, 24 feet long and 1}-inch diameter, having 16 outlets for {-inch sweaters.
One double manifold, 3-inch diameter, having 16 outlets for }-inch sweaters.
One single manifold, 3-inch diameter, having 12 outlets for #-inch sweaters.
Each leg of the double manifold is 12 feet long and has eight outlets. The base has a connection for a 3-inch canvas supple hose which takes water from the 6-inch distributer pipe. Plate IV, B, shows a general view of the double manifold.
The single manifold is 24 feet long, and has connections for 16 sweaters. It is useful when thawing across the bed of the stream flowing from the dredge pond, as it avoids frequent wading by the workman making the set-up. Plate IV, A, shows a view of the single manifold.
The double manifold for the 3-inch sweaters is constructed the same as the double manifold for the j-inch sweaters, except that it is made from 3-inch pipe. This larger pipe is necessary because of the additional volume of water necessary to supply the sweaters. Plate V, A, shows this manifold.
The single manifold for the }-inch sweaters is a 12-foot length of 3-inch pipe, capped at one end and having a connection for the supply hose at the other. Plate V, B, shows this manifold.
Construction And Number Of Sweaters Used.
The sweaters are made of pipe, of the diameter required, in lengths ranging from 4 to 26 feet, and have a T threaded on the upper end. One side of the T is capped and the other has a connection for the supply hose. The T is also used as a driving cap in case a bowlder is encountered. The lower end of the sweaters is not pointed but discharges full bore. During the experiments described there were 64 of the 23-inch sweaters and 28 of the }-inch sweaters in use, or a total of 92 used in one thaw, Plate IV, C, shows a general view of the dredge with the sweaters in operation.
TIME REQUIRED FOR SETTING POINTS AND THAWING GROUNDS. After the water is turned on the sweater sinks almost immediately through the first
foot of thawed muck and with a little working by hand from side to side it will penetrate about 1 foot into the frozen material. From this point the progress is slower,
80 Thawing Frozen Gravel.
but usually the sweater will sink to bedrock without driving in four to six hours. Two men have no difficulty in sinking 92 sweaters in an eight-hour shift. The sweater is allowed to remain until water ceases to bubble up in its vicinity, indicating that the water from the points has found an underground channel and the thaw is complete. This usually requires 36 to 40 hours.
Two men on a shift can take care of 100 sweaters and make such changes as are required. From the tests made at Fairbanks a sweater of j-inch diameter under a high head was found more satisfactory. The smaller sweater thaws a larger area with less water and is also easier to sink to bedrock than one of larger diameter. There is no doubt as to the advantage of thawing with cold water as compared to thawing with steam. Although with cold water the time required to thaw the ground is somewhat longer, this is offset by the thoroughness of the thaw and the saving in cost.
Conclusions.
The different experiments and work done in cold-water thawing demonstrate that under favorable conditions the process is an assured success. It is proving of great value to the gold-mining industry of the far North and is encouraging investigation and exploitation of areas of frozen gravel that had been considered of too low grade for profitable mining.
Much remains to be learned regarding the best method of applying the water to the frozen ground. The article by Edward Pearce (p. 19) should encourage similar experiments. Undoubtedly further improvements will be made as work is attempted on a larger scale and operating costs will be reduced as better methods are developed.
Water Supply—Supplementary Steam Plant.
A number of factors must be considered in judging the feasibility of any mining venture based on the thawing of frozen ground by the use of cold water. The most important factor, other than a high enough gold content and the general conditions governing successful dredging or mining operations, is that a large quantity of water delivered at small cost is required for thawing frozen ground by the method as now used. At the plant of the Yukon Gold Co. on Gold Run, where the ground was 25 to 30 feet deep, 1,000 points were necessary to thaw the ground in front of a 74-foot dredge. Water for these points was furnished by a large pump delivering 3,000 to 3,500 gallons per minute. About 45 men were required to operate the plant, set points, etc. Thawing with cold water was practicable during only three months of the six months' season and a supplementary steam-thawing plant was necessary in the early summer and late fall. This has been the experience of the Yukon Gold and other companies that have tried cold water.
Edward Pearce in his early experiments found it advisable to use steam in addition to water under normal temperature, but has since developed a method of artificial drainage to supplement the cold water applied by points. It is possible that a large plant might thaw
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Conclusion. 81
enough ground in three months to last until the end of the dredging season, but the experience of the companies mentioned is that steam thawing is necessary at least in beginning operations in the spring. Ground that is thawed but not mined at the end of a season is generally frozen solid again during the winter and requires rethawing.
The number and size of dredges will depend upon the size, depth, and character of the ground to be worked; the same considerations govern the size of a thawing plant, and to some extent the cost of thawing. The latter, however, will also depend largely on the quantity of water available and whether it can be delivered under natural head or whether pumping will be necessary.
It should be understood that on some creeks where mining is carried on the water available during the greater part of the season would be insufficient to thaw ground for large-scale work, even though the available water was constantly reused. The Yukon Gold Co. found that 3,000 gallons or more of water per minute was required to thaw ground ahead of a 74-foot dredge. Figures on the quantity of water required at other operations are not at present available.
It is obvious that when water under natural head is not available, or is obtainable only through a great outlay for the construction of ditches, the cost of pumping water in districts where labor and fuel costs are high might prohibit the profitable thawing of frozen gravels low in gold.
The success of thawing frozen ground by the application of cold water (water at natural temperature) under pressure depends, therefore, on securing a large quantity of water that can be delivered to the ground at low cost.
Need Of Comprehensive Data.
Tracts suitable for dredging in the far North will contain areas that differ as follows:
1. Wholly frozen.
2. Wholly thawed.
3. Thawed from the surface to within a few feet of bedrock and from there on completely frozen.
4. Frozen some distance from the surface and to within a few feet of bedrock and from there on thawed.
5. Containing irregular blocks of frozen and thawed gravel extending from surface to bedrock.
Enough information on the action of cold water on ground having these different characteristics is not yet available. Little is known regarding the amount and the circulation of underground water. The water delivered to bedrock by a point may not rise near the point, but may pass several points in succession and come to the
32 Thawing Frozen Gravel.
surface 50 feet or more away, indicating an underground circulation. The result of this circulation is to leave blocks of unthawed ground which require the placing of additional points between those first set. It is in such cases that steam thawing has proved useful in supplementing water thawing.
Some experiments in cold-water thawing have been made on ground that has previously been mined by drifting or other methods, or on frozen ground adjoining thawed spots or close to banks exposed in open pits. It is not clear what influence these conditions would have in aiding the thawing of the frozen gravel. It would seem, however, that the thawing with cold water of areas where there is little or no underground circulation would require a longer time than the thawing of frozen ground close to thawed areas and open faces.
Most frozen areas that are mined contain irregular patches of frozen and thawed ground, as mentioned by McCarthy in describing his experiments in the Yukon. Aside from this, the circulation of underground water in these areas seems to have received little study, yet it is a matter likely to have considerable bearing on the successful application of cold-water thawing.
Existing surface streams may be used to advantage by preparing drainage ditches instead of allowing the water to find a natural course through the ground.
In the Nome district it was noticed that considerable water was lost from surface ditches by seepage through the frozen muck and gravel. This water, forming or following natural channels of circulation through the frozen ground, gradually cut its way to bedrock. The effect of this thawing was in part responsible for an ambitious scheme proposed about 1910 or 1912 for draining a large part of the Nome tundra. The proposal had many advocates at the time, but the cost of the work prevented its being carried out. It is evident, however, that the thawing of frozen ground can be greatly aided by a drainage system of some kind, either following natural underground channels of circulation or by forming artificial drainage ditches.
A thorough study of drainage either by ditches or by pumping, or by some system combining both methods, should be made in any further experiments of thawing frozen ground with water under natural temperature as it seems that in this direction improvements can be expected and expenses reduced.
Temperature Of Water.
Any method for thawing frozen gravel necessarily depends on the application of heat in some form. The experiments discussed in this paper indicate that the use of large quantities of water at a tempera-
Conclusion. 33
ture of 50° to 60° F. at intake and 10° to 12° F. less at outflow is the cheapest and most efficient way thus far known of applying this heat.
It must be recognized, however, that in the far North the thawing season, when the natural temperature of the water is 50° F. or more, is short; it lasts only three months, from the middle of June to the middle of August. In some seasons the highest temperature of the water in ditches and ponds has averaged only about 40° F. and in experiments with cold-water thawing during those seasons it was necessary to heat the water before applying it. Gibson (p. 17) had to raise the temperature of the water from 44° to 54° F. in order to obtain satisfactory results, as did the Yukon Gold Co. in some of its work. Burrall points out that during the summer the temperature of the water ranged from 40° to 70° F., and that although cutting down the velocity of the water caused the temperature of the discharge to fall below 40° F. greater efficiency was obtained by having the temperature of the discharge water above this figure.
Statements as to the percentage of ice in a cubic yard of the frozen gravel do not agree and therefore estimates of the number of heat units required to thaw the frozen material differ. It should be recognized that the figures are generally only estimates, and while they are useful in indicating approximately the heat units required they are useless for any refinement of calculations.
Cost Of Cold-Water Thawing.
The cost of cold-water thawing is a matter of great interest, but unfortunately the data available are insufficient. At the Yukon plant, where pumping was necessary but the company had available a large amount of power, the bare operating cost indicated was 10 to 14 cents a yard, without any charges for overhead or the amortization of the plant. In the Nome district the estimated cost for large-scale operations is 4 cents or less per cubic yard, this estimate assuming the availability, with no charge for it, of a large quantity of water under natural head. Estimates in other districts under varying conditions run from 5 to 10 cents a yard. As in all mining operations, the cost will depend largely on local conditions, and chiefly whether water is available under a natural head or will have to be pumped. The recent work of Pearce, following improvements in his method of thawing ground, shows a much lower operating cost, the figures he gives being less than 14 cents a yard for 20,000 yards thawed.
Need Of Investigation.
Each of the features mentioned, as well as all others that govern successful placer mining, should be given a careful study by the examining engineer or mine operator. When mining investment is pre-
34 Thawing Frozen Gravel.
ceded by a careful investigation of every phase of the venture there can be reasonable expectation of profits. At times, even after fairly careful though hurried investigation by well-intentioned men, the returns fall short of anticipations, but without careful investigation by men of experience a so-called investment in a mining venture becomes merely a wild gamble. Especially is this true when the investor is invited to participate on the basis of glowing terms used by an irresponsible promoter.
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