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STABILITY OF FOOTDATIOSB nw PEBUAHENTLY FBOZEB GROOHD haery CARLSQK Corps of Engineers, St. Paul District

Sn MMAHT The stability of foundations on permanently frozen ground depends on the soil characteristics, the quantity of ice in the 6oil, a nd the depth of thawing below the foundation. Heaving and settling common to fine - grained or frost action in temperate zones are also common to similar soils in and re­ gions which are subject to seasonal freezing and th awing. Tests of insulating materials and deep - fills indicated that they did not appreciably reduce heat transfer into the ground under structures. The use of piling and an air space under heated structures appears at this time to be the most effective method of in­ suring the stability of such structures constructed on permanently frozen ground which loses upon thawing.

During the recent war years, numerous frozen, even though the composition, texture, structural failures of runways, , and and moisture content are the same. The pres­ buildings occurred at military installations ence of underlying frozen ground affects the in and northern . Generally, the physical properties of the unfrozen ground failures were caused by the melting of perma­ above. Ice fills some or all of the voids be­ frost (permanently frozen ground) due to the tween the soil particles in frozen ground and removal of the natural ground insulation in­ acts as a cement. The strength of frozen cident to construction operations and, in the ground therefore approaches that of ice. The case of buildings, the introduction of arti­ compressive strength of frozen ground in­ ficial heat to the ground. An investigation creases as the temperature is reduced and it is now being made by the Corps of Engineers, also varies with the amount of voids filled St. Paul District, for the purpose of develop­ with ice, in general, increasing until the ing methods for the design and construction voids are filled. of airfields in arctic and subarctic regions. In permafrost regions the relative in­ Permafrost is found at depths of one to tensity of heaving of ground during freezing many feet below the surface in Alaska, Canada, is determined by the texture of the soil,the and other parts of the world where thickness of the soil layer above permafrost, the mean annual temperature is below 0° C. the availability of a ground supply,and Seasonal freezing may penetrate to permafrost the duration and intensity of cold weather. in many locations, thus providing a continu­ Essentially, the causes and effects of freez­ ously frozen layer from the ground surface to ing and thawing are the same as in temperate the bottom of permafrost during the winter zones for similar conditions. Where the layer season. Permafrost exists as a continuous lay­ of soil above the permafrost is relatively er or as discontinuous layers. thin, as in northern Alaska, there is gener­ The natural temperature of the ground ally no appreciable heaving when the ground follows a cyclic variation with the seasons. freezes during the winter. Farther south The amplitude of these cyclic variations is where the soil layer is thicker and more greatest at the ground surface and decreases ground water is held above the permafrost, to zero at some depth below the surface.There more heaving can be expected where other con­ is an increasing time lag with depth for sur­ ditions are similar. Generally, the effect of face temperatures to penetrate into the and thawing is more severe in arctic under natural conditions. When the natural in­ and subarctic regions than in temperate reg­ sulating cover is removed and an artificial ions because of the greater intensity and cover of relatively high thermal conductivity duration of cold weather. such as a or runway surface is placed on When frozen fine- grained soil thaws, it the ground, the cyclic temperature variations generally becomes very plastic and the excess are generally increased. Where a heated struc­ water flows out from under any building or ture is placed on the ground, seasonal temper­ pavement load which has been placed upon it ature variations may be largely eliminated resulting in settlement and damage to the due to the nearly uniform heat transfer into structure. The magnitude of the ensuing set­ the ground during all seasons of the year.The tlement depends on the soil characteristics, general effect of the replacement of the na­ the quantity of ice in the soil, and the depth tural ground cover with an artificial cover of thaw. The rate of settlement depends large­ is to cause an increase in the amount of heat ly on the rate of thawing as as on the transfer into the ground a consequent in­ type of construction and dimensions and orien­ crease in the normal ground temperature. Thus, tation of the structure. There is a definite where permafrost existed prior to construct­ tendency for structures to settle most on the ion, the effect of the construction is to south side because the ground on this side cause a lowering of the permafrost surface. receives the normal amount of solar radiation Where the permafrost is of a silty character plus the reflection from the south wall of and melting occurs below the foundation level the structure. Ground on the shaded side of a of a structure, the stability of the structure structure receives less solar heat and, as a is endangered. result, the depth of thaw as well as the re­ Physical properties of frozen ground de­ sultant settlement is less thaA on the south pend on its composition, texture, ice content, side which is directly exposed to the sun. ice distribution, and temperature. Thawed Sliding or flowing of surface ground ma­ ground has somewhat different physical char­ terials is common in the permafrost region acteristics than ground that has not been during the summer when it is thawed and in a 52

Construction of foundation supported by steel piles in permafrost. Note ground ice in back­ ground .

4 April 1946. crack In roadway shoulder at Northway Airfield, Alaska.

4 April 1946.Differential settlement of garage building at Northway Airfield, Alska.

permafrost, temperature of the ground, and the type of foundation construction to be em­ soil after stripping of ployed. rihere the soil is largely silt or very and . Note water from thawing of frozen fine sand, elimination of the frozen condition ground, will convert the ground into a plastic and un­ stable mass upon which construction operations plastic condition. Such actions occur on hill­ are extremely difficult. sides There there is a concentration of ground Methods used in arctic and subarctic moisture at shallow depth immediately above regions for construction of foundations on the permafrost or seasonal frost level. Con­ ground which does not heave are not appreci­ sideration should be given to the possibility ably different from those which are used in of sliding or flowing of ground, especially temperate . In planning a foundation in connection with the location of roads. that will rest on or in permafrost, the bear­ Removal of the natural vegetative cover, ing capacity of the underlying ground should such as trees, grass, and , through the be determined both when frozen and when thaw­ action of fire and man, results in greater ed. Unless positive measures are taken to pre­ heat transfer into the ground, melting of the serve the permafrost, foundations must be de­ ground ice, and settlement or caving of the signed for the bearing capacity of the founda­ ground. Such action produces depressions of tion material in a thawed condition, where it various sizes and shapes varying in width is desired to preserve the permafrost under a from a few inches to several miles and up to building, it is necessary to provide an air many feet in depth. space between the floor of the building and In building structures in permafrost the ground and to control the circulation of areas, the permafrost may be retained or eli­ air in this space. Vent shields are sometimes minated depending on the character of the constructed around the base of a building be­ soil, ground water conditions, thickness of tween the ground and the floor. The vents are 53

Settlement of floor in motor pool building at Horthway Airfield, Alaska. Floor orinally flush with top of pipe coupling. Condition on 11 April 194- 7. grained fill material in the base courses. This raised type of construction allows the Settlement of floor in motor pool building at wind to remove the snow and provides a well Northway Airfield, Alaska. Floor originally drained roadway and base. In many cases, it flush with top of pipe coupling. Condition an is more economical to place fill material 4 April 1946. over the natural ground than to strip the ve­ opened in winter to permit cold air to circu­ getation or remove soil which is unstable late above the ground surface, thus permit­ when thawed. However, the tests reported on ting deep freezing. They are closed in summer herein indicate the inadequacy of insulating to retain the cold in the ground and to keep qualities of such fills which are composed of the permafrost from thawing. In some cases, coarse- grained materials. Ground and surface structures are placed directly on fills of water should be intercepted and diverted a*ay gravel or other nonfrost action materials. from the earth structures or carried across Tests have been made of the heat transfer in insulated subdrains or culverts. from buildings placed on various thicknesses The following comments are based on the of gravel fill as well as on gravel fills results of certain tests made by the Corps of containing various insulators. Results of Engineers, St. Paul District, in connection these tests indicate that the insulators and with its investigation of airfield construc­ fills are not very effective in preventing tion in Alaska. In order to conserve space, heat transfer into the ground. Where piling only typical examples of general interest are are used in soils overlying reported herein. During the past year, tests permafrost, it is necessary that the piling have been made at a specially constructed re­ extend into the permafrost a sufficient depth search area near Fairbanks which contains na­ to provide anchorage to overcome the upheav­ tural and disturbed ground areas, 26 runways ing forces developed in the seasonally frozen test sections, and 11 small test buildings. ground. Anchorage in permafrost is obtained Air temperatures during the test period are by roughening or notching the part of the pil­ shown in Fig. 1. The tests of the natural ing embedded therein and permitting it to and disturbed ground areas show that there is freeze into the permafrost. As it is impos­ less thawing under natural cover of trees, sible to drive piling into frozen ground,a brush end surface vegetation than were such hole must be made by thawing the ground and growth has been partially or entirely removed. placing the piling therein. The depth of em­ Where the trees and surface vegetation are bedment in permafrost must be sufficient to present, permafrost is very close to the resist the upheaving force developed in the ground surface. !Phe shallow layer of thawed portion of the overlying 60il layer subject ground above the permafrost freezes rapidly to seasonal freezing. Generally, the depth of in the fall and the cold is conducted through embedment is made twice the thickness of the the solidly frozen ground more rapidly and to soil layer subject to seasonal freezing. a greater depth than in unfrozen ground be- , As previously indicated, the stability cause of its greater thermal conductivity.The of roads, runways and other earth structures removal of vegetation from the ground surface in the permafrost region depends largely on causes deeper thawing and the cold does no.t proper site location. More emphasis is placed penetrate as deeply in the winter as it does on foundation conditions 6uch as , under natural conditions. These conditions ground water, surface drainage, and possible are shown in Figs. 2, 3 and 4. The effect of icing action than on the location of the most pavement surfaces on heat transfer is shown direct route in the case of roads. Sidehill in Figs. 5» 6 and 7 for 50- foot square gravel, locations should be avoided where practicable, asphalt, and concrete surfaces, respectively. due to the possibility of sliding and flowing Temperatures in the ground immediately below of the soil and ice formation from ground wa­ both gravel and asphalt surfaces are higher ter under pressute issuing from the hillside. throughout the year than under a concrete Hoads and runways may be built up above the surface. The depth of freezing during the natural ground surface by placing coarse- winter of 1946- 47 was greater under the con- 5*

FIG.1

FIG. 2

Crete pavement surfaces than under the other prevented the flow of a substantial amount of surfaces; however, the depth of thaw during heat. The difference in the maximum depth of the summer of 1947 was essentially the same thaw between insulated and uninsulated sect­ under all surfaces. The effect of insulation ions of otherwise similar characteristics was in the base course is to retard the flow of less than one foot in 1947. Typical results heat slightly. None of the insulators tested are shown in FigB. 6 and 8. The anticipated 55

FIG.3

FIG.4 56

FIG.5

FIG.6 57

PERMAFROST INVESTIGATION FIELD RESEARCH - FAIRBANKS, ALASKA AREA NO.2 GROUND ISOTHERMS Q MMEMENT ELEVATIONS RUNWAY TEST SECTION RN-17 COOP S Of ENGI NEERS. ST RAUL,MINN. OCT I94T

FIG .7

- ZERO ISOTHERM ISOTHERMS SHOWN W OESMCS OCNTMftAOC. PERMAFROST INVESTIGATION . isotherm s OTHER THAN ZERO FULO RESEARCH - RfctftBANKS, ALASKA (ABOVE OR SELO* AS MARNEO) A RIA NO. t CROUND ISOTHERMS I PAVEMENT ELEVATIONS RUN «AX TEST SECTION W H ______COR»» 0» kNOiNEERS. S* «ftUk . «m * > .1 .»«»

FIG.8 58

FIG.9

FIG.10 59

FIG.11

FIG.12 60

FIG.13

FIG.14 61

OENERAL NOTES: PERMAFROST INVESTIGATION ISOTHERMS SHOWN IN OtSREES CtNTlWUflt. TEMPERATURES ANO ELEVATIONS OBSERVED NORTrtWAT AJRFlClO, ALASKA NEAR CENTER 0* KIILDIH*

FIG.15

summer of 1947. The greatest relative vertic­ reduction in the depth of thaw through increas­ ing the base course thickness was not realized. al movement in the three buildings was noted in Building No. 7. Figs. 14 and 15 show the See Figs. 6, 9 and 10 for the effect of 4, 8, and 12- foot fills. In fact, thawing penetrated ground temperatures and floor elevations in the 160' x 210' hangar at Northway Airfield, to approximately the same depth in the origin­ al subgrade regardless of the thickness of the Alaska during a period of about two years. The building was heated during the winters coarse- grained fill placed upon it. The prob­ except during the period October 1946 to Jan­ able reason for the rapid heat transfer through the fills is that they were coarse­ uary 1947 when the heating plant was out of operation. The depth of thaw has increased grained, compacted to high density, and well drained being placed above the natural ground progressively near the center of the building during the period of record. Settlement of level. The depth of thaw under the test build­ the floor near the center has also been pro­ ings depended largely on the area of the floor or footings in direct contact with the ground. gressive, with a rapid rate during the last In the case of Building No. 1 at the Fair­ half of 1946, then retarding abruptly appar­ ently as the result of the temporary termina­ banks Field Research Area No. 3 ( Fig. 11), tion of heating of the building. the 16- foot square concrete floor is entirely in contact with a 4—foot thick sand and gravel The conclusions of the tests to date are fill placed on the natural silt subgrade. The as follows: depth of thaw has lowered gradually during 1) Less thawing and more freezing takes place the period of record and a fairly uniform tem­ in ground covered by natural vegetation, perature gradient has been developed below especially tree growth, during the course of the ground surface. Building No. 2 ( Fig. 12) a year than in ground which has been disturb­ in the same test area has a 16- foot square ed by construction operations or other causes. insulated wood floor with a 3- inch air space 2) Artificial insulating materials and in­ between the floor and the 4—foot sand and creased heights of fills of coarse- grained gravel fill which is placed on the natural materials placed on the natural ground are not subgrade. The insulation and small air space appreciably effective in reducing heat trans­ under this building are apparently quite ef­ fer into the subgrade. fective as the ground temperatures reflect 3) Concrete surfaces apparently reflect more the outside air temperature variations rather solar heat radiation received than do gravel than the relatively uniform temperatures in or asphalt surfaces. However, the difference the building. Building No. 7 ( Fig. 13) has a is not great. 2- foot dead air space between the 16- foot 4) The most effective method of preventing square insulated wood floor and the natural heat transfer into the subgrade and thus ground surface. Ground temperatures here are insuring the stability of heated structures also affected, principally by the outside air placed on soils which are unstable when thaw­ temperatures. The depth of frost penetration ed is to use piling and an air space under into the natural subgrade under both Buildings the structures. 2 and 7 was approximately the same during the The investigation of airfield construc­ 62

tion in arctic and subarctic regions upon gineer, and the general direction of Colonel which this paper is based is being made under Walter K. Wilson, J r ., Corps of Engineers, the supervision of Mr. Henry J. Manger, En­ District Engineer, St. Paul District.

•0«0*0- 0*0“0*0-

Y l d 7 ENGINEERING IN PERMAFROST IM CANADA'S MACKENZIE VAT.T.KY

R.A. HEMSTOCK Edmonton, Alberta, Canada.

Progress in Canada's Northland continues es in ordinary living quarters was recorded in steadily as the vast natural resources are six months. developed to serve the nation. This develop­ Attempts were made during the second sum­ ment brings to the fore many engineering prob­ mer after erection, to shim up the buildings lems, not the least of which is the erection in the hope that equilibrium had been reached of structures on the permanently frozen ground and that further settlement would not occur. of the North. However, frost heaving around the outside of Norman is a small but important oil the building of course continued and measure­ centre in the lower Mackenzie Valley. From its ments showed too that settlement under heated wells and refinery come the petroleum products buildings also went on although at a reduced to heat, light and. power the mining camps, the rate. A total of fifty- two inches settlement river transport, and the planes of the whole under the locomotive type heating boilers was Mackenzie District. At the refinery the avail­ recorded in the two years after construction. able area for the erection of living quarters, In 1943 several important installations the shops and the refinery consists of a fine were made at the field, among them the erection frozen silt with occasional thin layers of of two repressuring stations. The heavy reci­ gravel and . Moisture content of the silt procating machinery and high pressure piping may run up to ninety percent. Tests have shown required substantial foundations that would not the ground to be frozen to a depth of at least be subject to heaving or settling. Test pits one hundred and forty feet— seasonal thaw may sunk at the locations selected showed the soil he from one to five feet depending on the to be a fine silty material with inclusions of ground cover. Yearly mean air temperature aver­ ice. Layers of clay and some gravel were en­ ages about twenty- five degrees Fahrenheit. countered below twelve feet. A weak sandstone One of the problems at the refinery is to occured at about forty feet. No machin­ erect structures on this frozen ground in such ery was available to carry the foundations to a manner that they remain stable. It can easi­ bedrock, so it was decided to drive wooden ly be seen that thawing of frozen silt with piles well into the permafrost, to insulate such high moisture content will result in slud around the tops of the piles and to pour con­ with no whatever. In addition, crete blocks founded on the piles. The active the problem of frost heaving comes to the fore, layer was taken off and work was carried on the (that is the layer of season­ at the original top of the permafrost to give al freeze and thaw) is very susceptible to better drainage of the final job, and to allow frost heaving and the damage to structures is greater penetration of the piles. Examination in many cases more evident than that caused by of this foundation after four years, the build­ settlement. ing was heated for the first eighteen months The first large expansion of Norman Wells and unheated after that, showed no measurable occured during the war when the Canol project movement, and no visible deterioration of any was begun. It was in this emergency that many part of the foundation. buildings were erected without due regard to Since 1943 all important construction ar foundations. In most cases a fill of silt and Norman Wells has been erected on piles. During dirty gravel from one to three feet thick was this time there has been an opportunity to im­ run over the , and on this were put the prove the pile driving methods, to study the small pads and posts for the frame buildings. behaviour of piles and generally devise satic- A small town was erected in* this manner , the factory engineering methods for the North. only variations being that in shops and power Since the ground is frozen, holes must be plants a slab of concrete was poured for a jetted out for piles. Experience showed that floor directly on the fill. the most economical method was to use a steam The first six months after construction jet, a three quarter or one inch pipe about proved that permafrost must be dealt with in a one foot shorter than the desired pile depth different manner. Heat from the buildings, caus­ is used, a steam pressure of fifty to eighty ed settling of the interior of the buildings. pounds or greater seems most satisfactory. Ex­ That is the permanently frozen ground was grad­ perience and a careful study of the ground ually thawed and due to its very high water will aid in getting economical results. Some content settling was inevitable the four inch pertinent points for similar conditions are: concrete floor of the camp boiler house settled 1) If the ground has dry layers, some water thirty inches in the first winter. At the same with the steam will speed jetting. time these conditions led to a large supply of 2) Holes may be left up to three weeks in sum­ water under the building and so set up ideal mer before driving piles, but in freezing conditions for frost heave around the circum­ weather should not be left more than a week. ference. Differential movement of fourteen inch­ 3) Except in large gravel special bits, e.g.