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Canadian Geotechnical Journal, 1, 1, pp. 27-42, 1963-09
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Volume changes in undisturbed clay profiles in western Canada
Hamilton, J. J.
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https://nrc-publications.canada.ca/eng/view/object/?id=3344e55e-4a47-46cd-bb64-511eb33e6fc4 https://publications-cnrc.canada.ca/fra/voir/objet/?id=3344e55e-4a47-46cd-bb64-511eb33e6fc4A N A L Y Z E D
Reprinted from Canc~dian Geoteclznical Journc~l, Vol. I , No. 1, Sept. 1963
VOLUME CHANGES IN UNDISTURBED CLAY PROFILES I N WESTERN C A N A D A
As part of a n over-all study of the per- formance of building foundations i l ~ highly plastic soil areas of western Canada, the Division of Building Research has been measuring ground movernents a n d changes in soil moisture conditions ill grass-covered, i ~ ~ l d i s t u r b e d soil profiles under climatic con- ditions ranging fro111 sub-11u1nid to semi-arid. Results of measurements begun in 1951 in Winnipeg, Manitoba, and Inore recently those in Regina, Eston, and Tisdale, Sas- katchewan, are reported. Results of a theoretical soil nloisture depletion calcula- tion, based on Thornthwaite's potential evapo-transpiration concept, a r e presented. I t is proposed a s a more rational way of
measuring the vegetation-climate factor in hulllid to sub-humid clinlates than simply comparing air temperature a n d precipitation with long-term averages. E ~ n p i r i c a l relation- ships are suggested between calculated soil moisture depletion, the depth of free water table, a n d the depth of frost penetration under similar thermal conditions b u t different soil moisture conditions. Shrinkage ,in sit16 of ~lndisturbed, unsaturated soils a t tcmpera- tures \\,ell below 32" F has been observed a n d is attributed to thermal air-void volunle change.
Dans lc cadre d'une Ctude d'ensenlble concernant le comportement des fondations daus les regions d e l'Ouest ayant des sols estr61nement plastiques, la Divisio~l d e s recherches en construction a eriectue des mesures relatives a u s mouvenlents du sol e t a u s changements des conditions d'humidit6 du sol dans des prolils d e sols non r e ~ n u e s e t recouverts d'herbe. Ces Inesures sent faites dans des conditions climatiques allant d e la sous-humidit6 B la semi-aridite. On donne da11s l'6tude les resultats d e mesures effect~16es en 1951 B Winnipeg a u Manitoba e t plus r6cemment B RCgina, B Eston, e t ?I Tisdale en Saslatchewan. On trouvera 6galement les r6sultats d'un c a l c ~ ~ l th6orique sur 116puise- ment d e I'humidit6 du sol fond6 s u r le con- cept d e Thornthwaite ?I l'6gard de 116vapo- transpiration possible. Ce c o ~ ~ c e p t est sugger6 comme une n16thode plus rationnelle d e mesure du facteur v6g6tatio11-climat dans les zones climatiques d e sous-hurnidite que la simple cornparaison d e la teinperaturc d e l'air et des precipitations avec les moyennes B long terme. Des relations e~npiriques sont sug- gerees entre I'&puisement calcul6 de 11humidit6 du sol, la profondeur d e la penetration du gel dans des conditions thermiques semblables ~ u a i s dans des conditions d'h~1midit6 du sol diff6rents. Une contraction i n situ de sols non r e m ~ 1 6 s e t non satur6s B des temperatures bien inf6rieures ?I 32" F a 6t6 observee e t elle a 6tC attribuee A un changernent de volume des espaces vides par voie thermique.
RiIovements of engineering structures founded near the surface of deposits of swelliilg and shrinlting clay soils constitute a serious problem in large areas of western Canada. 'The severity of damage, a n d therefore public awareness, increases during periods of extremes in weather coi~clitions a n d wanes during periods of more or less average weather conditions. Because these moveinents are closely linked with cycles in climate, engineers and architects cannot afford t o overlook this important factor in the design of structures for these areas.
Although the probleill has been recognized for many years, relatively little factual field evideilce has been obtained t o illustrate t h e magnitude of ground "Presented a t the Sisteenth Canadian Soil Mechanics Conference, Septenlber 12-14, 1962, E d ~ n o n t o n , Alberta.
tRcsearch Officer, Prairie Regional Laboratory, Division of Building Research, National Research Council, Saskatoon, Saskatche\\,an.
movements, their relationship t o depth, or their correlation with climate and soil type. Early work, sponsored by the Division of Building Research in co-operation with the University of i\/Ianitoba, was concerned primarily with the perforinance of basement-less houses (Baracos and Bozozul<, 1957) and the unusually poor performance of waterrnains buried in the Winnipeg clays (Baracos et al., 1955). In 1959, I was able to devote myself to these problems; this paper reports some of the worlc developed since then and reviews some of the observations dating back to 1950.
The general areas of swelling and shrinlcing soils in western Canada are reasonably well delineated now by geological reports (Geol. Assoc. Can., 1938; Elson, 1961; Christianson, 1961), agricultural soil surveys (Ellis, 1038; i\?itchell et al., 1944, 1950), and by engineering construction experience. T h e problenl soils of the eastern Prairies can be divided into two main groups. T h e oldest group is the highly overconsolidated Cretaceous shale which underlies much of southwestern i\/Ianitoba (i\/Ianitoba Dept. of Industry and Commerce, 1960) and populated Saskatchewan (Saskatche\van Research Council, 1960). This hard soil, or soft rock, provides soine extremely difficult engineering problenls due to uncertainties in assessing its shearing strength and, therefore, the long-term stability of man-made anel natural slopes, and because of its marked volume change characteristics. The other group of soils includes the more recent Pleistocene lacustrine clay and claj-ey-silt deposits. I t is possible t h a t some of these materials inay have been derivecl from the Cretaceous shales and redeposited on the floor of glacial lakes. Partly because of the high agri- cultural productivity of this soil, many of the population centres ol the I'rairie provinces are located on these deposits.
The original ground rnoveinent installations a t the University of IIanitoba have been described by Baracos and Marantz (1932). lIeasurements, begun in 1951, were continued until 1958 when the test area was required for a new building. A well-point type of piezometer was added t o the test arrangements during 1933.
Since 1939, new installations have been inacle a t different sites. IIeasure- ments of soil moisture changes and vertical ground lnovenlents are being made a t present a t four locations in Manitoba and Saskatchewan (Figure 1). Each of these test sites is located in undisturbed lacustrine soils having grass cover only. The description and classification of these soils, along with water coiltents nleasured a t various times siilce the installation of the gauges, are shown on Figure 2. T h e regional cliinate of Wilulipeg and Tisddle would be classified as sub-humid; that of Eston would be semi-arid; while that of Regina would be borderline between sub-humicl and semi-arid.
T h e open-field test plot in the Elinwood district of Winnipeg was established during the summer of 1950 in colljunction with continued studies of the physical factors influencing the performance of buried waternlains. -1 set of
Frtivas 1. Open field test plot locations in glacial lacustrine soil deposits
modified multi-rod vertical ground moveinent gauges, a deep bench nark
(Bozozul~ ct al., 19G2), and a thermocouple string were installed, as shown in Figure 3. During the late fall of 1961 a surface gauge (placed just under t h e sod, 3 in. below the surface) was installed t o furnish information on move- ments of the soil fro111 the surface to a depth of 2 ft. A complete record of ground n~ovements and temperatures has been kept since t h a t time. During the early summer of 1962, a neutron nloisture meter access tube was added to the instrumentation a t the site.
A group of gro~ind movement gauges was installed in Regina during t h e summer of 1960 in coiljunctioll with studies of the performance of slab-on- grade houses. T h e installation consisted of vertical ground movement gauges nleasuring a t depths of 1, 3, 5, 10, and 15 ft. A neutron moisture meter access tube was added a t this location during August, 1962.
Y
E
R
C!:TEN, t o D R Y WT,6tTTERBE[i L I M I Z~~~~~
I -3 -
0 - - 0 JUNE 1961
I 0 - 4 OCT. 1962 15
FIGURE 2. Soil profiles a t test plots: (A) University of Manitoba test plot, established 1951;
(B) Elmwood, Winnipeg, test plot, established 1959; (C) Regina test plot, established 1960;
FIGURE 3. E l ~ l l w o o d , \Yinnipeg, test plot instrumentation
Similar gauges were installed in test plots chosen to be representative of the lacustrine soil deposits a t Eston and Tisdale, Saskatchewan, in 1961. These installations consist of vertical ground movement gauges measuring a t depths of 1, 3, 5, 7, 10, and 15 ft., a deep bench marlt, and a neutron moisture meter access tube.
A new test plot was established a t the University of A'lanitoba during t h e sumnler of 1962. A study of the influence of a large tree on the soil moisture conditions and vertical ground movenlents under various surface covers h a s been initiated, in co-operation with the Department of Civil Engineering. A
large number of vertical ground ~llovernent gauges, neutron nloisture nleter access tubes, tensiometers, piezoineters, thermocouples, and deep bench marlts have been installed a s well a s reference points on nearby asphalt and concrete pavements. This instrumentation was con~pleted a t the end of a period of ~lnusually d r y weather, and i t is hoped t h a t valuable information will b e gained as soil moisture returns to a lllore normal (wetter) condition.
Before discussing some of the results of these field measurements, some of the factors ltnown t o affect volunle changes in clays will be reviewed.
Research worliers in the fields of c l i ~ ~ ~ a t o l o g y , soil science, and hydrology have long been concerned with changes in soil moisture conditions caused b y the effects of climate and vegetation (Lassen et al., 1952). T h e rate of evapora- tion from a soil surface is affected b y temperature, wind movement, vapour pressure gradient, depth of the water table, and soil texture and structure. T h e nloisture utilized by plants through transpiration processes is affected by t h e solar radiation received by the leaf surfaces, t h e vapour pressure gradient, air
temperature and moven~ent, the depth and spread of the root system, the stage of growth of the plant, and the moisture-retention characteristics of the soil.
The soil moisture retained by various types of soils, but available for plant utilization, depends upon the textlire of the soil: a fine sand may store 0.5 in. of water per foot depth of soil, a silty clay may store 3 in. or illore of water per foot depth of soil, and a clay may store as much as 4.3 in. per foot. In 1948, Thornthwaite presented a formula for calculati~lg potential evapo- transpiration for a grass-covered area from ineasured values of nleail tempera- ture and length of day. The Thornthwaite eq~iatioil and a full description of the calculation technique for Winnipeg is included in a paper now in preparation (IIamilton). Briefly, the procedure ~ised in developing a cumu- lative record of theoretical soil moisture change for Winnipeg, over the period oi years 1930 to 19G3, was as follows.
Using the mean daily teillperature and daily precipitation from ineteorological reports, the moisture-change condition ior each d a y was calculated, in sing Thornth~vaite's forinula. When potential evapo-transpiration exceeded the precipitation for the day, the difference represented a theoretical increase in the "soil moisture depletion." Conversely, when the precipitation for the day exceeded the potential evapo-transpiratioi~, the "soil moixture depletion" decreased. By assessing the probable run-off, a cumulative soil inoisture depletioil representing long-term climatic cycles was developed. T h e datuin chosen for these calc~ilations was 11Iay 1950, because of the general flooding of the area which occurred a t that time. The cuinulative plots of theoretical soil m o i s t ~ ~ r e depletion are included in Figures 4 and 7.
T h e type and degree of development of the secolldary or macro-structure of
a soil may greatly infl~ience its volume-change characteristics. Soil inorpholo- gists have recognized various natural agencies responsible for changing soils from coherent inasses of single-grain structure into friable inasses of aggregated paclcets of soil particles. Some of the ilat~iral agencies that may cause or aid such aggregation are root growth and decay, nlicrobial activity, and swelling and shrinking due to wetting and drying or freezing and thawing. Jennings (1963) has dealt with the phenomena of "micro-shattering"" and speculated on possible volunle changes which inay occur when a soil with such a secoildary structure is soalced after having previouslji been dried to a n unsaturated condition. In his words, "It is possible that soalcing of the soil under load will result first in a rapid reduction in volume due to reinoval of intergranular 'bonds', and then to a slow increase in volume due to particles taking ~ i p water."
A detailed discussion follows of the results of vertical ground movement studies in two undisturbed soil profiles in Winnipeg, as coinpared with ineasured and theoretical soil inoisture changes. The results of silnilar studies i11 Regina, Eston, and Tisdale are not yet conlplete enough to allow detailed discussion, but general observations to date on the inag~litude and depth of rnovement related to measured soil inoist~ire changes will be given.
(a) Test Plot at the University of Manitoba
Figure 4 presents the results of vertical ground movernent measLire~nents a t the University of Mailitoba test plot from October, 1951, to September, 1957, a s well as the cumulative plot of calculatecl soil ~lloisture depletion froin the 1950 datum. T h e dashed line represents a theoretical soil moisture depletion datum established for conditions a t the time of the installation of the gauges in October, 1951.
Apart fro111 the sullllller of 1952, a generally wetter-than-average trend was experienced during the period October, 1951, to July, 1958, as indicated by tlze curves for soil moisture depletion. During the winters of this period, a general vertical upward movement of gauges was measured to depths greater than 7 ft. The general relationship between change in soil moisture depletion and vertical movenzent seems to be good. The apparent anomaly in this relationship during the summer of 1952 is explained by Baracos ancl AIarantz (1952) as tlze result of reduced evapo-transpiration losses a t tlze site due to the area's having been covered by a wooden box.
During the severely dry summer and fall of 1955, s11rinl;age was measured t o depths of 7 ft. For the short period of ~neasure~llents in 1957 there appears t o be good general correlation of ground movement and calculatecl soil nzoisture depletion.
March April Moy June July August Sept. Oct. Nov.
FIGURE 5. Plots of calculated soil moisture depletion and water table depth for summer of 1955, University of Manitoba test plot
Figures 5 and G show the measured depth of "water table" from July 8 through November 1955, and June through October 1957, and the calculated theoretical soil moisture depletion. During 1935, an encouragingly good relationship was indicated for change in depth of the "water table" with calculated soil moisture depletion. After the piezometer had reached equili- briunz with in situ conditions, a calculated soil moisture depletion increasing froin 9 in. t o 17.5 in. of water seemed to correlate with n l e a s ~ ~ r e d change in depth of the "water table" of fro111 2.5 ft. to alnlost 5 ft.
March April May June July August Sept. Oct. Nov.
-
04
12
X
FIGURE G . Plots of calculated soil moisture depiction and \\rater table depth for sumtner of 1957, University of Manitoba test plot
I
Within this depth range, a change in calculated soil ~lloisti~re depletion of
4 in. of water seemed to be equivalent to a "water table" lowering of 1 ft. The response interval from rainfall to measurement of change in "water table" was between 6 and 7 days for this arrangement. In view of the very low permeability, determined on moulded samples of these soils, this rapid response time is considered to be indicative of the importance of a highly developed secondary structure in relation to nloisture penetration and storage.
The correlation between soil moisture depletion and change in water table depth is not nearly so good for 1957. I t is thought that, in seasons cl~aracterizecl by frequent rainfall, closure of fissures and joints in the soil's macro-structure may appreciably affect seepage of ~lloisture into the subsoils, with Inore moisture being held in storage near the surface. In this case, increased evapora- tion and run-off from the surface and shallower root penetration of annual plants may affezt the relationsl~ip between theoretical soil moisture depletion and change in water table elevations.
Calculated freezing indices, depth of snow cover on the ground, and depth of penetration of t i e 32' F isotherm are als-, presented in Figure 4. Exa~llination of these plots reveals interesting relationsl~ips. The depth of frost penetration in an undisturbed profile seems to be greatly affected by the soil inoisture condition prevailing before freeze-up and by the depth of snow cover. I t appears that the effect of the fornler may be equally inlportant as that of the latter for undisturbed Winnipeg soil profiles. For instance, during t h e winter of 1952-53, following a dry summer resulting in a calculated soil moisture depletion of 21 in. of water, frost penetration reached a maximurn depth of 6 ft. with a maximum freezing index of 2,500 frost degree days. During the winter of 1953-54, having s u b s t a ~ ~ t i a l l y the same calculated freezing index and depth of snow cover, frost penetration reached only 4.5 ft. In the winter of 1954-55, following another wet ),ear resulting i l l a further reduction in soil
- 16
WATER TABLE
24 DEPTH
moisture depletion to 10 in. to water a t freeze-up, frost penetration reached the 2-ft. depth early in January, and remained a t this depth until early April. During the same period, frost heaving in excess of 0.75 in. was measured a t a depth of 1 ft. The conlbination of availability of moisture, as indicated by the low soil moisture depletion, and slow steady heat extraction, as indicated by the freezing index plot and the existence of a continuous snow cover during the period, was conducive to frost heaving a t shallow depths. T h e value of a n abnornlally deep, undisturbed snow cover, accu~nulating early and remain- ing throughout the winter, in reducing frost penetration is well illustrated by the records for the winter of 1955-56.
Hutcheon (1958) has reported that "frost-induced pressure gradients within the liquid phase will not cause any appreciable flow of water when soil moisture conditions are drier than those characteristic of moisture tensions in the vicinity of one atmosphere." Under such conditions, the rate of frost penetration will be governed by the thermal characteristics of the moisture and solids in situ with no additional latent heat being made available due to inigration of moisture to the freezing plane, unless ~lloisture flow in thevapour phase becomes significant. Whether frost heaving due to ice lens growth will occur a t shallow depths will depend in large measure on the soil rnoisture tension prevailing a t the time of freeze-up.
The usefulness of the soil moisture depletion calculation in predicting the depth of potential frost heaving or shrinkage, as the case may be, for various winters is also suggested. In the winters of 1951-52 and 1952-53, a vertical shrinkage of approxin~ately 0.75 in. was measured a t the 1 ft. depth and 0.3 in. a t the 3 ft. depth, during the period in which ground temperatures were well below freezing. During both of these periods the depth of the "water table" suggested by soil moisture depletion calculations was more than 4 ft. Con- versely, during the winter of 1954-55, following a soil ~ n o i s t ~ l r e depletion of 10 in. a t freeze-up and an implied depth of "water table" of a little nlore than 2 ft. a frost heave a t the 1 ft. depth of 0.75 in. was measured. I t appears that the soil moisture depletion calculation for Winnipeg is useful in a n under- standing of the depth and direction of vertical ground n~ovements in un- disturbed profiles during winter months.
The maximum amplitudes of vertical ground movements measured a t the University test plot are shown in Table I.
TABLE I
Maximum Groiund Movements a t the University Test Plot Maximum Ground
Depth Movement, in. Retnarlcs
I ft. 2.8 Made u p of a maximi~m shrinkage of 0.7 in. and a maximi~m heave of 2.1 in. relative to the October 1951 datum.
3 ft. 0 . 8 Made u p of a ~nnaximrl~n shrinkage of 0.3 in. and a maximum heave of 0.5 in.
5 ft. 0 . 4 Maliimi~m shrinlcage of 0.1 in., maximum heave of 0.3 in.
7 ft. 0 . 3 Made up almost entirely by a rnaximiun heave of 0.3 in.
( b ) Test Plot at Elmwood, Winnipeg
The results of vertical ground movement studies a t the Elmwood test plot in Winnipeg are presented in Figure 7, including the cumulative plot of soil moisture depletion from the 1950 datum. A dashed line represents the d a t u m shifted for gauge installations in July, 1959.
The years 1960 and 1961 were very much drier than the average for Winnipeg, and the general trend in vertical ground movements was shrinkage. Severe shrinkage developed a t depths greater than 8 ft. when the calculated soil moisture depletion exceeded 32 in. of water in September, 1961.
The amplitude of the movement of the ground surface after freeze-up during the winter of 1961-62 is considered to be of special interest. During the below- freezing weather, a shrinkage of 0.8 in. a t the ground surface and 0.3 in. a t the 2 ft. depth was measured. These measurements confirmed earlier results indicating appreciable vertical shrinkage near the ground surface, apparently related t o below-freezing temperatures in unsaturated soil.
The maximum amplitude of vertical ground movelnents measured a t t h e Elmwood test plot to date are shown in Table 11. As a t the University
TABLE I1
Maximum Ground Movements a t t h e Elmwood Test Plot Maximum Ground
Depth Movement, in. Remarks
Ground surface 2 . 9 Maximum shrinkage
(Nov. 1961 t o Feb. 1963) maximum heave
2 ft. 1 . 8 Maximum shrinkage maximum heave 4 ft. 1 . 3 Maximum shrinkage maximum heave 6 ft. 0 . 8 Maximum shrinkage maximum heave 8 ft. 0 . 3 Maximum shrinkage masimi~rn heave 0 . 8 in. 2 . 1 in. 1 . 3 in. 0 . 5 in. 0 . 8 in. 0 . 5 in. 0 . 5 in. 0 . 3 in. 0 . 1 in. 0 . 2 in.
plot, the theoretical soil moisture depletion calculation indicated the depth t o which vertical ground movements would be expected; t h a t is, when curnu- lative soil moisture depletion reached 32 in., vertical ground movement became appreciable a t the 8 ft. depth.
( 6 ) Test Plot at Regina
Owing to a borderline sub-humid semi-arid climate, the soil in Regina is below saturation to depths of 15 ft. or more. Little work has been done o n lneteorological records t o try to find an empirical relationship between t h e potential evapo-transpiration and precipitation balance with actual soil moisture storage.
The r n a x i ~ n u ~ n amplitude of ground movements measured to date since September, 1960, are given in Table 111. In grass-covered, undisturbed areas in Regina, the maximum depth of annual soil moisture change due to preci- pitation and evaporation appears to be 8 ft.
5000 4000 A
-
r li 0 3000 15 8 "Y w 2000 101 e 3 P 0 1000 w w"w
O m 0 L3 0FREEZING INDEX
-
SNOW COVER ... FROST PENETRATION----
SOIL MOISTURE DEPLETION
b
CUMULATIVE FROM APRIL, 1950.
2.0 1.5
VERTICAL GROUND MOVEMENTS 1.0
E L M W O O D , WINNIPEG T E S T PLOT 0.5 0 -0.5 0.5 0 -0.5 - 1.0 0 -0.5 0.5 0 - 0.5 0.5 0 -0.5 0 -0.5 38
TABLE 111
Maximurn Ground Movements in Regina Test Plot Maximum Ground
Depth Movement, in. Remarks
1 It. 1 . 2 Maxirnunl shrinkage 1 . 2 It. maximum heave less than 0 . 1 in. 3 It. 0 . 3 Maximum shrinkage 0 . 2 in.
maximum heave less than 0 . 1 in.
5 ft. 0 . 1 Essentially all heave
10 It. 0 . 1 Essentially all shrinkage
15 ft. < 0 . 1 Shrinkage
( d ) Test Plot at Eston
As for Regina, the long history of soil moisture deficiency a t Eston h a s left the soil below saturation water content to great depths. Although only one full year of records had been obtained a t the time ofwriting, the amplitudes of movement a t various depths are listed in Table IV. A slight increase i n soil m o i s t ~ ~ r e content has talcen place since installation of these gauges i n June, 1961.
TABLE IV
Maximum Ground Movements a t the Eston Test Plot Maxi~num Ground
Depth Movement, in. Remarks
1 It. 1 . 1 Essentially all heaving
3 It. 0 . 4 Essentially all heaving
5 It. 0 . 2 Essentially all heaving
10 ft. 0 . 1 Essentially all heaving
Preliminary results of studies in Regina and Eston indicate that the long- term ecluilibriu~n moisture content in the top 10 ft. to 15 ft. of heavy clay is well below sat~lration water content. Precipitation is rarely effective i n penetrating more than G ft. to 8 It. in undisturbed soil profiles, even in long wet cycles. Thro~lghout these depths, evaporation and vegetation are effective in withdrawing most if not all of the readily "available" water in storage each season.
(e) Test Plot at Tisdale
i\iIovements t o date a t the Tisdale test plot have been very small; maximum movements of 0.4 in. a t the 1 ft. depth, and 0.2 in. a t the 5 f t . depth have been measured. Very little change in soil moisture conditions has been measured to date.
FICDI~E 7. Plots of calculated freezing index and soil moisture depletion, measured frost penetration, snow cover and vertical ground movements 1959-62, for the Elmwood, Winnipeg,
test plot
The modified soil moisture depletion calculation proposed in this paper has been successful in qualitatively estimating the depth t o which soil moistures change and lnoveinent take place in two grass-covered, undisturbed soil profiles in Winnipeg. If consideration is given to the influence of other types of vege- tation or surface cover and local drainage, it inay be possible to speculate qualitatively on the nature of movements that may result in undisturbed areas due to other conditions, such as growth or removal of trees or other vegetation, or covering the surface with a pavement or impermeable mem- brane. Building movements can be better understood if reference is made to the cun~ulative soil moisture depletion a t the time of construction and con- sideration is given to the long-term effect of the structure on soil moisture conditions.
T h e influence of soil moisture conditions on the depth of frost penetration is more easily seen when reference is made to soil moisture depletion. Appreci- able vertical shrinkage in unsaturated soil profiles a t temperatures well below
32" F takes place in winters following extended drought. A t the two test
plots in Winnipeg, the shrinltage following d r y seasons has been equal to or greater than the measured frost heave following wet seasons. T h e magnitude of these winter movements could well be of significance to engineering structures such as roadways, sidewalks, parking lots, and shallow foundations under unheated structures, especially where these adjoin more deeply founded structures such as manholes, bridge abutments and buildings, because differential movement may be experienced.
During winters following dry seasons, horizontal shrinkage resulting in severe vertical cracking of heavy clay profiles has been observed to take place after ground temperatures drop well below freezing. This cracking may allow deep and uneven penetration of water a t spring break-up and inay be detri- mental to highway subgrades and other structures on heavy clay. Infiltration through these cracks may reduce spring run-off appreciably, even though the soil may still be frozen. I t is suggested that air-void volume change illay be a contributing factor in this shrinkage. I t is also conceivable t h a t air-void volurne change may develop under buildings on unsaturated soils d u e t o newly imposed temperature conditions which raise the mean subsoil temperature. Further investigation of the effects of temperature change on the voluille of unsaturated clays is planned.
Since the vertical dimension changes of undisturbed soils i n situ have often been found to be less than would be predicted from a knowledge of t h e change in water content, it is suggested t h a t further study of the part played by macro-structure in volu~ne change is necessary. I t is possible that shrinltage of the smaller units (nuggets or crumbs) with lowering in moisture content, may not contribute to an increase of bulk dry density but cracks and fissures in both the vertical and horizontal direction may result in the crumb-like packets of clay particles acting like silt or sand grains. In this condition it is conceivable that abnorn~ally low d r y densities may result, similar in effect to the bulking of sand.
Contact moisture may hold the crumbs in this low state of dry density until immersion destroys the water menisci responsible for this bulking action and a densification takes place because of adjustments in the secondary structure. Occasionally, when a house foundation has undergone settlement due t o desiccation of the subsoils, watering has caused further settlement rather than the desired swelling. Compaction due to vibration or loading of a highly macro-structured, desiccated soil may result in greater subsequent heaving with soil moisture increase than might have been experienced if left in its loose, bulked state.
The. above considerations indicate the important influence on soil charac- teristics of type and degree of development of its secondary or macro-structure and negative pore water pressures. Studies now underway, or planned for the future, include n~easurenlents of soil moisture suction and the effects of secondary structure on the engineering characteristics of undisturbed clays.
The magnitude of seasonal movements in undisturbed soil profiles, due t o both thermal and desiccation shrinkage, frost-action, heaving, and swelling of clay, are of significance t o many types of engineering structures. Changes in the dynamic equilibrium of open field conditions due to changes in vegeta- tion, surface cover, micro-climate or drainage can magnify the amplitude of these movenlents and increase the damage potential.
iblovements a t the surface of undisturbed soil profiles having native grass covers only, due to swelling and shrinkage of clays, have been measured t o be in excess of 3 in. Under severe drought or highly moist conditions, move- ments of significance t o engineering structures have been measured a t depths exceeding 8 ft.
A soil moisture depletion calculation using meteorological data has been found t o be a useful tool in the understanding of soil moisture and volume changes a s affected by climate and vegetation.
The depth of frost penetration in undisturbed clay profiles is greatly affected by soil moisture conditions prevailing before freeze-up. T h e soil moisture depletion calculation is also useful in predicting whether frost heaving or shrinkage is t o be expected and in speculating on the possible depth and magnitude of these movetnents.
As the author has reported some of the results of a field investigation t h a t has been underway for Inore than ten years, and to which rnany students and staff of two universities and other organizations have made considerable contributions, he feels greatly indebted to many fellow workers, although space does not permit individual acknowledgment. I n particular, however, he wishes to express his appreciation to Professors A. Baracos and 0. Marantz, of the University of Manitoba, and C. A. Noble of the University of Saskatchewan; Messrs. F. G. Denson, Engineer of Waterworks, Sewerage and Street Maintenance, City of Winnipeg; B. J. Mickle- borough, Special Projects Engineer, Saskatchewan Department of Highways; and G. Raeburn of Eston, Saskatchewan, for continued co-operation and assistarlce in these studies over t h e years. The author also wishes to record his appreciation for direction and encouragement given by his colleagues in the Soil Mechanics Section and by the Director of the Division of Building Research, with whose approval this paper is published.
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