Influence of geology and geological processes on the geotechnical properties of a plastic clay

Engineering Geology, 22 (1985) 109--126

109

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

INFLUENCE OF GEOLOGY AND GEOLOGICAL PROCESSES ON THE GEOTECHNICAL PROPERTIES OF A PLASTIC CLAY

J. G R A H A M

and D.H. S H I E L D S

Department of CivilEngineering, University of Manitoba, Winnipeg, Mar~ R 3 T 2N2 (Canada) (Received M a y 3, 1984; accepted after revision December 19, 1984)

ABSTRACT Graham, J. and Shields, D.H., 1985. Influence of geology and geological processes on the geotechnical properties of a plastic clay. Eng. Geol., 22: 109--126. The complex properties of postglacial clay at Winnipeg, Canada have major impact on civilengineering construction in the city. Problems include house foundation movements, low stabilityof riverbanks, poor highway pavement performance, difficultexcavation, and a high incidence of watermain breaks. The research in this paper presents an understanding of the geologic factors that cause these problems, and h o w to ameliorate their effects in engineering design. The geotechnical properties of the clay were measured from carefully sampled, 76 m m diameter triaxialspecimens. The clay is nonhomogeneous, anisotropic, active, overconsolidated and fissured. These properties are related to the depositional and post-depositional geology of the area. The depositional geology of the extensive Lake Agassiz clays is straightforward and well known. However, it does not fully explain the measured geotechnical properties. Attention is then directed towards post-depositional geological processes such as regional groundwater hydrology, cementation, aging, freeze-thaw and desiccation.Consideration of effective stressstrengths, quasi-elasticanisotropy, the variation of overconsolidation ratio with depth, yielding and porewater pressure generation, suggests strongly that the clay is cemented by carbonates and sulphates from upwards movement of salt-richgroundwater. Freeze--thaw disturbance causes marked changes in behaviour, especially at low stresses. INTRODUCTION

Geotechnical properties of glaciolacustrine clays arise from both depositional and post-depositional geologic environments. This paper examines these relationships for Lake Agassiz clay at Winnipeg. The stress-strain-time behavior of clays result from (1) their mineral composition and grain-size distribution, (2) the chemical and environmental conditions when they were deposited, and (3) the various processes experienced subsequent to deposition. These processes include diagenesls, changes in porewater chemistry and groundwater levels, and the influence of climatic effects. A considerable amount of reliable data is available concerning the properties of the plastic glacial Lake Agassiz clay that underlies much of 0013-7952/85/$03.30

© 1985 Elsevier Science Publishers B.V.

110

Winnipeg (for example Graham et al., 1983a, b; Graham and Houlsby, 1983). The clay is nonhomogeneous, anisotropic, active, overconsolidated, fissured and swelling. All of these properties have important engineering implications. The purpose of this paper is to provide an integrated overview that combines (1) an understanding of the conditions under which the clay was deposited, and (2) the various geological and climatic processes which it has since experienced, (3) its geotechnical properties and (4) the extensive experience of its behaviour in many different types of engineering construction. GEOLOGY

Winnipeg is in the centre of Canada at the confluence of the Red and the Assiniboine rivers, 100 km north of the U.S.--Canada border. The stratigraphy divides conveniently into four units: (1) Paleozoic dolomitic carbonate bedrock; (2) dense basal tills overlain locally by loose waterlaid till; (3) plastic clay from proglacial Lake Agassiz; and (4) an upper complex zone of mixed sands, silts, clays, organic soils and urban fill. The geology of the bedrock and till units was described recently by Baracos et al. (1983) and will be reviewed here only because it influences the properties of the clay layer. Ordovician and Silurian dolomitic carbonates dip at low angles of about 0.3% to the west, and lie unconformably on Precambrian basement rocks. Successive periods of uplift and erosion produced erosion surfaces containing numerous cavities, open joints, sinkholes and channels in the bedrock. Glaciation caused further joint openings and fragmentation in the upper layers of the carbonates. Tills were deposited during a series of advances and retreats of the Wisconsin ice sheet (Teller and Fenton, 1980). They generally contain hairline fractures and irregular seams of silt, sand and gravel that contribute significantly to their mass permeability. The lower tills are typically well graded and dense to very dense, with moisture contents from as low as 4--6% to perhaps 15% as the overlying clay is approached. The clay content and plasticity are low. The youngest tills contain fewer cobbles and boulders, and are irregularly loose or soft. They are believed to have been deposited through water during the earliest stages of Lake Agassiz. Sporadic deposits of glaciofiuvial sands and gravels occur below, within and above the tills. These originate from earlier tills that were reworked by advances of the ice. The sands and gravels form occluded aquifers that regularly cause problems during excavation. The hydrogeology was described by Render (1983). Industrial groundwater usage and altered infiltration rates have caused a 20 m deep drawdown cone under the city centre. Artesian conditions that are controlled by occurrence of the compact till are still frequently experienced elsewhere. The groundwater is recharged from infiltration; from localized fluviogiacial deposits, particularly to the northeast of the city; and more importantly from uplands to the west through a series of aquifers in the carbonate sequence. High concentrations of sulphates and chlorides are encountered locally in the groundwater.

111 The clay that provides the principal topic for this paper was deposited in proglacial Lake Agassiz between about 11,500 and 8000 years ago (Quigley, 1980). At m a x i m u m extent, Lake Agassiz was the largest lake in N. America (200,000 k m 2) and its deposits at various times cover about 520 000 km 2. The average thickness o f the clay at Winnipeg is 10 to 12 m, with a range from zero to 20 m. The clay-size fraction is often as m u c h as 75--80%, while the clay minerals are mostly interlayered montmorillonite--iUites (Quigley, 1968; Baracos, 1977). Average geotechnical classification data are summarized in Table I. The clay m a y be described as a highly plastic (CH), compressible, swelling clay o f low sensitivity, often fissured visibly to depths of about 6 m. At greater depths, the clay contains numerous angular pebbles, sand, and pebblesized white-grey inclusions which probably fell into the lake b o t t o m sediments as ice-rafted frozen lumps of silt-size dolomite. Upper layers of the clay are usually brown in colour. They display evident nonhomogeneity and anisotropy in the form o f fissures and laminations with veinous, crosscutting streaks (Baracos, 1977). Lower levels are grey. They are less fissured and less obviously laminated than the brown clay, but contain m a n y more inclusions. The colour change f r o m brown to grey is not obviously associated with changes of mineralogy, clay content or plasticity. Baracos (1977) showed that in order o f decreasing occurrence, the clay minerals are smectite, illite, and kaolinite with some mica. The n o n , l a y minerals are quartz, dolomite and calcite, with some gypsum and feldspar. These constituents are compatible with Precambrian crystalline rocks to the north and east over which the ice advanced southwards; and with erosion and hydraulic transport of Paloeozoic carbonates and Cretaceous shales to the south and west. Discordant veins in the upper levels consist of gypsum, magnesite and dolomite. TABLEI Geotechnical properties of Lake Agassiz clay at Winnipeg Geotechnicai property

Unit weight (kN/ms) Moisture content (%) Liquid limit (%) Plasticity index (%) Clay size fraction (%) Sensitivity (fall cone test) Overconsolidation ratio Compreuion index Ce Swelling pressure (kPa) Undrained shear strength (kPa) Angle of shearing resistance: peak (degrees) residual (degrees) Modulus/undrained strength, Eso/s u (triaxiai)

Range lower

upper

15.7 37 65 40 55 2 1 0.6 0 50

18.1 60 10 75 90 4 5 1.2 75 125

17 8 230

23 12 860

112

They are thought to result from salt-rich groundwater evaporating in the zone of desiccation. BASIC

GEOTECHNICAL

PROPERTIES

As is c o m m o n in glacio-lacustrine clays, the plasticity decreases with increasing depth from about Ip = 60% at 3 m depth to 45% at 12 m depth. It is clear from the geomorphology of the area that no significant erosional off-loading has occurred since deposition of the clay. Strengths might therefore be expected to increase with overburden pressure, that is to say, with depth. In fact, the observed undrained strength distribution is generally quite different. The strength decreases markedly below a desiccated crust to about 80 kPa at 4 m depth, and then continues to decrease (but more slowly) to about 40 kPa towards the b o t t o m o f the deposit. Careful tests show (Fig.l) that the clay has been affected by processes whose effects decrease with increasing depth, and which cause the clay to behave as if it had been apparently overconsolidated by geological loading and offloading. {This is sometimes called "apparent overconsolidation" (Bjerrum, 1967), "quasi-overconsolidation" or "pseudo-overconsolidation". The distinction in terminology is now frequently ignored). It is important to note firstly in Fig.1 that the clay yields in one-dimensional compression at axial stresses ' (see the Notation) higher than the vertical overburden pressure O'vo and a vc then, secondly, to consider the processes that have produced this behavior. Later sections examine the various geological and climatic effects that have caused overconsolidation in the Winnipeg clay. 0

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113 NOTATION Cohesion w.r.t, effective stress Compression index from slope of log o v vs V E h, E v Compression moduli in horizontal, vertical directions K , G , J Bulk, shear and anisotropic moduli Slope of failure envelope in q, p'-space = 6 sin ¢'/(3-sin ~) M Overconsolidation ratio o~c/a~o OCR p,p' Mean principal (effective) stress (ax + 2as)/3, (a'l + 2a'3)/3. Deviator stress al -- % q P Effective angle of shearing resistance, normally consolidated ~b', ~bnc t Vertical effective (overburden) pressure O'rv~av 0 Preconsolidation pressure avc Effective laboratory axial consolidation pressure alc CF

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MINERALOGY AND ENGINEERING PROPERTIES

The content of smectite in the Winnipeg clay has important engineering significance. Smectites such as montmorillonite for example, have a very low coefficient of interparticle friction, so that the residual strength of Winnipeg clay is low. The smectites "smear" into a continuous, low-friction film along landslide surfaces, even when the failure surface is steeply inclined to the bedding planes. Representative values of the Mohr--Coulomb parameters for the residual strength of Winnipeg clay are about c' = 4 kPa and ¢' = 8 ° (Baracos and Graham, 1981). The low residual strength of Winnipeg clay has made it difficult to stabilize riverbank slopes once they start to move and fail. New construction often reactivates former slides. Slope flattening, toe loading and crest unloading usually have little effect in arresting slope movements. Improved drainage measures which lead to worthwhile gains in effective stresses on the failure plane do have a useful, measurable effect. However, this improved drainage is only possible where topographic conditions are suitable. One difficulty has been to ensure the efficient working of drains throughout the cold Winnipeg winters, and particularly at the time of spring melting when safety factors are often most critical. The presence of non-clay minerals such as quartz and dolomite produces rather higher effective stress properties in previously undisturbed clay in the normally consolidated range of stresses. Reported values of ~'nc range from 17 ° to 24 °. One reason why there is apparently no unique value of ¢'n¢ for the relatively uniform Winnipeg clay is that the normally consolidated Mohr--Coulomb strength envelope for smectites is curved (Mesri and Olsen, 1970; Trainor, 1982). Graham et al. (1983a) report a critical state strength M = 0.67 (¢n¢= 17.5 °) for the stress range ale 500--900 kPa (Fig.2). The stress axes in this figure have been normalized by the vertical preconsolidation pressure a'vc. This pressure may be taken as a convenient measure of the processes that produced the properties of the clay. The section A B in Fig.2 represents the "frictional" strength of the fissured macrostructure of the clay, BC represents the overconsolidated or cemented strength of the micro'

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structure, and C represents normally consolidated strength. The " + " symbols represent the undrained shearing resistance after large strains. Another negative effect associated with the clay's mineralogy is its h i g h shrinkage and swelling potential. The montmorillonite in Winnipeg clay is in the form of small irregular flakey particles which are very thin and have large specific surface areas. Large, reversible volume changes are caused by wet and dry climatic cycles and the moisture demands of nearby vegetation such as trees. These volume changes produce major problems with house foundations and road pavements in the region (Hamilton, 1969; Domaschuk, 1983). The city experienced more than 2200 watermain breaks in 1983 due to a combination of corrosion of the pipes and soil movements. The current frequency of breaks is 1.1 break/km of total system per year, and the annual budget for repairs and replacements is $14 million (Canadian). Excavations for footings that are allowed to stand open for some time may subsequently experience dramatic foundation movements. If the b o t t o m of the excavation is dried by h o t summer sun, subsequent repletion of the moisture can cause the footing to heave 10 cm or more due to swelling pressures as high as 75 kPa. Conversely, if water is allowed to pond in the excavation, the softened clay will compress under the weight of the structure. Settlements can be appreciable. Fig.3 shows these effects in laboratory oedometer tests (Graham and Au, 1985). There are marked differences in compressibility and apparent preconsolidation pressure between samples (A) whose swelling was inhibited by stressing them initially to about their swelling pressure, (B) which were permitted to swell freely with access to water at low stresses, and (C) which experienced several cycles of freezing and thawing before being tested. Fig.4 shows that allowing the Winnipeg clay to swell and soften reduces its

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116 strength markedly. Here, the strengths (A) and yield stresses (B) are from 76 mm dia triaxial samples which were allowed to swell freely to equilibrium before testing commenced. The undisturbed envelope (C) from Fig.2 is included for comparison. SEDIMENTARY ORIGIN LEADINGTO TRANSVERSELY ISOTROPIC PROPERTIES Given the changeable size of proglacial Lake Agassiz as the ice retreated northwards, and the accomparlying variations in river flows and surface runoff, there would have b e e n a complex depositions] pattern as the clay was laid down. At times only finer clay particles would reach a certain area -- due perhaps to high lake levels and long distances to river mouths, or due to low winter flows in the rivers. At other times, say during spring runoff or during low lake levels, coarser particles such as silts could reach the area in question. The brown upper clay at Winnipeg is visibly laminated with alternating clay and silt bands. Even where it has no visible laminations, electron microscopy has shown (Baracos, 1977) that particles in clay-rich layers are preferentially oriented in the horizontal direction. Silt-rich layers contain closely packed, uniform sub-rounded particles. The clay has generally not been reworked so it is reasonable that the preferred depositional orientations remain. The clay therefore exhibits transversely isotropic (also known as cross-anisotropic) physical properties which are constant in any horizontal direction, but the vertical properties are different. An awareness that a clay is anisotropic is important for the design of tunnels and tunnel linings, prediction of the stability of highway embankments, estimation of the likely settlement of foundations, and numerous other engineering projects. Strength anisotropy in the Winnipeg clay has been examined in terms of total stresses by Loh and Holt (1974) and in effective stress terms by Freeman and Sutherland (1974). In both cases, cylindrical samples of clay were trimmed with their axes at various inclinations to the horizontal. The elastic anisotropy of clays can be tested in a more fundamental way, if samples are subjected to various patterns of stresses (stress paths), with the axis of vertical symmetry preserved (Graham and Houlsby, 1983). Yielding of clays has traditionally been examined in log o~ vs. specific volume V curves such as those shown in Fig.5a. In recent years, close attention to sampling, trimming and testing procedures has led to a realization that clays exhibit much more linear stress--strain behaviour at pre-yield stresses than was previously realized (for example Mitchell, 1970; Crooks and Graham, 1972). This is especially true if elasticity of the soil is examined in terms of its tendency to change volume and to change shape (Fig.5b). This justifies the increased attention that has recently been paid to elastic soil properties, and to computational methods such as the modified Camclay model of soil behavior (Wroth and Houlsby, 1980). Elastic anisotropy may be expressed (Graham and Houlsby, 1983) by the relationship:

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where K and G are the bulk and shear moduli respectively, and J is a parameter which expresses the cross dependence o f the shear strain on the mean effective pressure p', and of the volumetric strain on the shear stress q. Values of the normalized moduh," K/ovc ' = 14.5, G/a'vc = 8.5 and J/a've = --5.5 imply that the ratio of horizontal to vertical compression moduli En/E v = 1.8. Lower stiffness in the vertical direction results from the relative compressibilities of clay-rich and silt-rich layers. Anisotropy o f the clay structure can also be expressed by the magnitudes o f the porewater pressures generated during shear. In lightly overconsolidated clays these increase initially b u t then begin to decrease when the samples start to dilate and soften. If a clay is isotropically elastic, the change in pore pressure AU equals the change in mean principal stress Ap, (with &~ = 0). In the tests on Winnipeg clay the measured values of Au were consistently greater than Ap during the linear (elastic) portion of the tests (Graham and Houlsby, 1983). These results support the conclusion in the previous paragraph that stiffness in the horizontal direction is greater than in the vertical direction. DEPOSITIONAL ENVIRONMENT

Lake Agassiz was formed when the Wisconsin ice sheet blocked the natural drainage o f the northern Great Plains region to the Arctic Ocean through

118

Hudson's Bay. Rivers continued to bring in water from the south and west. This combined with melt-water from the ice sheet and local rainfall to form proglacial Lake Agassiz. Most of these waters were undoubtedly "fresh", although the lake probably received some inflow from regional, salt-rich groundwater movements (Teller, 1976). In a cool, wet period foUowingthe ice retreat, significant concentration of salts in the upper soil layers was unlikely. It is also unlikely that any significant salt concentration could derive from an influx of seawater from Hudsons' Bay in the North. From current information (Teller, 1976; Fenton et al., 1983) it seems that Lake Agassiz was "fresh" rather than "salt" in the Winnipeg region, although the cation concentration was probably sufficient to permit the clay particles to be deposited in a fairly loose flocculated state. Evidence of this is seen in the high porewater pressures that are observed in triaxial compression tests compared with values from other clays (Graham and Au, 1985). Despite the high values ofA~ = Au~/q~ in Fig.6, the sensitivity of the clay is low, only 2 to 4 from fall cone tests. Lake Agassiz clay is therefore quite different in character from the highly sensitive marine clays in Eastern Canada and Scandinavia. POST D E P O S I T I O N A L G E O H Y D R O L O G Y

In other clays, oedometer results like those shown in Fig.5a have been attributed to cementation. From the preceding discussion it is unlikely that this t o o k place in Winnipeg clay during deposition. It is more likely to have been caused by post-depositional changes in pore fluid chemistry (Bjerrum, 1967; Torrance, 1975). This implies that the high post-yield compressibility

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Fig.6. I n f l u e n c e o f o v e r c o n s o l i d a t i o n ratio o n p o r e w a t e r pressure parameter A f : A = "undisturbed"; B - f u l l y s o f t e n e d ; C - f r e e z e - - t h a w c y c l e d ; D = Weald clay (Henkel, 1 9 5 6 ) , E = Belfast c l a y ( C r o o k s and Graham, 1 9 7 2 ) .

119 in Fig.5 (C e = 1.08, range 0.6--1.2) need not depend only on its environment at the time of deposition. A strong case can be made at Winnipeg for postdepositional porewater alteration. After the ice finished retreating northwards towards Hudson's Bay and Lake Agassiz had drained to its present extent (Lake Winnipeg and Lake Manitoba), conditions were right for an upward flow of groundwater into the clay from the underlying carbonate rocks. In addition to the regional supply of groundwater from the Manitoba escarpment to the west, there is a major esker-kame complex about 20 km northeast of Winnipeg which also connects directly to the carbonate bedrock. Both sources still provide artesian groundwater pressures in outlying areas of the city and have caused severe construction problems in deep excavations. Records of the high frequency of former landslides in Winnipeg riverbanks (Baracos and Graham, 1981) suggest strongly that high, artesian groundwater pressures that are still measured outside the center of the city caused upward seepage of salt-laden water through the clay. This resulted in the introduction and deposition of sulphates and carbonates into the clay. The sulphates attack normal Portland cement if it is used in below-ground construction in the city, and sulphate resistant cement must he used instead. Deposition of the salts was irregular since the water flow was controlled by local bedrock joint patterns, and by the thickness and permeability of the intervening till. Larger amounts of salt in the clay close to the ground surface (indicated by the veins of gypsum, magnesite and dolomite mentioned earlier) are probably due to high summer evaporation rates lowering the groundwater table in the clay on a seasonal basis. Upwards groundwater migration would also be aided in winter by pore fluid gradients at the freezing front in the soft. Freezing and desiccation both lead to the fissuring that is so evident in the brown near-surface clays. In turn, the fissures increase the mass permeability of the clay and provide the openings in which the salts were deposited. Whether or not these salts are responsible for causing cementation in the Winnipeg (and so causing its particularly brittle, compressible behaviour) is still a matter for conjecture. Electron microscopy has not permitted close examination of the small contact areas where cementation bonds might form. Some evidence of secondary, post-depositional structures consisting of pyrites have been inferred (W.M. Last, pers. comm). Recrystallization of the carbonates could be reasonably expected (Townsend, 1965). Additional indirect evidence of cementation comes from the clay's strength behaviour (Fig.2) and the porepressure response of the clay during shear (Fig.6). Cementation may also contribute to the preconsolidation pressure profile shown in Fig.1 which is discussed in more detail in the next section. Evidence of overconsolidation, and hence of cementation, is shown by the clearly defined normalized yield envelope reported by Graham et al. (1983a) and shown again in Fig.7. Stress changes inside the yield envelope produce stiff, linear, quasi-elastic behaviour. Stress changes that go beyond the current yield envelope produce compressible, non-linear, time-dependent, plastic

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behaviour that has been observed for example below several oil tanks and grain elevators in the city (Domaschuk and Valliapan, 1975). Yielding states to the right o f and below the critical state point B in Fig.7 produce plastic strain hardening; yielding states to the left o f B correspond to the overconsolidated strength, and are strain softening. POST DEPOSITIONAL CLIMATE, CLAY COMPRESSIBILITY AND STRENGTH

The wanning which led to the melting and retreat o f the Wisconsin ice sheet around ten thousand years ago has persisted to the present time. The climate was colder than at present just after the ice retreated, and warmer on other occasions, notably about 4000 years ago, and again about 1000 years ago (Prest, 1961). Winnipeg currently has a mid-continental extreme climate in which temperatures o f - - 3 5°(3 and + 35°C are regularly experienced. An average freezing index of 1900°C days gives about 1.8 m of frost penetration. Rainfall and evaporation rates are such that there are small net moisture depletions over periods of, perhaps, 5 to 7 years, followed by several years of net moisture gain (Hamilton, 1969). Groundwater levels in the clay range from the ground surface after the spring melt to perhaps 6 m depth at the end o f dry summers. Extensive, wide (10 ram) cracks are caused by combinations of frost action and desiccation.

121

As in other clay deposits, the higher near-surface strengths and the high apparent preconsolidation pressures close to the ground surface in Fig.1 can be attributed to weathering processes. At various times in the past, frost penetrations and the depths of summer desiccation were greater than at present (Last and Teller, 1983). However, there is no evidence that the Winnipeg clay has ever been frozen or desiccated to itsfulldepth. The effects of physical weathering are evident only in the upper portion of the deposit -the clay is brown in colour and heavily fissured in depths ranging from 5to9m. The frequent drying out and subsequent re-wetting of the upper part of the clay, and the effects of frost must have led to a marked change in its compressibility and strength. It is therefore reasonable to expect that the physical properties of the brown, fissured clay will be different from the grey clay. Recent studies by Graham and A u (1985) show that the effects of desiccation and of freeze-thaw are in some ways contradictory (Figs.3, 4). The influence of climate on the clay is not as straightforward as might appear on firstconsideration. The thermodynamics of the freezing process draws water towards a region which isjust on the point of freezing (MacKay, 1974), and causes decreased moisture contents at greater depths. The water concentrates into a reticulate series of closely spaced planes of ice that surround nuggets of frozen clay having reduced moisture content. The process is not reversible, and a fissured structure remains after thawing. Strongly developed fissure structures with spacings of 5--10 mm are frequently seen in shallow excavations in Winnipeg. The process significantly disturbs and alters the arrangement of the particles in the original clay, and therefore affects the clay's stress--strain response. This altered response has recently been examined in a series of tests on samples of intact undisturbed grey clay from depths of 8.5 m and 11.5 m (Graham and Au, 1985). The samples were carefully prepared for triaxial testing and then subjected to five freeze--thaw cycles. For this study, the samples were placed in a freezing chest at --5°C or --25°C and allowed to freeze from all sides without separate access to a supply of water. They were then thawed at room temperature. The duration of the freeze--thaw cycling was one to four days. It is understood that this does not fully represent the conditions of freezing in the field. The process produced a heavily modified clay structure with fissure spacings of 2 to 4 ram. During the freeze--thaw cycling, the samples shrank in an axial direction by 3 to 6%. The influence of freeze--thaw on the stress--strain behaviour of the clay is shown in Figs.3, 6, 8 and 9. Although the moisture content decreases, the process appears not to build overconsolidated behaviour into the clay. Instead, because of the fissures, it gives the appearance of being considerably disturbed and its structure destroyed. It shows high pore-pressure response to loading (Fig.6) and only a "virgin" compression curve results in oedometer tests (Fig.3). The compression index of fissured clay is larger than that of undisturbed clay at low stresses, and smaller at higher stresses. Freeze--thaw effects also alter the strength and yielding of the clay, especially at low stresses (Fig.9). Marked

122 0.3 I

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Fig.8. Stress--strain-porewaterpressure curves from anisotropicallyconsolidated samples (Ovc 4 0 0 kPa, ozc 2 Ovo/3). A = "undisturbed"; B = softened; C = freeze--thaw cycled.

reductions in strength occur at the stress levels encountered both at the toe and at the crest of riverbank slopes. Natural freeze--thaw processes have therefore important implications for slope stability analyses in Winnipeg. Since geological evidence shows that the present ground level at Winnipeg is essentially as high as it has ever been in postglacial times, the high preconsolidation pressures in the upper levels of the clay (Fig.l) must be due to causes other than erosional off-loading. As explained earlier, these "other causes" cannot include the effects of freeze--thaw directly, although moisture migrations towards the freezing front must clearly cause decreased water contents and higher stiffnesses at lower elevations. Cementation, desiccation and aging remain as possible causes for the apparent overconsolidation. Cementation and desiccation have been discussed previously. If aging alone were responsible for the pseudo-preconsolidation pressures, the lower levels of the clay (the grey clay) would be equally affected as the upper brown clay, and the overconsolidation ratio would be constant with depth. In this context "aging" is the process described by Bjerrum (1967) whereby slow, delayed compression takes soft postglacial clay to specific

123

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volumes which are lower than those measured in short-duration consolidation tests.Fig.1 shows that the preconsolidation pressure reported at the lowest levels of the grey clay is nearly coincident with the existingvertical effectiveoverburden pressure, so aging alone cannot have contributed significantly to overconsolidation. In addition (Fig.5a),the post-yield states of laboratory samples (,4)lie below the locus of the preconsolidation states (B) obtained directly from samples of natural clay from different depths (Graham et ai.,1983a). That is,the post-yield compression line does not correspond with the line defined by the yielding of natural samples consolidated and possibly cemented in situ at differentmoisture contents. Both of these p h e n o m e n a - the fact that the overconsolidation ratio is not constant and the position of the post~yield states-- lead to the conclusion that aging is not the principal cause of the apparent preconsolidation pressures measured in the clay, although it clearly has some effect (Graham et al.,1983a). The fissured,overconsolidated nature of the clay produces complicated strength properties (Fig.2).W h e n water is available,as it would be in drained triaxiaitests,or if lateralstrainingis permitted, say in trench excavations, then the clay tends to collapse prematurely unless lateralsupport is provided. That is,the strength envelope A B at low stressesin Fig.3 liesclose to (but below) the zero confining stressline (o~ = 0) separating compressive and tensilestresses.

124 The unconfined compressive strength of the upper brown clay is large enough to allow fairly deep excavations to stand open for a short time with unsupported vertical sides (say for sewer and watermain trenches). Unfortunately for workmen, the fissured nature of the clay allows pore-pressure redistribution to occur relatively quickly and unpredictably. This fast response, coupled with the low stress, effective stress behaviour of the drained clay shown in Fig.2, has led to numerous trench collapses and loss of life. At intermediate stress levels the inherent strength BC of the particle structure of the overconsolidated clay provides the limit to its effective stress behaviour. At stresses greater than the apparent preconsolidation pressure the effective stress strength becomes normally consolidated at C. However, such stress levels should be strenuously avoided in design if problems of excessive displacements, high porewater pressures and slow consolidation rates are to be avoided. The presentation in Fig.2 of strength results norrealized by preconsolidation pressures is emphasized. This format decreases the customary attention to normally consolidated strengths that are in fact least relevan~ to practical problems. It emphasizes the strengths of (i) the fissured macrostructure at low stresses, and (ii) the overconsolidated interparticle bonds at intermediate stresses. SUMMARY AND CONCLUSIONS Winnipeg clay has been shown to be anisotropic, fissured, swelling, brittle and overconsolidated by a variety of environmental and climatic processes. All of these properties, and additional characteristics of engineering significance such as the appreciable unconfined compression strength at shallow depth, the complex shape of the effective-stress Mohr-Coulomb envelope, and the low residual strength, arise from the geology of the deposit. They all have important implications for construction. Despite this, the simple stratigraphy of the Winnipeg deposits has meant that the role of geology in geotechnical engineering has generally not received sufficient attention. This situation must change as the city develops more underground structures; the public becomes aware of the growing importance of groundwater; and greater demands are made for smoother streets and houses that are less susceptible to heaving and settlement. ACKNOWLEDGEMENTS

The research was supported by Grant Nos. A3712 and A1470 from the Natural Sciences and Engineering Research Council, Canada; and by a grant from the Manitoba Department of Economic Development and Tourism. Information for this paper was collected under our supervision by V.C.S. Au, B.H. Kjartanson, K.V. Lew, E.C.C. Li, M.L. Noonan, and N. Piamsalee.

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REFERENCES Baracos, A., 1977. Compositional and structural anisotropy of Winnipeg soils -- a study based on scanning electron microscopy and x-ray diffraction analysis. Can. Geotech. J., 14: 125--143. Baracos, A. and Graham, J., 1981. Landslide problems in Winnipeg. Can. Geotech. J., 18: 390--401. Baracos, A., Graham, J., Kjartanson, B.H. and Shields, D.H.S., 1983. Geology and soil properties of Winnipeg. In: R.N. Yong (Editor), ASCE Special Publication on Geological Environment and Soil Properties. pp.39--56. Bjerrum, L., 1967. The engineering geology of Norwegian normally consolidated marine clays as related to the settlement of buildings. G~otechnique, 17: 81--118. Crooks, J.H.A. and Graham, J., 1972. Stress--strain properties of Belfast estuarine clay. Eng. Geol., 6: 275--288. Domaschuk, L., 1983. House Damage due to Shifting Foundations. Unpubl. report to Canada Mortgage and Housing Corporation, CR File No. 6585/D17, Civil Eng. Dept., Univ. of Manitoba, pp.l--35. Domaschuk, L. and Valliapan, P., 1975. Nonlinear settlement analysis by finite element. ASCE J. Geotech. Div., 101 (GT7): 601--614. Fenton, M.M., Moran, S.R., Teller, J.T. and Clayton, L., 1983. Quaternary stratigraphy and history in the southern part of the Lake Agassiz basin. In: J.T. Teller and L. Clayton (Editors), Glacial Lake Agassize. Spec. Pap. 26, Geological Association of Canada, Toronto, 1983. Freeman, W.S. and Sutherland, H.B., 1974. Slope stability analysis in anisotropic Winnipeg clays. Can. Geotech. J., 11: 59--71. Graham, J. and Au, V.C.S., 1985. Influence of freeze--thaw and softening on a natural clay at low stresses. Can. Geotech. J., 22(1). In press. Graham, J. and Houlsby, G.T., 1983. Anisotropic elasticity in a natural plastic clay. G~otechnique, 33: 165--180. Graham, J., Noonan, M.L. and Lew, K.V., 1983a. Yield states and stress--strain relationships in a natural plastic clay. Can. Geotech. J., 20: 502--516. Graham, J., Crooks, J.H.A. and Bell, A.L., 1983b. Strain rate effects in soft natural clays. G~otechnique, 33: 327--340. Hamilton, J.J., 1969. Effects of environment on the performance of shallow foundations. Can. Geotech. J., 6: 65--80. Last, W.M. and Teller, J.T., 1983. Holocene climate and hydrology of Lake Manitoba. In: J.T. Teller and L. Clayton (Editors), Glacial Lake Agassiz. Spec. Pap., 26, Geological Association of Canada, Toronto, 1983. Loh, A.K. and Holt, R.T., 1974. Directional variation in undrained shear strength and fabric of Winnipeg upper brown clay. Can. Geotech. J., 11: 430--437. MacKay, J.R., 1974. Reticulate ice veins in permafrost, Northern Canada. Can. Geotech. J., 11: 230--237. Mesri, G. and Olson, R.E., 1970. Shear strength of montmorillonite. G~otechnique, 20: 261--270. Mishtak, J., 1964. Soil mechanics aspects of the Red River Floodway. Can. Geol. J., 1: 133--146.

Mitchell, R.J., 1970. On the yielding and mechanical strength of Leda clays. Can. Geotech. J., 7: 297--312. Prest, V.K., 1961. Geology of the softs of Canada. In: R.F. Legget (Editor), Soils in Canada. R. Soc. Can., Spec. Publ. No.3, Univ. of Toronto Press, Toronto, Ont. Quigley, R.M., 1968. Soil mineralogy, Winnipeg swelling clays. Can. Geotech. J., 5: 120--122. Quigley, R.M., 1980. Geology, mineralogy and geochemistry of soft soils, and their relationship to geotechnical problems. Can. Geotech. J., 17(2): 261--285. Render, F.W., 1983. Hydrology. In: A. Baracos, D.H. Shields and B.H. Kjartanson

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(Editors), Geological Engineering Report for Urban Development of Winnipeg. Geological Engineering Department, University of Manitoba, 76 pp + 18 plates. Teller, J.T., 1976. Lake Agassiz deposits in the main offshore basin of southern Manitoba. Can. J. Earth Sci., 13: 27--43. Teller, J.T. and Fenton, M.M., 1980. Late Wisconsinan glacialstratigraphy and history of southeastern Manitoba. Can. J. Earth Sci., 17: 19--35. Torrance, J.K., 1975. O n the role of chemistry in the development and behavior of the sensitive marine clays of Canada and Scandinavia. Can. Geotech. J., 12: 326--335. Townsend, D.L., 1965. Discussion on "Geotechnical properties of three Ontario clays" by L.G. Soderman and R.M. Quigley. Can. Geotech. J., 2: 190--193. Trainor, P.G.S., 1982. A n Investigation and Review of Oedometer and Triaxial Tests on Winnipeg Clays. Unpublished M.Sc. Thesis, University of Manitoba, Winnipeg. Wroth, C.P. and Houlsby, G.T., 1980. A criticalstate model for predicting the behaviour of clays. Proc. Workshop on Limit Equilibrium, Plasticityand Generalised Stress-Strain in Geotechnical Engineering, McGill University, Montreal, pp.592--627.