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Canadian Geotechnical Journal, 6, 1, pp. 65-80, 1969-02
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Effects of environment on the performance of shallow foundations
Hamilton, J. J.
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A N A L Y Z E D
EFFECTS OF ENVIRONMENT ON THE PERFORMANCE
OF SHALLOW FOUNDATIONS1s"
J. J. HAMILTON Prairie Regional Station, Diuision of Bt~ilding Research,
Notional Research Council of Cann(lc1, Sasliatoon, S a ~ k a t c l z e ~ n n
I n addition to its structural role, the foundation usually separates the Inan- macle indoor climate from the actual en- vironment of the ground. The implica- tions of this seconclary role are sometimes not fully appreciated by the designer, with serious consequences. The structu- ral success of shallow foundations on volume-changing clays may be governed Ly their net effects on the subsoil en- vironment. T h e significance of changes from natural ground conditions by con- struction ]nust be appreciateel at least qualitatively by the founclation designer. Natural and artificial changes in the thermal, the nloisture, and the stress environnlents of subsoils are discussed in this paper, with particular emphasis on the clay soils and the climatic conclitions of Western Canada.
Outre leur r6le structural, les fonclations habi- tucllcment skparent ainsi le inicroclimat int6- rieur crC6 par l'homme de l'environnernent naturel cle sol. Les inlplications cle ce r61e secondaire sont parfois nCgligCes par le maitre d'aeuvre, entrainant dcs conskquences f8cheu- ses. L C SLICC~S, clans le clomaine structural, cles fonclations superficielles assises sur des lits d'argiles expansibles peut Ctre clCtern~inC par lcur action globale sur le lnilieu sous-jacent. L'importance des modifications imposCes au nlilieu nature1 par la construction clevrait Ctre apprCciCe au mains qualitativement par l'ingC- nieur du gros-aeuvrc. L'auteur ktuclie clans le p~i'scnt article les modifications naturelles e t artificielles cles conditions thcrmiques, hygro- mktriques, et mCcaniques clcs sols d'assise, par- ticulii-rcment clans le cas cles coucl~cs cl'argiles clc l'ouest canadien.
INTRODUCTION
A fouildation usually forms part of the total building enclosure, in addition to its structural role, sei~arating I V the man-made. indoor climate from the natural environment of the underlying and surrounc~ing soil or rock. The structural success of relatively shallow foundations on volume-changing clays may, in fact, be governed by the net efiects of the foundation on the natural soil environment. Natural and artificial changes in the thermal, moisture, and " stress environments in the soil prior to and during construction, and through- out the life of structures, will b e discussecl in this paper, with particular refer- ence to clay soils and the climatic conditioils of Western Canada.
lNRCC No. 10519.
2Presentecl at the 21st Canadian Soil Mechanics Conference, Winnipeg, Manitoba, Septem- ber 12-13,1968.
CANADIAN GEOTECHNICAL JOURNAL MEAN ANNUAL 37. AIR TEMF!
I
I I I IJ
20 40 60 80 100 TEMPERATURE , OF.0-
- -
0 NATURAL SNOW COVER , OPEN FIELD ,UNDISTURBED SOIL 0 - - - - ---A NATURAL SNOW COVER, RECOMPACTED CLAYBACKFILL I N WATERMAIN TRENCH
0--0 SNOW CLEARED CONCRETE PAVEMENT,
GRAVEL BACKFILLED SANITARY SEWER NEAR OF STREET
F I G . 1. Typical maximum, minimum, and mean annual ground temperatures, 1959-1963,
for a typical street cross-section in Winnipeg, Manitoba. 0 = natural snow cover, open
field, undisturbed soil conditions. = natural snow cover, recompacted clay backfill in waternlain trench. = snow-cleared concrete pavement, gravel backfilled sanitary sewer near centerline of street.
THERMAL ENVIRONMENT
Mean annual temperatures in undisturbed ground that is subject to natural snow cover usually range between 5 and 7 O F above the mean annual
air temperature (Crawford and Legget 1957). In the major urban centers in the Prairie Provinces, the mean annual air temperature ranges between 35 and 40 OF; the mean annual ground temperatures are between 40 and 47" F. The amplitude of diurnal and seasonal ground temperature variation becomes smaller with depth, becoming insignificant at depths exceeding 25 ft.
Typical curves for maximum, minimum, and mean annual temperatures in Winnipeg for an undisturbed soil, a compacted clay backfill, and a gravel backfill are shown in Fig. 1. The influence on temperatures created by back- filling, paving, snow clearing, and operation of service pipes is apparent. At the 6-ft depth, minimum soil temperatures usually occur during April, as they lag behind minimum air temperatures by 2 to 3 months.
HIIMILTON: EFFECTS OF ENVIRONMENT ON SHALLOW FOUNDI~TIONS 67
-16'-
FIG. 2. Typical ground temperatures adjacent to a slab-on-ground house with edge insulation, late winter and late summer conditions.
E FEB -MARCH
I
MEAN AIR TEMP 12'E65' 60' 55' 50' 45O
Heated buildings significantly change subsoil temperature conditions. Almost without exception, because of the relatively cool natural ground temperatures, there is a continuous heat flow away from heated structures in Western Canada during all seasons. Figure 2 shows typical ground temperatures for late winter and early fall near a slab-on-ground house with conventional perimeter heating and insulation. The significance of the very steep temperature gradient (greater than 40 O Fper f t ) beneath the slab perimeter will b e discussed later. A heated
basement introduces a new high soil temperature at depth and steep horizontal temperature gradients are imposed where previously only vertical gradients existed. Figure 3 illustrates the ground temperature conditions around an
k
I
MARCH 26,1954 SEPT 30,1955
EARLY SEPT. MEAN AIR TEMP 58'F.
FIG. 3. Ground temperatures adjacent to an uninsulated basement, late winter and early fall conditions.
68 CANADIAN GEOTECI-INICAL J O U n N A L
F I G . 4. Effect of on-grade perimeter insulation on heat flow pattern in late winter at the
edges of a heated slab-on-ground.
7'- M A R C H 13,1967
uninsulated concrete basement for late winter ancl fall conditions (Solvason and Hanclegord 1959).
In selecting the appropriate type, amount, and location of insulation for foundation walls, slabs-on-ground, ancl grade beams, designers should b e aware of the loacl placed on the heating system of the building as well as the resultant soil temperatures and their possible implications with respect to moisture redistribution in clay subsoils. Vapor and liquicl flow systems can be set up in partly saturated subsoils clue to steep thcrmal graclients (Hutcheon 1958).
Numerous slab-on-ground foundations in Regina, with 2 in. of insulation covering the thickened edges of slabs, have undergone severe perimeter heav- ing, clue, at least in part, to moisture migration caused by steep thermal gra- dients (Fig. 2 ) . An alternate design, using a 4-ft width of 2-in.-thick expanded polystyrene insulation, laid beneath pavement or topsoil, creates a better heat flow pattern under the edges of slabs-on-ground as shown in Fig. 4. The ther- mal gradient is not as steep and the zone of soil cooler than 35 OF is well beyond the edge of the slab. h/Ioisture barriers placed above and ( o r ) below the insulation can further stabilize soil moisture co~tditions under the edge of the slab.
OETAIL- 2X SCALE BELOW
I t is preferable in many instances to place insulation on the outside of base- ments and grade beams to obtain masimum insulating benefits, and to elimi- nate cold spots where condensation of water vapor may lead to the redistribu- tion of soil moisture and cause h e a v i n ~ of shallow foundations. When insula-
C - - - - -
,
// SNOW
0
tion is
laced
on the insicle of basement walls the surface of contact with the 25°F:30"
---
- - -
32"35"
ground'is kept cold; this condition is conducive to adfreezing between the wall and the ground. Insulation on the outside, however, effectively stops adfreezing to the underlying structure, ancl ininimizes any transfer of move- ments between the ground and the building.
HAMILTON: EFFECTS OF ENVIRONMENT ON SHALLOW FOUNDATIONS 69
OPEN FIELD -GRASS COVER
TREE AND GRASS COVER
W
b TREE AND FALLOW
2 5.0 0 3 o 4.0 u w 2 3.0 0 2.0 > 1.0 0 1.0
TREE AND CONCRETE PAVEMENT
8 PAVEMENT SURFACE
0
1962 1963 1964 1965 1966
FIG. 5. Calculated soil moisture depletion ( A ) and vertical ground movements at various depths in undisturbed soil; ( B ) in an open field with grass cover; ( C ) in a grass-covered plot, 226 f t from a tree; ( D ) in a fallow plot, 223 f t from a tree; ( E ) under a concrete road pavement centerline, 273 ft from a tree.
70 CANADIAN GEOTECHNICAL JOURNAL
MOISTURE ENVIRONMENT
In undisturbed clay covered with natural vegetation, soil moisture conditions are governed mainly by climate and vegetation. Plants and trees act as efficient pumps in removing moisture from the soil through processes of evapotranspira- tion, Many heavy clays, which have been seasonally frozen or cycled through wetting and drying, have a well-developed secondary structure in the form of fissures and cracks, which assist infiltration of surface water. Tlle extremely low permeability of the more massive, unstructured clay subsoils at greater depths, inhibits soil moisture drainage or recharge from below.
Figure 5 shows typical ground movements measured at various depths in undisturbed clay subsoils under the influence of grass, trees, fallow, and pave- ment, from July 1962 to June 1966. From June 1965, the influences of chemical fertilizer and irrigation were superimposed on the natural factors; the ground movement measurements (Figs. 5C and 5D) reflect these additional influences (Mindess 1965). Typical ranges in soil moisture content and degree of satura- tion for the periods of observation are shown in Fig. 6.
RANGES OF MOISTURE CONTENT AND DEGREE OF SATURATION, O/.
FIG. various . - OPEN FIELD- GRASS COVER I TREE AND FALLOW
6. Ranges of measured water contents conditions. 12 TREE AND GRASS COVER 121 I TREE AND CONCRETE PAVEMENT
HAMILTON: EFFECTS O F ENVIRONMENT ON SHALLOW FOUNDATIONS 71
MOISTURE CONTENT, % (DRY WEIGHT BASIS)
C O
j
tf14 U-. F 8 6r:~
0T b
8i d
M
I 0 'I -0'5 AUG. 21,1962 SEPT. 4,1962 JULY 16,1964 0 SEPT, 13,1965FIG. 7. Vertical ground movements and typical water contents at various depths in an undisturbed soil, Regina.
15'
' - q i L d > o L ' J' '
'
'd
' 'J' ' b' '1965
Figure 7 shows measurements of field ground movements in an undisturbed Regina clay for a period of 8 years, and corresponding minimum and maximum moisture contents. Much above normal precipitation during the summers of 1963 and 1965 is reflected in the measured ground movements and water content profiles.
The probable extreme range of water contents to be expected in heavy clay subsoils in Regina for various conditions is shown in Fig. 8. The extremely dry conditions (Curve A ) might develop in crawl space subsoils exposed for many years to an average temperature of
75
O F and relative humidity of 25% or less.The extremely moist condition (Curve B ) could develop if the surface of the soil was kept continuously saturated and at above freezing temperatures for many years. Wetting from the usual field moisture contents would probably result in ground surface heaving of the order of 8 in., whereas a settlement of more than
5
in. could be expected from drying to the condition indicated by Curve A. Curve C shows measured moisture contents resulting from 10 years of drying in an unprotected crawl space subsoil. Curve D shows the results of5 years of heavy watering of a subsoil below a lawn area.
Figures 9A and 9C show two cases of rapid heaving of natural soil due to flooding from the surface. Figures 9B and 9 D are water content profiles before and after flooding. Although these soils were similar in type, their initial
' ' ' ' J' '
'A
' 'J' ' 0 ' 1966 t o U 0 L V L d & , - b ~ J J ~ ~ ) J' ' 'A ' 'J' ' '0 ' 1967 0 0 J' ' 'A ' 'J 1968CANADIAN GEOTECIINICAL JOUnNAL
MOISTURE CONTENT, % (DRY WEIGHT BASIS)
F I G . 8. Probable extreme ranges of water content for unclisturbed Regina clays.
( A ) extremely dry conclitions resulting from many years of exposure to 7 5 " F and 20%
relative humidity at the surface (hypotl~ctical); ( B ) extremely moist conditions resulting from many years of continuous surface flooding (hypothetical); ( C ) soil moisture con- clitions aftcr 10 years of drying in an uncovered crawl space (actual); ( D ) soil moisture conditions after 5 ycars of heavy lawn watering (actual).
lnoisture contents were different. The soil for the second case was initially mucll drier and considerably more cracked than that in the first case. After flooding for over 2 months, the moisture contents at various depths became similar, indicating that the drier soil had taken on considerably more water. In spite of this difference in water intake, the amount and rate of heaving for the two soils were remarkably similar.
It is postulated that thc differences in secondary structure, rather than any differences in soil type, are responsible for the difference in the relationship between moisture content cl~ange ancl volume change or vertical heaving. For the drier case, the increase in moisture is largely taken up in resnturation and refilling of internal cracks and fissures, and contributes little or nothing to net heaving or horizontal swelling against structures.
STRESS ENVIRONMENT
During prolonged periods of drought, or when subject to freezing, high negative pore pressures may develop in subsoils. The pore space in he?
' Y
clays may remain approsimately saturated to suctions over p F 5.0 ( i s . approxl- mately equivalent to 100 tons/sq. f t ) . The wilting point of most plants is in the range of p F 4.2 (i.c, over 15 tons/ sq. ft ). Penner (1957) and Williams ( 1966), in studying soil suctions developecl during freezing, have measured suctions above p F 4 ( 1 5 tons/sq. f t ) at coinmoilly encounterecl field temperatures and moisture contents. Tllese stress levels are at least one order of magnitude larger than the usual stresses ~mcler foundations of small structures. They represent substantial driving forces for moisture movement, which in turn, may cause significant shrinkage of compressible soils.EIA3IILTON: EFFECTS O F EN\'lROhlhlENT O N SIIALLOW FOUSDATIOPI'S 73
MOISTURE CONTENT . (DRY WEIGHT BASIS) ,O 2 0 40 - 60 80 I --CONCRETE FLOOR -GRAVEL SUBFLOOR
-
2 - ,HIGHLY PLASTIC REGINA CLAY 4-
I- W k? 0-
C a8
W ? 0 8-
?-8
0 AUGUST 23,1962MOISTURE CONTENT, % (DRY WEIGHT BASISI
FIG. 9. Heaving of natural soils due to flooding. ( A ) hcaving of subsoil below slab-on- ground floor causecl by plumbing lcak; (13) soil moisturc profiles before and after heaving; ( C ) heaving of a clay clue to intentional Roocling during above-freezing weather; ( D ) soil moisture before and after flooding.
Natural soils often contain a small percentage of entrapped or dissolved air to depths exceeding 20 ft, well below the water table or inasimum depth of frost penetration. This air may go in and out of solution and change volunle in response to changes in pore water pressures or temperature. Thus sinall quanti- ties of adsorbed and occluded air in fine-grained soils inay impart cornpression and expansion properties not otherwise exhibited, e.g., elastic reactions to stress change and thermal volume change.
At shallow depths, large cluantitics of air are containecl in fissures, cracks, root holes, and other voids inherited from previous periods of frost action, high suction shrinkage, and from earlier plant life. This air is generally at atmospheric pressure, but may occasionally become occluclecl following a heavy rainfall, surface flooding, or ~ v h e n overlying soil is frozen. These large quantities of trapped air inay cause substantial vertical ground movements related to change in soil temperature.
74 CANADIAN CEOTECHNICAL JOURNAL
~t has been observed that, after a dry autumn, undisturbed soils under natural grass cover shrink and can cause $ to 1 in. settlement of the ground surface. In Winnipeg, for example, the soil temperature in the top 3 ft of natural soil dropped 20 OF below the mean annual soil temperature during the winter of 1963-64. With the pore space saturation ranging between 80 and 90%, the ground surface in the open-field, grass-covered test plot settled $ in. At the same time in a fallowed test plot, 224 ft away from a large tree, the ground surface settled slightly more than 1 in. (Figs. 5B and 5 D ) . In laboratory studies of freezing shrinkage of partly saturated compacted clays, A. B. Hamilton (1966) measured volume shrinkage ranging up to 10% as a result of freezing heavy clays at degrees of saturation from 90% down to less than 60%.
PRECONSTRUCTION CONDITIONS
Attention can now be given to the influence of construction, keeping in mind the dynamic equilibriums established under more or less natural climatic and vegetation conditions. Changes in effective stress related to changes in soil suction may be an order of magnitude larger than the stress changes due to foundation pressures. Quantitative methods of measuring these stresses in situ and of predicting subsequent volume change are not well established. Qualita- tive methods of assessing present conditions have been proposed for areas for which climatological data are available. Plots, showing the cumulative depar- ture from long-term average annual precipitation for Winnipeg, are helpful in assessing present conditions and in predicting subsequent foundation per- formance, e.g., Fig. 10. Other records, showing cumulative soil moisture at various times prior to construction, are similarly useful.
50 I I I I I I I I I
91 YEAR AVERAGE ANNUAL PRECIPITATION
FOR WINNIPEG, MANITOBA (1874-1964) = 20.53" -
-
-
3
I 0 L-
1 I I I I I I I I 1870 lW0 1890 1900 1910 1920 1930 1940 1950 19601
FIG. 10. Cumulative departure from long-term average annual ~recipitation for Winnipeg, Manitoba.
HAMILTON: EFFECTS O F ENVIRONMENT ON SHALLOW FOUNDATIONS 75 The usefulness of the soil moisture depletion method has been treated more fully in an earlier paper (J. J. Hamilton 1966). A designer can use the theoreti- cal soil moisture depletion as an indication of present soil moisture conditions, provided he has an understanding of the various factors that create differences between "actual" and "theoretical" soil moisture depletion.
CONDITIONS AT T H E TIME O F CONSTRUCTION
As construction proceeds almost without interruption throughout the year in Western Canada, varying temperature and moisture conditions are encoun- tered during the installation of foundations. In warm weather construction, either excessive drying of the subsoil or wetting may result, depending on the vagaries of the weather and the extent of exposure of subsoils, Moist subsoils, left exposed in excavations to hot summer weather or to artificial heating in enclosures for winter construction, can dry out significantly to depths of several feet. The rate of evaporation from an exposed soil surface can exceed
+
in, of water per day, depending on the temperature, relative humidity, rate of move- ment of air over the soil surface, and the availability of soil moisture. I t is apoor but common practice to place house basement floor slabs many weeks after excavation. The cumulative drying of the exposed excavation may b e great. Several inches of water may be lost by drying during normal construction periods prior to the placing of some effective vapor barrier on the subsoil surface.
Basement excavations are occasionally flooded during construction by heavy rainfall or for other reasons. This may b e beneficial in reducing the subsequent heaving to b e expected under lightly loaded floor areas, but the flooding is seldom allowed to proceed for sufficient time prior to placement of floor slabs to eliminate future heaving. Excessive softening of near-surface strata may occasionally b e harmful to the bearing capacity of shallow-spread footings placed on such subsoils.
The placement of a relatively clean gravel sub-base or a vapor barrier over the subgrade shortly after excavation will greatly reduce the drying of the subsoil. Where vapor barriers are not placed over subgrades, it is desirable to place the granular sub-base as early as possible and to keep it damp with a small amount of moisture (in excess of the amount lost by evaporation) until the floor slab is placed.
The freezing of previously unfrozen heavy clay subsoils during the construc- tion period has been observed to develop a nuggety structure in the clay. This secondary structure appears to persist for some time after thawing, and pro- vides channels for rapid subsequent movement of moisture in and out of the subsoil. Fissures and cracks in unsaturated clays may b e effective in increasing the rate of infiltration and distribution of water to considerable depths when flooded with water during or follo\ving construction. Very high negative pore pressures may result from either freezing or drying out of subsoils during construction.
POST-CONSTRUCTION CONDITIONS
For most small buildings with full basements (including all conventional one- and two-storey frame houses), the w e i g h of soil removed from basement excavations is considerably greater than the total weight of the structure and
CANADIAN GEOTECHNICAL JOURNAL
FIG. 11. Total stress redistribution due to combined effects of excavation and house foundation loads.
foundations placed on the subsoils. Figure 11 slio\vs the net effect of unloading due to escavation and reloading arising from foundation loads for a typical one-storey wood-frame house wit11 cast-in-place concrete foundation walls. The influence lines can be collverted to change-in-stress contours by multiply- ing the influence nuniber by the unit weight of soil removed and by depth of excavation. For perimeter footings, the upper 6 ft of subsoil undergoes increased stresses, while tlie remainder of the subsoil below 6 ft has undergone a net unloading. Under the interior column footings, only the top 2 ft of subsoil experiences increased load d u e to the structure above, and tlie underlying sub- soil has been substantially unloaded. For esample, under interior footings at a depth equal to one-half the width of the basement, the net unloading is of the order of $ ton/sq. ft. Immediately below the floor slab, in areas centrally located between footings, the net l~nloacling is of the order of ton/sq. ft.
Gilchrist (1963) and Noble (1966) have demonstrated the effects of unload- ing on the swelling of Regina clay. It is apparent that large volume changes take place when quasi-saturated clay gains access to free water under low
II.-\hIILTON: EFFECTS OF ENVlRONhIENT ON SI-IALLOW FOUNI>ATIONS 77 confining pressures. In Winnipeg and Regina, and to a lesser estent in other urban centers in Western Canada, perimeter and interior footings and floor slabs experience heaving within n short time after construction. The rates of
heaving depend on the extent of unloading and upon the availability of mois- ture to permit change in effective stresses within the soil (Hamilton 1965).
Further stress reduction may be expected if the soils have previously been desiccated to considerable depth and exist in a state of large negative pore pressures. Removal of deep-rooted vegetation and covering the subsoil with an effective vapor barrier allows soil moisture conditions to rise to new, higher equilibrium values with an attendant reduction in effective stresses. Under such conditions in Winnipeg, it Ilas been observed that perimeter foot- ings have heaved several inches and lightly loaded interior footings and floor slabs have heaved even greater amomnts, causing serious damage to the base- ment and superstructure. The unloading rebound efl'ects appear to take from 5 to 10 years to reach new equilibrium conditions. Follo\ving that phase, differential shrinkage of the subsoils, due to the growth of trees in the vicinity of the founclations, appears to become relatively more important. The long- term performance of shallow foundations is governed by vegetation, natural climate, and the effects of man's artificial watering of the ground surface.
In crawl spaces, significant soil moisture redistribution is caused by thermal gradients and moisture movement through the soil ancl vapor movement from warmer areas with condensation on cooler areas. klcasurements in several crawl spaces in Regina, without vapor barriers ant1 unventecl to the outside atmosphere, have sl~olvn that the average relative humidity of the air is con- trolled by the surface with the loniest temperature. I t is common practice to attach batt-type or rigid expanded polystyrene or polyurethane insulation on the inside of grade beams or foundation walls. lnsuficient attention is often paid to providing effective vapor barriers on the warm side of this insulation. The insulation may become detached from the structure after a fell7 months of service. The exposed surface of the grade beam and the nearby soil, just insicle the cralvl space, may rcach very lour temperatures in midwinter. h/lois- ture from the crawl space atmosphere collects o n thcse cold surfaces and seeps down into the underlying clays. Severe heaving of the shallolv perimeter foot- ings and settlement of interior footings caused in this way halie been observed in a number of cases. Where deep pile foundations have been provided, large swellillg pressures have developecl against grade beams. Uncontrolled evapora- tion from subsoils sometimes causes excessive humidity in thc building abovc, wood rot, and serious accumulation of salts harmful to concrete.
Where steep suction gradients are set up in saturated or near saturated soils due to rapid evaporation losscs at the soil surface, apprecial~lc flow of soil moisture may take place, even against thermal gradients. In crawl spaces n7itl1- out adequate vapor barriers, acculnulations of solu11le salts havc been left at the soil surface. TVhen concrete is in contact with soil on one side ancl esi~osed L on the other side to warm, dry air in heated structures, it may transmit appre- ciable moisture containing clissolved salts. This moisturc often evaporates at or just below the concrete surface, leaving behincl its dissolvecl salts. Serious attack of concrete piles, grade beams, foundation walls, and floor slabs has resulted when sulfates are the principal anions (Hurst 1968; Hamilton ancl Handegord 1968 )
.
78 CANADIAN GEOTECHNICAL JOURNAL
observed even in crawl spaces with vapor barriers on the ground surface. Much of this moisture movement appears to take place through the soil from warm to cooler regions. These cases suggest that provision of vapor barriers over the soil may not be sufficient to maintain constant moisture distribution under crawl spaces unless insulation is used effectively to minimize tempera- ture gradients in the soil.
SOME DESIGN CONSIDERATIONS
Uncontrolled addition of moisture to or loss of moisture from subsoils can be guarded against througll design details and enlightened construction and operation of buildings on shallow foundations.
Soils in an unsaturated condition exhibit significantly different engineering properties than similar soils in the fully saturated state at the same dry density. Differences in thermal, moisture, and vapor conductivities make it highly desirable that the state and degree of unsaturation be known to foundation designers. The existence of secondary structure also imparts important charac- teristics to subsoils. Whenever possible, a complete description of secondary structure of a soil and state of saturation should be included as part of the foundation investigation ( Aitchison 1956).
Well-developed secondary structure in the form of fissures and cracks imparts characteristics not found in massive clays. Rapid infiltration of surface water into natural profiles is aided by these shrinkage cracks. Soils which have under- gone a large number of cycles of swelling and shrinking appear to develop a higher effective shrinkage limit than that determined in the conventional laboratory test on remoulded samples. Below this effective shrinkage limit further net volume decrease stops, although the air content and crack frequency and spacing continue to increase. Above this effective shrinkage limit, crack filling approaches completion and net swelling begins. In Regina clay, the effective shrinkage limit for undisturbed soil profiles may prove to be close to the plastic limit.
Moisture barriers and insulation may be placed around the perimeter of foundations to establish more uniform environmental conditions in the sur- rounding subsoil. In many cases, insulation placed on the outside of founda- tions will be more beneficial than if placed on the inside. Improved surface drainage by grading and paving the ground surface is usually helpful and can be quite acceptable from the aesthetic point of view. Underground plumbing that is trenched into volume changing clays should be avoided whenever possible in areas of net unloading, or where substantial change in soil stresses can be expected, as negative slopes and broken lines may result. When sub- grade plumbing cannot be avoided, the piping and joints should be flexible and strong enough to resist large strains and stresses. Steeper than usual slopes should b e specified for all gravity-flow plumbing below grade, such as drainage tiles and sewer lines, to ensure rapid and complete drainage.
Foundation specifications should include some minimum standards of pro- tection and control of subsoil moisture content during the construction phase. These might limit the amount of exposure to wetting or drying, or require application of water to maintain a specified subsoil moisture content. Protec- tion of-subsoils from freezing should also be required. Shrinkage upon freezing of unsaturated clays is an effect not commonly recognized by designers and is
quite opposite to the more conventional effects of frost heaving in water-bearing silts. Freezing shrinkage of clay subsoils may cause settlement of foundations during construction. Fissures developed in the clay during freezing may greatly increase infiltration and subsequent heaving after thawing takes place.
CONCLUSIONS
1. Foundations for most structures have an important secondary function as part of the total building enclosure. When placed on subsoils sensitive to moisture changes, they must accomplish this separation of environments with- out jeopardizing their primary requirement of stability.
2. A general understanding of the changes in stress and moisture conditions imposed on subsoils by various structures is a prerequisite for satisfactory foundation design.
3. Control of heat and moisture flow in subsoils can be achieved through the use of insulatioll and vapor barriers. In many situations these are most effective when placed on the outside of the foundations and should be installed at early stages of construction.
4. Large volume increases result from relatively small reductions in confining pressure on swelling clays. These effects extend deeply into the underlying soils and cannot be corrected by shallow treatments. Additional large volume changes may also occur when deep-rooted vegetation is removed and if water is available due to irrigation.
5.
The establishment and maintenance of reasonably uniform stress and mois- ture conditions in clay subsoils are principal requirements for satisfactory performance of shallow foundations.ACKNOWLEDGMENTS
The writer is indebted to several of his colleagues at the Prairie Regional Station of the Division of Building Research for their assistance in carrying out the studies reported in this paper. The constructive criticism of Messrs. C. B. Crawford and C. F. Ripley, during the preparation of this paper, is also gratefully acknowledged. This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the approval of the Director of the Division.
REFERENCES
AITCHISON, C. D. 1956. Some preliminary studies of unsaturated soils. Proc. Australia- New Zealand Conf. Soil Mech. Found. Eng., p p 173-191.
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8 0 C A N A D I A N GEOTECEINICAL JOURNAL
MAXLILTON, J. J., and HANDEGORI), G. 0. 1968. T h e performance of concrete in Prairie soils of high sulphatc content. I n "Pcrformancc of Concrcte, resistance of concrete to sulphate and other environmental contlitions" ( A syinposiuin in honour of Thor- l ~ c r g u r Thorvaldson, April 1 9 6 7 ) . Univ. Toronto Press, Toronto, pp. 135-158. HURST, W. D . 1968. Espericncc in thc Winnipeg area with sulphatc-resisting cement
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