Permafrost: a digest of current information

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Permafrost: a digest of current information

National Research Council of Canada

Associate Committee on Soil and Snow Mechanics

PERMAFROST

A Digest of Current Information

Prepared

by

Directorate of Engineer Development Army Headquarters

Ottawa Canada

1949

Issued as

Technical Memorandum No.

49

of the

Associate Committee on Soil and Snow Mechanics

Ottawa August

1957

PREFACE

The developments which have taken place in the northern regions of Canada during recent years have focused attention on the engineering problems associated with

permafrost or perennially frozen ｾｯｵｮ､ - a terrain condi-tion enro untered over one-half the land area of this coun-try. These developments have led to a steadily increasing demand for information on the properties of permafrost and methods of construction in areas where permafrost occurs.

In recognition of the importance of permafrost in the develonment of the North, the National Research Council has been active for some years in the study of this phenom-enon through the work of its Division of Building Research. In

1952

this Division established a Northern Research Stat-ion at Norman Hells to provide a centre for this specialized field research.

Prior to the start of this work the Canadian agency mo st actively interested in permafrost was t he Department of National Defence. Much of the early major construction in Northern Canada was for defence purposes and in

1949

the Dir-ectorate of Engineer Development of the Department of Natio-nal Defence compiled and issued a digest of cur-r-ent .information on permafrost to aid in a better understanding of the problems encountered with construction in the North.

It is that document which is pUblished here as a Technical Memorandum of the Associate Committee on Soil and Snow Mechanics. The Associate Committee in its role as co-ordinator of permafrost studies in Canada is privileged to reproduce this material with the permission of the Depart-ment of National Defence. With the approval of the Depart-ment, slight revisions have been made in the light of knowledge gained since the material was first published in

1949.

It is believed that this comprehensive study will be of considerable interest to those who must cope with the

problems imposed on construction by permafrost, and the Assoc-iate Committee wishes to record its thanks to the Directorate of Engineer Development of the Department of National De-fence for permission to circulate further this material on a

subject which is becoming increasingly important to many Canadians. Ottawa August

1957

Robert F. Legget, Chairman, Associate Committee on Soil and Snow Mechanics.

FOREWORD

For many years engineers have frequently en-countered extreme difficulties and have from time to time experienced professional embarrassment through failure of their works in areas of permanently frozen ground. In very recent years the engineering profession and scientists have been conducting active investigations to determine the pec-uliarities of this "permafrost" condition as well as to det-ermine the extent of the permafrost region in ｾｨ･ northern hemisphere. Experience has shown that timely recognition of the permafrost condition can save valuable time, effort, and money. Having recognized the condition, ｾｨ･ engineer who possesses an understanding of its peculiarities can readily select the economic and effective procedure for executing his works.

I fully realize that we can not all become ex-perts in the permafrost problem, but I do feel that this booklet can be of great assistance to all engineers in

dev-eloping a basic ｵｮ､ｾｲｳｴ｡ｮ､ｩｮｧ of the problems which we may all expect to encounter at some time or another.

This booklet is the result of an extensive study and search of existing papers and data, but there is still known to be a wealth of information in the minds of individ-uals who have encountered the condition but have not record-ed their knowlrecord-edge. Although the Directorate of Engineer Development can not continue to actively pursue these in-vestigations further, I request each of you to despatch any

information you may possess or gain to DED who will ensure that the proper scientific bureaus receive the data.

I recommend this digest to you and suggest that you consider it as your introduction to the problem with further knowledge to be gained through your personal efforts and. experience. W Love) Colonel Chief Engineer Army Headquarters, Ottawa, Canada. June 1949.

INDEX

SUBJECT

PERUAFROST - GENERAL CHARACTERISTICS 1. General 2. Origin of Permafrost 3. Thermal Regime 4. Recognition of ｰ･ｲｲｾｦｲｯｳｴ Areas Vegetation Topography Geology Field Method

5. Identification of Frozen Soils

6 .• Principal Soil Types and their Identification Gravel

Sand Silt Clay

Organic Matter

DESTRUCTIVE ACTION - FROZEN SOILS 7. General

8. Swelling of the Ground 9. Settling and Caving 10. Landslides and Slumps 11. Icing

ENGINEERING PROBLEMS IN PERMAFROST 12. Buildings

13. Roads 14. Bridges 15. Airports

16. Dams and Reservoirs 17. Water Supply

18. Concreting in Permafrost

FACTORS AFFECTING FOUNDATIONS UNDER PERrMFROST CONDITIONS

19. Texture and Structure of the Ground 20. Ground and Air Temperatures

21. The Hydrology of the Ground 22. The Depth of the Active Layer 23. The Depth of Permafrost Layer 24. The Ice Content of the Soil 25. Time Element in Construction

26. Adfreezing Strength of Foundation to Frozen Ground

27. Pilings

28. The Purpose of the Building 29. The Type of Loading

30. The Normal Internal Temperatures

31. The Rate of Heat Transfer from Structure to Ground

32. Depth of Foundation 33. Foundation Planning 34. Active Method

Bedrock

Gravel and Sand Fine Grained Soils 35. Passive Method

36. Small Buildings, Limited Life

37. Barracks, Boiler Houses, Power Plants and Medium Sized Buildings

38. Water Tanks, Pump Houses and Hangars

PAGE

1

7

12;

INDEX Continued

SUBJECT PAGE

FACTORS AFFECTING ROADS UNDER ｐｅｒｾｦｦｩｆｒｏｓｔ CONDITIONS 29 39. General

40. Surveys

41. Swelling and Heaving 42. Drainage 43. Sidehill Cuts 44. Fills 45. Cuts 46. Icing 47. Frost Boils 48. Snow Clearance 49. Road Surfaces 50. Road Maintenance

FACTORS AFFECTING BRIDGES UNDER PERMAFROST CONDITIONS 36 51. General

52. Supports for Wooden Bridges 53. Foundations for Bridge Piers 54. Freezing Belts

55. Drainage of Icing Water 56. Barriers

57. Deepening and Straightening of River Channels 58. Warmth Retention in Channels

FACTORS AFFECTING AIRPORTS AND RUNWAYS UNDER ｐｅｒｴｾｆｒｏｓｔ CONDITIONS 59. Selection of Site 60. Surface Drainage 61. Sub-Drainage 62. Icing 63 •. Airfield Surfaces 64. Heat Penetration 65. Frost Penetration 66. Insulation 67. Surfacing 68. Grading 45

FACTORS AFFECTING DAM AND RESERVOIR CONSTRUCTION

UNDER PERfMFROST CONDITIONS 47

69. Effect on Permafrost Table 70. Ice Lenses

PER¥..AFRO ST

GENERAL CHARACTERISTICS 1. General

1. The existence of permanently frozen ground, or permafrost, has been known for many years but relatively little systematic work has been done in the study of its effects. 1'1 summer a proportion of the surface soil, known as the active layer, thaws out above the permafrost. The unequal heaving, swelling and settlement which occurs within this active ]ayer is the cause of much of the foundation trou-bles which develop in permafrost areas.

2. The destructivo action of permafrost occurs in soils having a silt or clay content. Ice lenses forming in this type of material may increase the volume as much as ＲＰＰｾＮ Such surficial distortions develop stresses exceed-ing 28,000 psi, which is beyond the limit of economic struct-ural design. The recognition of such soil ｴｹｰ･ｳｾ together with the control of structural distortions induced by it provides the subject material for this chapter. Definitions commonly used in connection with permanently frozen ground are attached as Appendix A.

2. Origin of Permafrost

Permafrost is a legacy from the last great ice age (Pleistocene Epoch). I t occurs generally on this con-tinent in areas having a mean annual temperature of below 30°F (See Fig. 1), and underlies about half of Canada's area. Towards its southern f'r-Lnge , permafrost mS.'J be but a few feet in depth, but beds have been found in excess of 1500 feet deep. Although permafrost areas in Canada have been extensively drilled for ore bodies, sub-surface temp-eratures have not been taken simultaneouslY9 and thus has much valuable information been lost.

3. Thermal Regime

1. In undisturbed areas a temperature equilibrium has been established between the oermafrost and surface known as the thermal regime. Construction operations invol-ving the stripping of the surface disturbs this regime, nec-essitating a readjustment, causing swellings, settling or caving to occur with consequent damage to foundations and road surfaces.

2. The ground in permafrost areas separates itself into two divisions. The upper section, which alternately thaws and freezes with the seasons, is called the active layer, and the lower layer, which remains frozen continu-nusly, is the permafrost. The depth to which the active layer penetrates varies with t he locality, insulating vegetation cover, type of soil, water content and movement and the exposure of the ground to the sun. Figura 2 shows typical cross sections in permanently frozen ground.

4.

Recognition of Permafrost Areas 1. Practical ｣ｯｮｳｴｲｵ｣ｴｩｯｾ methods depend upon the recognition of permafrost on any construction site under consideration. The successful execution of such a task depends upon an early reconnai s s anc e , Hec o gnftion may be effective either from the air or from the gr-cund , Many readily distinGUishable surface features present valuable clues to t he condition of the unoer-Ly Lng material.

2 -DOMINION OF CANADA SHOWING APPROX I MATE SOUTHERN LIMIT OF PERMAFROST BP.J.JJ7 20 Vegetation

(a) Thick Moss and Hummocky Tundra - indicate a water bearing zone above a high permafrost table and very poor drainage, even on terraced areas.

(b) Aspen poplar - will not grow on frozen ground. (c) Birch - usually indicates thawed ground.

(d) pine and Fir - commonly grow where permafrost is either low or absent with well-drained granular soil.

(e) Poplar - usually grows on reasonably dry well drained ground.

(f)

Spruce - may indicate wet poorly drained ground and a high permafrost table.

(g) WilloVis - usually indicate underground water and an area in which fields of surface ice may probably form.

3. Topography

(a) Large polygonal soil structure - the formation of broad cracks forming a polygonal pattern over the surface of an area indicates the existence of considerable ground ice and fine grained soil.

-3-FROST ZONE

AND ACTIVE ZONE

IDENTICAL GROUND ALTERNATELY FREEZES AND THAWS ｾＭＭＭＭｐｅｒｍａｆｒｏｓｔ

fROST ZONE EXTENJS TO PERMAFROST

FROST ZONE UN'RO'UIt GROUND (TALIK) PERMAFROST TABLE ICE VEIN---",,"r GROUND ALTERNATELY FREEZES AND THAWS ACTIVE LAYER ＭｶＮＮＺｾｾＭＭＭＭ ICE LENS ＯＭｌ｟ｾ \ . y " ' - - - - -PERMAFROST

CONTINUOUS PERMAfROST CONTAINING GROUND ICE

FROST

UNFROZE" ｇｒｏｕＢｄＭＴｾｾｾ

(TALlK) ｾＢＳＧ］ｬＭＭＭＭＭ PERMAFROST_ISLA"D

ISLANDS OF PERMAFROST IN UNFROZEN GROUND

'ROeT PERMAFROST UNfROZEN GROUND (TALIIC) UNFROZE" tROUND ｾｾＭｍａｙ OONTAIN tROUND WATER ACTIVE LAVEre LAYERED PERMAFROST

TYPICAL CROSS SECTIONS IN GROUND CONTAINING PERf.1AFROST

ＭＴｾ

(b) Springs and icing areas - direct evidence of ground water.

(c) Soil flow or creep - is evidence of wetted slip planes within the ground.

(d) Exposed sand. gravel and rock - on h111s adjac-ent to lowlands form an excelladjac-ent intake for surface water and indicate the pOSSibility of ground water in the lowlands.

(e) Flood plains - the margins of rivers and lakes frequently have large layers of unfrozen material containing ground water. These usually form poor construction sites, for two ｲ･｡ｳｯｮｳｾＭ

(i) through the danger from flooding and (ii)because fields of surface ice frequently

form over such apeas when the river and ground water become confined tlITough winter frosts.

(f) Ground slope - when surface conditions are unl-ｦｯｲｭｾ unconfined ground water above the perma-frost generally flows in the direction of the surface slope.

(gY Ground relief - the depth of the water table at any point is influenced by the ground rellef. Ridges are likely to be better drained and more free of surface ice formations than are flat low areas.

(h) Exposure to sun - north slopes ordinarily have a higher permafrost level than do south ｳｬｯｰ･ｳｾ

and therefore are more free of unconfined ground water.

4. Geology

(a) Soils - coarse soils usually have a low par-ma-frost table and transmit water readily. Fine grained soils tend to create a high permafrost table and oppose the flow of water.

(b) Rock - seepage and fields of surface ice may occur near outcrops of pervious or fractured rocks. Ground water usually flows along the contact between s oil and r oc k , forming springs where the plane is exposed in ｣ｵｴｳｾ terraces, scarps and bluffs.

5. Field Method

A practical method of' locating the permafrost table is as follows: A pointed steel rod of approximately

tit

in diameter is driven thr-ough the surface frost and/or active layer with a sledge hammer to refusal. When the refusal point is reached.' a Stillson wrench is applied to the rod and if it turns freely without any back spring being felt at the handle of the wrench9 it is probable that the rod has been driven to rock. ｉｦｾ however. a back spring is felt at the handle. it is considered that the tip of the rod has penetrated for a short distance into the permafrost and. therefore. the depth to the permafrost table is readily determined.

5

-5.

Identification of Frozen ｓｯｩｾ･

1. Before attempting any type of construction work which is expected to last beyond the spring breakup and into summer, a clear assessment of the soil conditions must be made. Frozen soil with fine grain characteristics have many appealing constructional qualities which soon disappear as the temperature rises above freezing.

2. The choice of a site may all too frequently depend upon a very limited and cursory examination because of the urgency of most types of military construction. Neither time nor facilities are available for a complete laboratory analysis of the soils encountered. Therefore, reliance must be placed upon the identification of soils by visual inspection and simple classification tests in the field.

6. Principal Soil Types and their Identification

1. Gravel - gravel is readily identified by inspection, having bUlky pebbles larger than one-quarter inch in diameter. Next to solid rock well graded gravel makes the most stable solid foundation and is little affect-ed by either moisture or frost action.

2. Sand - sand consists of mineral grains ranging from one-quarter inch to .002 inch in diameter. It is easily identified by inspection except in cases in which the part-icles are uniformly small. In such instances care is re-quired to distinguish between fine sand and silt. Dried sand does not hold together and feels gritty. Well graded sharp angular sand is a desirable foundation material. Water in sand simply turns to ice and the volumetric change is about nine per cent. Ice lenses do not form in such material.

]. Silt - silt consists of natural mineral grains of very fine-rexture possessing little or no cohesion when dry. Two simple field tests afford positive

identification;-(a) Shaking Test - Prepare a pat of wet soil, adding water if necessary; then shake horizon-tally in the palm of the hand. With typical organic silt, this action causes water to come to the surface which now appear glossy and rather soft. Squeezing the sample be-tween the fingers causes the water to disappear from the surface. The sample quickly stiffens and finally cracks and crumbles.

(b) Breaking Test - Allow the sample to dry; test the cohesion (ability to hold together) and feel by crumbling with the fingers. Typical silt has little or no cohesion when dry and possesses a smoth feel in contrast to the grittiness of fine sand for which it is sometimes mistaken. Frequently silts are misclassified as clays on account of their fineness and colour.

6

-All types of silt are ､｡ｮｾ･ｲｯｵｳＮ Silts and sandy silts with t heir relative

s

fine texture and high porosity permit the formation of ice lenses at a rapid rate. Silts are difficult to compact and to drain, and drainage is not effective in increasing their stability.

4.

cIa! - Clay is comprised of particles of microscopic gra n. Between certain moisture contents all clays display adhesiveness, characteristic of the physical property known as plasticity for classification purposes. Depending on the proportion of larger grains, clays vary from lean clays (low plasticHy) to fat clays (high

plasticity). Many clays, brittle in their undisturbed state, become soft and plastic when worked. Two field tests on clay are as

follows:-(a) The sample of soil is tested by working it with the fingers, adding water when the stiffness requires it. In this moistened condition its plasticity is evidenced by its susceptibility to kneading like dough.

Plasticity is also indicated by the ribbon test. The ribbon is formed by placing a ball of kneaded soil between the thumb and

index finger and drawing the index finger under the thumb as closing the hand. Clay will form a long thin flexible ribbon that does not break under its own weight. This test readily differentiates clay from silt. (b) The hardness of a dry sample clay is measured

by the intensity of finger pressure required to break up the sample. Much greater force is required to break dry clay than dry silt. The consistency of undisturbed clay is quite different in its characteristics from the soil being worked in the identification tests. As found in undisturbed beds it may range from hard through stiff, medium, soft and extremely soft depending upon the natural moisture content and degree of consolidation. Low resistance to deformation, high com-pressibility, imperviousness, and large expansion or con-traction with Changing moisture content are inherent qualit-ies of clay. As clays retain water tenaciously, sub grade drainage is ineffective.

5.

Organic Matter - Soils containing organic

matter are peat solIs consisting largely of partly decomposed vegetation and fine grained plastic and non-plastic sediments containing varying amounts of finely divided vegetable matter such as organic sandy soil, organic silt, organic silt-clay, or organic clay. In peaty soils organio matter is coarse and fibrous and identification is readily made by visual inspection. In organic silt or clays, however, organic matter is often so finely divided it cannot be detected Visually. In many cases the organic odour is strong enough to be detected. The odour may be intensified by heating a sample quickly. In the plastic range organic clays feel spongy as compared to inorganic clays.

All pea'y and plastic organic soils are

unsatisfactory sub grades because of their high compres-sibility and low resistance to deformation. However, their

ＭＷｾ

insulation characteristics made them oxtrenely important as a top covering in permafrost ar-e a s ,

4. The above field identification tests are not intended in any way to supplant laboratory tests which should be made if at all possible. They have been described to afford a ready method of field identification to facilit-ate the selection of the most advantageous site for any particular engineering task.

DESTRUCTIVE ACTION - FROZEN SOILS 7. General

The identification of the type of soil is probably the principal criterion governing the choice of a site, but there are several others which must be taken into consideration. The water content in the ground and the depth of the active layer are two additional factors that will affect the behaviour of the ground through the disturb-ance of the thermal regime by construction works.

o.

Swelling of the Ground

1. Swelling refers primarily to the volumetric increase of the ground mass. It is due in most cases to ice segregation in non-uniform soils, such as silts and sandy silts having insufficient permeability for good drainage but sufficient to permit the passage of water to form ice lenses. Uniform or widespread swelling usually causes little damage to foundations but differential swel-ling is very destructive. The three principal causes of differential swelling are as

follows:-(a) Unequal distribution of the load (as in a structure).

(b) Differences in the texture of the ground. (c) Differences in water content from which

. ice will form.

2. Ground composed of coarse solid fragments of rock does not swell. Ground composed of gravel and coarse sand with an admixture of fine particles (clay or silt) in either a moist or saturated condition will swell a little.

3. The conditions favourable to the swelling of ground

are:-(a) Occurrence of permafrost at shallow depth. (b) Fine grained texture of the ground.

-8-(c) The moistening of fine grain soils by either capillary or ground water.

(d) The freezing of shallow ground water which is backed against underlying ｰ･ｲｭ｡ｦｲｯｳｴｾ

9. Settling and Caving

1. When ground of fine texture thaws it

becomes more or less plastic and will not sustain the loads supported when frozen. If thawed ground becomes sufficiently plastic it will flow or ooze out from beneath the structure, causing damage by settling and caving (see Fig. 3). The magnitude of the settlement depends

upon:-(a) The composition of the ground. (b) The amount of ice in the soil.

(c) The depth of the seasonal thaw (active layer). (d) The amount of heat transmitted from the

building to the ground (see Fig.4).

2. Normally, a building settles more on the south side than on the north. The ground along the south wall receives the reflection from the wall of the building in addition to the normal direct solar radiation and will therefore thaw to a greater depth than that on the north side, which will be shaded (see Fig. 5).

10. Landslides and Slumps

1. Landslides and slumps generally occur along steep slopes and are of variable movement. In permafrost areas they may commonly affect roads and buildinss. Solidly frozen ground in the winter which is susceptible to sliding or flowing viII remain quite stablo until thawing takes place.

2. Methods of

prevention:-(a) Control of the surface and underground waters by the maintenance of drains, diversion of seepages and improvement of general drainage.

(b) Prevention of deforestation.

(c) On slopes which tend to slide or flow, by planting trees on them.

(d) As a temporary measure, by the artificial freezing of slopes.

(e) By site selection.

(f) By use of vertical cuts which are less susceptible to erosion.

-9-LOAD ACTIVE LAyER-+-,-.... FINE GRAINED ORIGINAL ｆｏｕｎｄａｔｉｏｾＮ PERMAFROST Ｇ｜ＧＭＮＮＮＮＮＮＮ［ＮＮＮＢＧｲＺＭｾｲＭ LIFTING fORCE FOUNDATION ___LIFTED ｾｾｾｾｾｾｾｾ ACTIVE LAYER (FROZEN) ,...

-MATERIAL IN ACTIVE LAYER FREEZES ANP SWELLS. LIFTING FOUNDATION.

,"::

r

-ACTIVE LAYER

PERMAFROST

MOVING GROUND WATER THA 5 PERMAFROST BELOW FOUNDATION BASE AND SETTLEMENT OCCURS.

ACTIVE ｌａｙｅｒＫｾＢＢＢＢ FOUNDATION L 1FTE.D B'I' ICEＭＭＭ｜ＭＭＬＭＭＬＭＭＬＭＭＺＢｩＺｾ

PERMAFROST

SURFACE WATER ｓｅＮＮｅｾｏ｜Ｌｾｎ CONTACT SURFACE OF FOUNDATION AND FR!2EI2:e BELO.'; THE BASE. LIFTING

FOUrt()t\'j>10rJ •

-ｅｘａｍｐｌＮＮｅＮｾ OF S\'JELlIiJG, SETTLING AND HEAVING OF FOUNDATIOrlS ON PEPL\1AFROST.

ORIOIMAL PERMAFROST

LEVEL FOIJNOATlON---""'"

o /// /

/j,

/

ｾ

PERMAFROST

LEVEL AFTER

' - Ｎｾ ._. Ｎ｟ｾ｟Ｎ｟ｾＮ THAWED

EFFECT OF A STRUCTURE ON GROUND

...--_ _G.ROUNO

LEVEL

TEMPERATURES OVER A PERIOD OF

I YEAR

FIGURE 4 DEPTH

IN

Tt

i'

.st....

EFFECT OF AN EAST-WEST WALL

ON GROUND TEMPERATURES

FIGURE

5

3. In choosing a road or construction site, sharp drops in the permafrost table, should be avoided

(see Fig. 6). Where a choiae exists, quarries or gravel pIts should not be located close to any road or building, as the excavation of the gravel may CBuse a lowering of the permafrost table, causing an unstable condition favourable for a slide in the direction of the excavat-ion.

11. Icing

1. The term "icing" is defined as a mass of surface ice formed during tho winter by successive freez-Ings of sheets of water that may seep from the ground, from a river, or from springs. Unless controlled or pre-vented, this icing may engulf complete sections of roads, bridges or buildings.

2. Conditions favourab1l to icing are:-(a) Presence of ground wator in the active

layer.

QUARRY

ｾｬｬＭ

with a thin cover of snow during the early part of the winter (December and January) 0

(c) Proximity of the permafrost table to the surface of the ground.

(d) A thick cover of snow during the latter part of the winter (this does not affect the formation of icinGs bnt may greatly increase their intensity and duration). 3. There are several rnethod s of combating icing, which divide theMselves into those of a passive and active nature.

4. Passive methods ar-e j

-(a) Moving the s t.r-uctur-e to another location. (b) Cutting the ice around the structure. (c) Widening of the ditch near the springs

which are forming the icing.

(d) Divorsion of the course of the water forming the icing.

(e)

TIetention of the icing by the construction of holding embankments and barriers.

ROAD BUILT OVER A SHOULDER OF PERMAFROST TAflLE IS SUSCEPTIBLE TO LANDSLIDES AND DEFORMATION BY DIFFERENTIAL HEAVING.

...'.:' '".:;. ," Ｚｾ :'. 7'. " ' • · \••• : . . . · 1 ...: ［ｾＺＧＮ ROADBED

ＬＺｾＺ

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ＺｾＬｻＺＺｾ

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ｾ［ＮＺＮＺＺＯｾＨＺＺ＼ＺＺＮｾＺＢ＼ＺＺ｜ＺｾＮｾＺｾＺＯＺＺ［ｾＮｾｾＺｾ

' •• ' .. I.• ;ACTIVE LAYER •• _.

o&;.---1E.-:--_...

ＺＢＬｾＧＺＢＮｾＺｾｊＺＺＧＺＮｾｾＧＺｾＺＧ

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' ... : ",ＮＧｾ , : .. ': : :. '. .' : : '. I' .. . ' • • • • • • '• • I I

PROFILE OF THE PERMAFROST TABLE NEAR A RIVER OR QUARRY.

5. (a) (b) (c) (d)

-12-Active methods of combating icing ｡ｲ･ｾｾ

Improvement of the drainage of the arena By the construction of shallow trenches above the area to be protected$ creating frozen belts in which the frost penetrates the ground to form a aubter-r-anean dam which will divert the flow of water$ and therefore the icing$ away from the construction.

Deepening ｡ｾ､ straightening the beds of rivers or springs.

Warmth retention channels.

ENGINEERING ｐｒｏｂｌｅｲｾ IN PERMAFROST 12. Buildings

1. Engineering difficulties in permafrost areas stem from a disturbance of the thermal regime. Foundation problems of this nature have been encountered for hundreds of years. Early attempts were made to restrain the natural r'or-ce s induced by the frost through the use of stronger materials and more rigid designs. Results were far from satisfactory and continual repairs led to a costly maintenance program which never proved completely successful.

2. With a more thorough knOWledge of the problems two general methods of construction have been evolved, the passive and active methods$ to which a loose reference has been r:J.ade in the preceding section. prir.lar-ily. the passive method involves an attempt to avoid any disturbance to the existing thermal regime, if necessary by the insulation of the construction from the ground.

3. Where the permafrost table is high and ground conditions suitable, a more active method of control may be adopted. The ground texture must be such that it will adequately support the designed load in a thawing state, the bearing power of the ground being tested in this condition.

4. The decision to employ either the active or passive method of construction must be based upon the results of the preliminary investigation. Deductions will be mOre reliable if observations can be continued over several seasQns. This, however$ is seldom practic-able and the analysis is more commonly made as a result of the initial series of investigations coupled With a study of the weather records nearest the area for a

variat-ions, snow and rainfall records for such a period of years may serve to relate thfl year in which the ｩｮｶ･ｳｴｾ igation is being made with the series, and previous abnormal conditions may be interpreted with relation to their effects on those found on the ground. The durabil-ity and stabildurabil-ity of structures erected on permafrost

depend upon the selection of the proper type of foundation, the methods used in its construction and upon local condit-ions.

5. The choice of the active or passive method of dealing with the permafrost problems depends upon the following

:-(a) The hydrological condition of the frozen soil.

(b) The water content.

(c) The vegetation cover and relief in the locality.

(d) The construction and heating conditions within the structure.

6. Once the method of construction has been decided upon, normal principles may be applied, guided by a few basic rules to be explained later in this chapter.

13. Roads

The choice of a route in the permafrost area should not be dictated by the usual evaluation of topography or by the premise that the shortest and most direct route is the most satisfactory. The success of con-struction and particularly of maintenance of a road depends to a large extent on the location of the roadway with ref-erence to the slopes of the relief, the angle of these slopes, their thermal and hydraulic regimes, and the nature of the ground. ｃｾｲ･ｦｵｬ study should be given the effects which fills and cuts may have on the thermal regime. The use of aerial photographs will greatly aid

in the selection of a suitable route. 14. Bridges

1. The construction of bridge piers is essentially the sane as that of foundations of other large structures. It should be remembered, however, that the permafrost table is likely to have a permanent depres-sion under the stream bed due to the effect of the flowing water. Therefore, the principles of construction must be based on the condition of the active layer in a thawed

state and not to those of permafrost.

2. Precautionary neasures must be ｾ｡ｫ･ｮ to eliminate possible damage from river icings throughout. the winter due to the frequency with which icing condit-ions are encountered. A growing preference for the ＹｕＹｾ pension type of bridge is being shown.

15. Airports

The selection of a site suitable for a land-ing field is in all essentials subject to the same con-siderations as any other building site. but for ths facts

-14-that the ground must be relatively level and the site one of large dimensions. The great area required for the runways makes the location of a suitable site a much more difficult problem. Large areas possessing the homogeneity of ground conditions required are not easy to find in arctic or sub-arctic regions where glacial deposits of a wide textural range form so large a proportion of the ｳｵｲｾ face.

16. Dams and Reservoirs

Practical experience in dam and reservoir construction in permafrost areas is at present distinctly limited.

17. Water Supply

1. Summer landscapes in the north are featured by the preponderance of the water in ｳｷ｡ｭｰｳｾ lakes ｡ｲｾ ｲｩｶｾ ers. In such a ｲ･ｧｩｯｮｾ which seemingly abounds with waterp procurement of a suitable year-round supply of water may be-come a very difficult problem. Many lakes and rivers freeze to the bottom throughout much of the winter. Springs and shallow wells may freeze up. The most dependable source of water comes from deep ｷ･ｬｬｳｾ Which involves the problem of drilling through the ｰ･ｲｭ｡ｦｲｯｳｴｾ presenting many mechanical difficulties. As the thickness of the permafrost varies from a few feet to more than a thousand ｦ･･ｴｾ ｾｨ･ production of water from deep wells is not always practical. The tap-ping of ground water sources near the sea may also be hazard-ｯｵｳｾ as the water is not always palatable.

2. Water distribution systems present much the same problems as are encountered in other types of structure. The pipe lines must not be allowed to ｦｲ･･ｺ･ｾ yet must not transfer heat into the ground to disturb the thermal regimep

Which in turn would create an unstable foundation condition. 18. Concreting in Permafrost

1. The use of concrete in permanently frozen ground presents the usual problems involved in the placing and curing of concrete under cold weather conditions. Under such conditions it is normal procedure to maintain a suffic-iently high temperature in the concrete to allow it to cure properly.

2. In placing concrete on permafrost two addit-ional precautions must be taken. The heat from the concrete must not be allowed to escape and melt the surrounding soil; the cold from the surrounding 80il mu.st not be allowed to pene-trate the still green concrete. It has been found that the temperature ｷｾｴｨｩｮ such an excavation fluctuates be-tween 270F and 37 ｆｾ regardless of outside temperature. 3. The most dangerous effect of the penetrat-ion of the heat of the concrete into the solI exists OD

the surface upon which the foundation will bear. In order to avoid such conditions an insulating mat is used t.o sep-arate the concrete from the soil. 'l'h Ls mat is made of woodｾ for Which purpose 6" beams or Loc;s may be uae d , If logs are ｵｳ･､ｾ the crevices they forTI 1ust be well packed with moss or peat as an additional in21lator. The ｰｬ｡ｴｾ

form must extend at least 6" beyond the foundation 0'1 all sides and must be laid on a level surface covered with a

15

-thin compacted layer of moistened soil, the same material being used and compacted in backfilling. It is important to note that above freezing temperatures such concrete foundation work must be executed immediately after the excavation has been made. As soon as the concrete has set sUfficiently the forms are stripped, the side walls are insulated with moss, and the backfilling, as has prev-iously been mentioned, is filled with moist compacted soil.

4.

Satisfactory concrete may be made with

water containing salt in sufficient quantities to lower the freezing point below the ambient temperature. The addition of salt in such quantities will produce concrete of lower compressive and flexural strengths. It is

generally extimated that the strengths are lowered by approximately

33%.

There may be circumstances in which this expedient may be resorted to.

FACTORS AFFECTING FOUNDATIONS UNDER PERMAFROST CONDITIONS

19. Texture and Structure of the Ground

As has been previously mentioned, the type of soil upon which the :foundation is to be built is of primary importance. Buildings or other structures resting on clean gravel or sandy strata are not damaged by sett-ling or heaving action of the soil. Important foundat-ion problems may arise where they rest

upon:-(a) Alternate layers of sand and silt. (b) A thick stratum of silt.

20. Ground and Air Temperatures

The temperature of the air and the texture of t he soil both have a very direct bearing on the temp-erature of both the active layer and the permafrost layer. Ideally, air and ground temperatures for a period of at least one year should be available before the construction cormnences.

21. The Hydrology of the Ground

The amount of free water in the ground

through the presence of springs, seepage channels, etc., must be determined in order that adequate drainage may be provided or else t he foundation designed to resist the penetration of such moisture. The water content in the form of ice lenses and crystals is also important in the case of permanent buildings where an eventual disin-tegration of the foundations may result from the melting of this contained ice.

-16-22. The Depth of the Active Layer The active layer is the upper belt of the ground that alternately freezes and thaws each year and the difficulties in construction of foundations in permafrost areas are directly traceable to the seasonal actions of the soil within this active layer. It may change within a period of a few weeks ｦｲｯｾ a hard rock-like condition in winter to one of flowing mud in the spring. During the freeze-up period in the fall. when the frost again penetrates the ground, terrific stresses up to 14 tons per square inch may develop. The determin-ation of the depth of the active layer must be made in as many places as possible within the area under consideration. An incorrect assumption concerning the depth of this layer may result in the loss of the structnre within a matter of months. The maximum depths are obtained in ｾ･｡ｳｵｲｩｮｧ the active layer at the end of the summer after fall frosts have begun and the heat penetration in the ground is at its maximum.

23. The Depth of Permafrost Layer In erecting a structure on permafrost towards the southern margin of the permafrost area, it is important to know the thickness of the permanently frozen soil. The permafrost table is lower in any area under intensive cultivation. Where the permafrost belt is but a few feet in thickness, it is usually more feasible to melt this layer and use normal methods of construction.

24. The Ice Content of the Soil

1. In permanently frozen soils a cementing substance which unites the soil particles is usually ice in one form or another. It may exist either as minute crystals or as solid ice lenses varying in thickness from hair-lines to a foot or more, and in length from about an inch to many feet. If soil with a large ice content becomes thawed the whole structure may collapse.

2. There are two conditions which must exist simultaneously for ice segregation to take place in a frozen soil, assuming that the soil is of a texture which ｴｯｬ･ｲｾ

ates the formation of ice lenses. The first must be a source of ground water in continuous supply throughout the period of frost penetration. Secondly, the rate of frost penetration must be slowo

25. Time Element in Construction

1. The best time of the year in which con-struction in permafrost areas should be executed varies with the character of the task. Excavations are usually most easily made towards the latter part of the summer, as the active layer will at that time have its greatest depth. In passive methods of construction insulation should be laid down in the summer in a dry condition. If, on the other hand, active methods are used in which it is planned to keep the permafrost level high, insulation should be placed during the month of January or February when f'r-o st penetration is usually greatest.

2. piles should be driven between the months of February and June. The swelling of the ground in the previous winter ceases in June and should not begin again

-17-until about November.

3. If the foundation is to be constructed of concrete, the pouring should begin soon after the excavat-ion has been made in order to protect the frozen ground as much as possible.

26. Adfreezing Strength of Foundation to Frozen Ground

1. Adfreezing strength refers to ｾｨ･ grip of the frozen ground on a pile or foundation wall. piles or posts driven into the frost zone will be lifted as the act-ive layer swells during freezing, provided that the tQtal ad-freezing strength is greater than the load on the pile or post. On the other hand, if piles are sunk and frozen into the permafrost, the adfreezing strength developed in the permafrost tends to anchor them against uplift caused by their adhesiqn within the active layer. One method of red-ucing the uplift caused by frost action on concrete foot-ings is to taper their sides as is shown in Fig. 7.

PERMAFROST

ＯＯＯＯｾ

1'....o'--';..,L...ＺＺｏｕｾａｔｬｏｎ WITH SLOPED WALLS DEVELOPS DOWN-_/"--'::...:::... WARD COMPONENT OF SIDE THRUST, WHICH RESISTS

LIFTING FORCE.

A METHOD OF REDUCING UPLIFT CAUSED BY SWELLING OF MATERIAL IN THE ACTIVE LAYER,

FIGURE 7

2. The tangential adfreezing strength of frozen ground var.ies

with:-(i) The amount of moisture in the ground. (ii) The temperature of the ground.

(iii) The texture and porosity of the ground. (iv) The nature of the surface of the

foundation material (smooth or rough). (v) The porosity of the material near the

surface.

-18-3. In the absence of other information, the practice is to anchor pilings with 2/3 of the length of the pile within the permafrost.

27. Pilings

1. Most foundations in the permafrost area are supported on pilings of one type or another. The depth to which the piles must be driven depends on several fact-ors, such as the load on the pile, the adfreezing strength of permafrost to the pile, and the uplifting force due to the swelling of the active layer (see Fig. 8). It can readily be seen that due to the hardness of the permafrost layer normal methods of driving piles cannot be used. The permafrost must be pre-thawed (see Fig. 9) before the piles are driven. No load should be placed on the piles for a period of from two weeks to a month. In some areas it is recornmended that piling be left over a winter without load in order to allow the ground to completely refreeze. Steel pilings will refreeze much more rapidly than will wooden ones. The only alternative to that of allowing for such a refreezing period is the conduction of loading tests. 2. Three methods are in use to prevent piles from heaving, as is shown in Figs. lOa, b, c. In all cases a portion of the piling in the active layer should be smooth and smeared with pitch, grease or heavy oil after having first filled any cracks existing in this part of the pile with some type of filler.

THAWED GROUND DURI"G WINTER SEASON TO PREVENT UPLIFT. LOAD+Q lIlUST 8E GR!ATEA THAN b p . COMPRESSIVE STRENGTH OF PERMAFROST

A: AREA OF PILE

ｾ • ADFREEZING STRENGTH OF PERMAFROST TOPILE

Q • ADFREEZING STRENGTH OF ACTIVE LAYER TO PILE

FORCES ACTING ON PILE DRIVEN INTO ARTIFICIALLY

GROUND SURFACE -10-STEAM .. : .. • - I : . . . :.. ••••• Ｇｾ •._.I::.: .. I.•.:...:.: ...'..1:"Ｎｾ •••• : '•

.

ｾＮＺ

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..:

,:::.-:

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_{_ ::ACTIVE LAYER :::":}ｾ _ｾ｜Ｎ ,f., I • • • • • • • • • I• • •1 • • • • • • : • • • • • ' . . . . .••• •• •Jf••••: : .. ｾＧＺＮＺＮｾＮＺ

:::

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ａｐｐｒｏｘｉｾａｔｦＡ OMAP! 0' THAWED GROUND

tSANDY- SlLT SOIL) AfTER STEAM POINT

HAS REMAINED fOR 1'/2 HOURS IN A

14 FOOT HOLE.

FIGURE

9

3. If the active layer is being replaced with either gravel or sand, a frame approximately 6 ft. in diameter should be constructed of planks concentric with the pile before filling with new material.

4. The portion of the pile embodied -in the permafrost should be roughened and notched, as shown in Figs, 10, a, b, c.

28, The Purpose of the Building

Before deciding upon the type of foundat-ion for a proposed building, the nature, use and life of the building must be known, Deformation of foundations occur chiefly in structures, the interiors of which are kept at high temperatures, i,e. inhabited houses, boiler houses, workshops, etc. Structures such as warehouses and unheated hangars do not have so marked an effect upon the soil temperatures. Observations, however, have

shown that the erection of any type of structure on the soil surface will change the temperature behaviour of the solI (see Fig.4).

-20-INSULATION MATERIAL OP AND BOTTOM SEAL T WITH

r.

.=l E WRAP

GREASE PIL , R SURFAOE. PLANE AND EASE OUTE .Ia'Co'

GR FIGURE "10

NSllLATlON,

/

ｾｾｾｌＺＰＺＮｾｾｾｾ［［Ｍ［ｾ

BLANKET L1TOP AND

/ WO LOOSE L PLY ROOFING,AYERS AND SEA APPLY T WITH THREE- BOTTOM. 'C!I

OOVER FIGURE

ｾｏ

-21-29. The Type of Loading

The type of loading must be considered with groat care, since loadings on piles will have a lower heat transfer than will those on continuous foundations. The choice of loadings will depend

upon:-(a) The type of structure.

(b) The properties of the permafrost. (c) The composition of the soil.

30. The Normal Internal Temperatures

There is a general lowering of the perma-frost table underneath all structures. If this lowering of

the table continues over a long period of time, the building may rest upon thawed ground so unstable that it cannot sup-port the load. A heated building will accelerate this con-dition unless means are taken to prevent such heat transfer

into the ground.

31. The Rate of Heat Transfer from structure to Ground

There are two general methods of prevent-ing this heat flow. The first and most common is to insul-ate the flooring with rock wool or similar insulating minsul-ater- mater-ial. The second is most widely used in conjunction with the above and consists of the elevation of the structure to per-mit the free circulation of air under the flooring, which carries any heat away from the ground. The addition of peat or moss to the ground under the foundation will also aid in retaining the general permafrost table at its origin-al level. Where ｬ｡ｲｧｾ boiler units are an integral part of the building, special ducts may be required in the concrete foundation to carry out the excess heat.

32. Depth of Foundation

1. The depth of a foundation depends to a great extent upon whether the active or passive methods are used in its construction. If the passive method is used, the type of foundation is either piling or a gravel

insulating mat over the top of the permafrost.

2. Foundations constructed using active methods may extend down into the active layer. Any found-ation that is set into the ground in a permafrost area should be surrounded with a trench Which is filled with dried gravel or other non-swelling material. This prot-ection is required to counteract the effect of any

horizon-tal stresses which may be set up within the active layer. 33. Foundation Planning

1. The durability and stability of structures erected on permafrost depend upon the selection of the proper type of foundation, the construction of this found-ation, and upon local conditions. One of two basic prin-ciples may be applied in its

design:-(a) The principle of preserving the frozen condition of the soil (the passive method).

(b)

(a) (b) (c)

-22=

The principle of thawing out the permafrost during construction or while the building is in use (the active method).

The choice of the foundation depends upon: Temperature

The composition of the soil Its water content

If the active method is being used, unit values of the bearing power of the soil must be based upon its propert-ies wl1en thawed. AS normal types of foundations may be planned under this method, some points concerning types of ground are briefly described.

34. Active Method 1. Bedrock

A normal stable foundation can be built on bedrock irrespective of whether the permafrost regime is retained or eliminated, assuming the bedrock is on the sur-face. Adequate provision must be made to drain any water resulting from thaws, as such water may create icings, per-haps several years after the building had been occupied. 2. Gravel and Sand

Although the laying of foundations within a non-swelling active layer such as gravel and sand is fairly safe, in general it should be avoided in building structures of a permanent character. The periodic freezing and thawing of sand and gravel can cause the disintegration of such a mat. Since this method is ,based upon the assumption that the ground in its thawed state will support the structure, lenses of clay or silt occurring within a depth equal to the width of the entire building may cause local subsid-ences to occur. An impervious wall must be built around the building to prevent the movement of ground water into the thawed area beneath it (see Fig. 11). As the ground water is under hydrostatic pressure during the freezing period, it may escape into the thawed area under the build-ing, flooding the structure as ice is formed upon the

exposure of the water to the lower temperature of the atmos-phere (see Fig. 12).

3. Fine Grained Soils

Care must be taken in selecting a build-ing site on ground which is comprised of fine silts and sands. Assured results can only be obtained if based on a long series of observations of this type of soil, as the elimination of the frozen condition in most instances will convert" the ground into a plastic unstable mass.

35. Passive Method

Experience lends credence to the principle that the most suitable method of building on permafrost is one based upcn the least disturbance possible of the frozen ground. So long as the sub-grade remains frozen

-23-.MPEIWIOUS WALL FIGURE

it

'A' STEAM PIPES __ ｾ -z ｾ i': • - .. ;zＱＮｉＧｾｬＬＮＺ＾ｯＧＭＺｯ］Ｚｬ • •'D D

t:.

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SANO· •• 0 • • ...•• • ':1 ... , •. " . ' ,,0, .1"." "·BACKFILL •••••• ·,:::.;.:•••·:••ﾷＮ［ＮＺｾＺＺＺ ••.Ｇｾ•••ｾ•.•ﾷｾｉ.•·.ll.,;.•ｾＺＮＧＮＺＮＺ . • : - . 0_,0 · .,. · ... Dd • • • • : 0 • • •.It. -•••••

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.

ﾷＶﾷｄﾷＮｾﾷｯﾷ

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TYPES OF FOUNDATIONS ｒｅｃｏｍｾｅｎｄｅｄ IN

ACTIVE METHOD OF

CONSTnUCTION-WHERE PERUAFllOS'r IS ｅｌｉｾｕＧｴａｔｅｄ FIGURE U'Bt'

and is protected towards this end, the most stable found-ation conditions will result. The choice of the type of construction to be employed depends upon the nature, use and life span of the bu I Ld Lng , together with a study of the soil properties and climatic conditions at the site.

36. Small Buildings, Limited Life

For small buildings with a limited life span of from 15 to 20 years, the method shown on Fig. 13 has proven itself suitable. The ｢ｵｩｬ､ｩｲｾ will remain stable so long as the permafrost horizon ､Ｈｾｳ not recede beloW the base of the piles. Insulation ｾｳ placed between the

joists to slow up the rate of heat transfer from the building. The space under the building has no fluct-uation of air temperature, remaining fairly constant.

-24-s

Ｏｾｾ

Ｇ＼Ｍｾ

ｾ

WATER COMES TO SURFACE AND ＧＭＭｾＧＧＧＧＧＧＧ FREEZES ON EXPOSURE TO

'f"----,

COLD AIR

<;:-

_ＭＭＭｾ

SURFACE ICE MAY FORM UNDER AND AROUND HEATED BUILDING WHEN FLOOR INSULATION IS INADEQUATE AND COLD AIR IS NOT ALLOWED

TO CIRCULATE UNDER BUILDING IN WINTER. FIGURE

i2.

PERMAPROST

JOIST ,""'- SUB FLOOR

-25-Experience indicates that the permafrost level will recede slowly over a period of years. If piles are set with more than 10 ft. anchored within the permafrost, stabil-ity should be assured for at least the life span of the building.

37. Barracks, Boiler Houses, Power Plants and Medium Sized BUildings

For structures having or containing a continuous source of heat, a slightly different method of construction should be used, as shown in Fig. 14. The air space under the floor allows a free circulation of air, helping to maintain the permafrost at its natural level. The floor is well insulated to reduce the heat transfer from the ground. Piles, either wood, steel, or pre-cast concrete, should be set at least twice the depth of the active layer into the permafrost. The length of the pil-ing in the active layer should be surrounded with a layer of sand or gravel, the upper portion being well greased and encircled with a thick layer of tarpaper to prevent the ad-hesive forces from gripping the pile. The lower portion of the pile should be left rough and notched in order to in-crease the adhesive strength of the permafrost layer, thus decreasing its tendency to heave. Figs. 15 a, b, show examples of the foundation resting upon the permafrost table; there is no moving ground water and the frost zone is not subject to swelling.

38. Water Tanks, Pump Houses and Hangars

1. The method shown in Fig. 16 is well suited to large structures such as water tanks, pump houses and hangars. A layer-of clean sand is built up to a height of 3 to 4 ft. above the surrounding terrain, care being taken not to disturb the top soil except where the complete removal and replacement is necessary. The layer of sand must extend well beyond the limits of the building, beyond the limits of normal noon-day shadows. Such a fill is best placed on the frozen ground in the early spring when the depth of the active layer is least. Care should be taken to avoid the disturbance of any trees or other veget-ation within the area.

2. This method, sometimes called the "above screen"foundation, is well adapted to any type of building in the construction of which piling is neither feasible nor desired. Unless the natural surface on which this screen is to be laid is already covered with a good insul-ating blanket such as peat or moss, a layer of any low heat conducting material should be spread over the ground before the sand fill is made.

26

-TYPICAL DESIGN FOR STRUCTURE WHERE PER.UFROST IS TO BE PoAAIliTAINED BY PROPER INSULATION AND

VENTILATION FIGURE

14

-27-.0\

V

EXAMPLE OF REINFORCED-CONCRETE FOUNDATION RESTING ON A TIMBER BASE

FIGURE is'A'

WOOD PILLAR BOLTED TO nt.lBER FOUNDATION FIGURE i S ＮｾＮ

EXAMPLES OF FOUNDATIONS RESTING ON PERMAF"OST TABLE WHERE THERE IS NO MOVING GROUND WATER AND ACTIVE LAYER IS NOT SUBJECT TO

sWELLING.

TANK -28-STAVES

ｩｌＺＮＧＮｾＮＱＮＧ

,'1

, ' IEAW. BUILT UP ROOFINt END 0' STRINGER.

LEFT OPIN TO ALLOW

'OR AIR e1ReULATlON

Ｚｾ ｾＮｾＮＺＮＧ

:.::.:..;:-::;

ｾＮＺＺＮＺＮ［ＮｾＺ

.:

ｾＺＺＮＺ

:::.; .:

:..:...

ｾｾＺＮＺＮｾＺＮ

ｾＮＺＺＮｾ］ＭＺｾＮｾＮＺｾＮＮＮＮＮＮ

..

+"

: ＧＮｾｾＺＭ : •..: SANOY BACKFILL PAGKEDr:-.; :-••':'

;.._.' ｉＺＭＺｾＮＺＺ :-':'. :;:;.:::.::_{' : ;"': ' ..}IN ＴｈｌａｙｅｒＸｾﾷＺＬＮＮｴ_{ＺＢＬＺｾＧＬ .•.., ....• ',.. :'•.••••••... ACTIVE}••ｾＺＮＭＺ

='..

ｾＺＮＺ

.. : :...• '.' ' :.\:: , , LA ER

:- . ". : PERMAFROST L IHE .. ',': :.:.: ',.:' ..••; .. :

EARTH FOUNDATION FOR \'1ATER TANK.

29

-FACTORS AFFECTING ROADS UNDER PERMAFROST conDITIONS

39.

General

The application of normal construction methods to the building of roads over permafrost areas originally involved the stripping of the moss before the commencement of filling operations. This had the effect of removing the insulation from the frozen ground and disturbing the thermal regime, altering the

original hard sub-base to a soft plastic mass. Sections of road built by such methods required ｣ｯｮｾｴｮｮｴ main-tenance and encouraged the development of new methods of road construction through sucb regions.

40.

Surveys

1. As has previously been mentioned, the gov-erning factors in the choice of a road location through a permafrost area d Iffer in many respects from those ruling locations in more temperate areas. For roads of a permanent nature a much more detailed survey of under-ground conditions is required. This entails the sinking of many test pits or drill holes in order to determine the con-ditions which typify the route. No hard and fast rules can be given for the location of these test pits and drill holes, but the following list of locations may serve as a guide to likely points in which to make

investigations:-(a) Slopes with different exposures. (b) Slopes with varying inclines.

(c) Areas with different types of soil, vegetation and minor features. (d) Areas to be excavated.

(e) Areas to be filled.

(f) Swampy hollows and depressions.

(g) Sites of springs in fields of surface ice. (h) Cave-in lakes (a lake formed in a cave-in

depression produced by the thawing of ground ice).

(i) In the vicinity of landslides and slumps. (j) Area of ground ice.

(k) Along gullies and canyons. (1) Near lakes and rivers.

2. Test pits should be dug to a depth of at least

4

ft. but for greater depths the post hole auger or drill is more practical than digging.

41.

Swelling and Heaving

1. Landslides, swelling, settling and caving of ground due to the melting of ground ice are the major causes of damage to roads. Factors contributing

to the above conditions may be:-(a)

(b)

(c)

A layer of finely grained silty material near the surface.

Clay containing inclusions of fine sand or silt.

Medium or fine grained sands which are saturated with water and which cannot drain away becnuse of the permafrost below.

-30-2. Since the effect of a filled section of

the roadbed is to provide ｾｯｮ･ insulation for the ground on which it lieg, the permafrost table will rise bensflth it. Seasonal thaVling will also penetrate somewhat deeper under the fill than in the surrounding ｧｲｯｵｮ､ｾ particularly on the south side. This will cause a slight depression :tn the permafrost table on this side in which excessive moist-ure may accumulate to cause a landslide. Such a condition

may

be avoided by surfacing the south slope of a fill with an insulnting layer of peat: moss or brush.

L12. Drainage

1. As a general ｲｵｬ･ｾ where the natural

drainage system is altered, an artificial one with similar characteristics umst; be provided. Experience indicates that the less interference there is with the natural move-ment of water the less likely it is to cauue trouble. The exception to this rule is the condition in which the nutural drainage is causing icings.

2. Surface waters will freeze during the winter

to form ice which may cause trouble. Surface wate r-s may

be removed by the use of insulated ditches (see Fig.l?), which are covered With snow during the winter as a

prevent-ion again.st their freezing. Such channels must be pro-tected against erosion by the use of moss or sod. Deep

nar-r-ow cuLverts are preferable to wide shallow ones0

3. ',Vide ditches along a road provide a means of

storage of the snow cleared from a road.

4. The hydrologic regime of the area traversed

by a newly constructed road fill may become affected by that f i l l . If the direction of the f i l l more or loss coincides with the flow of surface and ground waters its effect may be slighto If the fill runs at right angles to the

direct-Lor, of the flow of vrater-, its effect can be c orne quite marked and may lead t.o serious troubles developing (see Fig.18).

4 "0 . Sideh1l1 Cuts

The use of sidehill cuts should be restrict-ed to 2. mLn Lmum as they may cause seepages in the winter and create a flow of ice 0'101' the road.

4'1:. Fills

The effect of tlw construction of a new f i l l upon the hydrologic regime of an area has been ､ｩｳｾ

cussed lJpon the provious page. If the construction of the f i l l ha s oo cur-r-ed in the late summer or early f'a L'I , it 'hill contain a considerable amount of stored heat and will re-tard t.h« st abfLi z a t.Lcu of the ther-ma L regime. Eventually this st ab i Lf.zat i on of the permafrost hor-Lzon wil::' take place at a hi.gner- level, usually moving up tnt o the newly mado fill (see Fig.1S).

SHOW EARTH BACK FILL TIMBER CULVIZRT LAVER 0' COARSE ROOK

EXAMPLE OF INSULATED ｭｊｄｃＺＺｒ･ａｏｕｾｊｄ CHANNEL

FIGURE

i

'7

45. Cuts

Road cuts present even more difficulties than do fills. The ground of the active layer is gen-erally either partly or complotely removed and the perma-frost layer itself may be penetrated. This will CRUSe a substantial lowering of the permafrost table and the ground that Vias frozen and firm will change into a. less stable plastic.: or freely flowing material which gives rise to landslides or slumps. If the area cannot be avoided in the road location the only solution may be the replacement of this unstable material.

46. Icing

1. A very serious difficulty whLch is often enc ountered is that of ic ings, or as t.he y are sometimes called, "Na Lye d s '", Icing is a mass of sur-f ac e ice formed during the winter by successive freezings of sheets of water which may. seep from the ground, from a river or from a spring. Unless prevented or controlled. the ice may completely engulf the road, rendering it useless. 2.

of icings

Conditions favourable for the formation

are;-(1) The presenca of ground water in the active layer.

(2) Low temperatures of the air with a thin cover of snow during the early part of the winter, December to January.

-32-ｉｬｮｾｳ ｇａｕｾＺＺＡ PERdAFR03T TAelE TO RIDI! ON EAOH

liD! 0' ROAD Fill ItEEPIf4G THE GROUND SOLIDLY

FROZEN

a

STADll (:C!fltlATH THl! ROAD filL. SUR.

FAOI WATER 18 DRAlll£!D TflROUOtt DITOH!!8

DRAlnAGI DITOH

AOTIVI lAYER

AN EXAt1PLE OF THE EFFECT OF A ROAD FILL ON THE PEm.3AFROST TABLE

-33-(3) Proximity of tho permafrost table to the surface of the ground.

(4) A thick cover of snow during the latter part of the winter.

(5) The removal of the top soil dur-ing construction.

THE EFFECT OF FILL ON THE ｐｅｒｙａｾ

FROST TASLE

FIGURE

i9

Up to very recently the only practical method of combatting icings was that of removing the ice either by hand or by machine. It was a very costlYI tedious and continuous pro-cess as the growth of the icing in itself was continuous. 3.

A

more practical method of treatment has since been found to consist of the transference of the icing to a place in which it will not be harmful. Where icings are formed by sub-surface water, the direction of the flow of the feeding ｳｰｲｩｮｾ should be determined and the water intercepted at some distance above the road. This may be done by removing part of the soil over the water bearing layer at the interception point to allow its rapid rreezing in cold weather. The water of the under-ground stream will then be blocked by the rrozen layer and forced to the surface to form an icing at the location desired. This method utilizes the principle of freezing belts (see Fig. 20).

4. Ir the icing has occurred where it was not anticipated or has appeared too late in the season to apply the freezing belt principle, the installation of a surrace barrier may be undortaken. permanent barriers may be erected where an icing is known to f'or-m year after year (see Fig. 21).

-34-GROUND WATER UNDER PRESSURE BREAKS (JUT HERE, FORMING ICE ON SURFACE 0'

GROUND

SUGGESTED METHOD OF CRE.ATING INDUCED FIELD OF SURFACE ICE.

==

_{fiGURE 20}

.5' TO .8'

1

ｒ［ｾｾｾｲＡＭＭＭＭＢ

ARTIFICIAL TINBER BARRIERS COVERED WITH SNOW FOR 'R01EOTI"9 STRUCTURES FROM ICINGS FORMED BY .ROUNDWATER.

FIGURE

al

5. Where fast flowing springs occur the water is led away from the road by menns of warmth retention channels (see Fig. 22).

6. If all of the above methods fail, the use of heat in the form of oil stoves at the source of an icing may be resorted to. The thawing of ice filled culverts by means of steam hoses can be used when un-expected icings occur or in areas where other methods are not practicable.

-35c;w

SNO_

fENCE

TWO METHODS OF WARtJTH nETENTION IN DRAINAGE ｄｉｔｃｾｓ TO PR5V1N ｔＮｴＱｉ［ｉｾ FREEZI," AND

FORM-ING I IHOS AT NO IR BL POINT.

FleUR!

2.2.

ＴＷｾ Frost Boils

1. Gravel surfaced roads are frequently made impassable in the spring due to the presence of frost boils. These are moisture saturated) semi-fluid pockets of

surfac-ing material which develop dursurfac-ing the sprsurfac-ing thuwp and when

broken through by heavy traffic form a quagmire. Their cause is attributed to the practice of keeping roads clear of snow during the Winter producing a deeper frost pene-tration beneath the travelled road. In the spring the road thaws at a greater rate than do the shoulders, which are usually protected by sod or snow. This rapid thawing pro-du c e s an excessive amount of undr-a Lnab Le moisture in the soil which traffic pressure may force up to form a fluid mass.

2. Many cures have been attempted) but nOne seem completely successful. The most satisfactory method developed consists of a trench aLong the centre of the road-way in which a perforated drain is buried with the gravel.

If a roadway has a good sub-grade of well drained rock and gravelp frost boils will be eliminated.

48. Snow Clearance

The problem of snow clearance is similar to that of the more temperate latitudes of Canada with the exception that in some exp05cd northerly areas in the barren lands high winds are prevalent which tend to cause hard-packed drifts.

49. Read Surfaces

Where a roadway is being construc;ted over permafrost consisting of silty s o Ll , ae ver-a I rules must be adhered to. 'I'he opel' at10"- ah.cu Ld :ake place late in the yearp after the active layer has fr-oz e n , A layer