Clay Minerals (1986) 21, 261 278

S O M E C L A Y - R E L A T E D P R O B L E M S IN E N G I N E E R I N G G E O L O G Y I N N O R T H A M E R I C A

J . E . G I L L O T T

Department of Civil Engineering, University of Calgary, Calgary, Alberta, Canada

(Received 12 March 1985)

ABSTRACT: Clay minerals are almost ubiquitous in soil and rock and are among the most reactive silicates. They affect the engineering behaviour of soil and rock both as materials of construction and as foundation materials. In the petroleum industry, clay affects the permeability of reservoir formations, it is a common cap-rock, and it is also a constituent of the fluids used in drilling operations. Engineering behaviour almost always involves clay-water interaction and in turn this depends on the nature of water and solutions and on the composition and microstructure of the clay. The importance of clay to specific problems from each of these areas is discussed. In foundation engineering its role in soil-moisture interaction is illustrated by reference to problems resulting from the geological history of some North American soils and from engineering activities. In building materials, reference is made to its effect on concrete durability in aggregate related problems. The importance of clay in petroleum engineering refers to authigenic clays in reservoir rocks, to clay behaviour in the Alberta oil sands and to the use of clay minerals as a geothermal thermometer.

The engineering geologist encounters clay in foundation materials, in materials of construction and in reservoir formations. Frequently the clay is associated with problems which more often than not result from its strong interaction with water. These engineering problems are invariably costly and sometimes lead to loss of life. A better understanding o f the nature of clay and aqueous solutions and the reasons for their strong interaction would make possible a more precise predict ion of behaviour, and many of the problems encountered in engineering practice would be avoided. The object of this paper is to review briefly the nature of clay, water and solutions and to illustrate the effects o f c l ay -wa te r interaction on behaviour of soils and rocks.

T H E N A T U R E O F C L A Y S , W A T E R A N D S O L U T I O N S

The strong interaction between clays and water results from the high specific surface area and structure of the clay minerals and from the polar nature of the water molecule. The high specific surface results from the small size and platy or fibrous shape o f the clay minerals. In smectites the surface area is of the order Of 760 m 2 g - i and in kaolinites it is about 15 to 40 m 2 g- l , placing clays within the limits of the colloidal state. The structure of 2 :1 type clay minerals makes possible unbalanced isomorphous substitutions among the cation population, giving many of the minerals a permanent negative charge principally associated with the basal surfaces. Edge surfaces probably acquire a reversible, pH-

�9 1986 The Mineralogical Society

262 J. E. Gillott

dependent, charge by adsorption of potential-determining ions from solution; the charge is positive in acid solution and negative in alkaline solution.

In the water molecule there is a tetrahedral charge distribution in which two corners are positively charged and the other two corners are negatively charged (Bernal & Fowler, 1933; Morgan & Warren, 1938). The result is that water has unusual physical properties. For example, its melting point, boiling point, specific heat, viscosity and dielectric constant are unusually high and there is a density maximum at about 4~ These abnormalities have been accounted for with varying success in terms of the molecular configuration and resulting structure. Thus the density maximum was explained by proposing a less dense ice-tridymite-like structure for water below 4~ and a more compact quartz-like arrangement of molecules from 4 o C to higher temperatures. Since these structures are not close-packed, intermolecular void-space must be greater than the minimum.

The translational freedom of the molecules suggests that the voids will continuously open and close and that at a given temperature and pressure there will be a certain average void-size distribution. It is thought that ions in solution are accommodated in these void spaces. The goodness of fit depends upon both void size and ionic size and on this basis ions have been divided into three groups. Group A includes ions smaller than the average void size, group B includes ions which are slightly larger and group C includes much larger particles such as organic molecules. Ion-water molecule association and diffusion mechanisms have been considered on the basis of these proposals (Sadek, 1983). The strongest association generally involves small polyvalent ions with a large charge to surface ratio. Ions with larger solid-state radii are therefore less strongly hydrated and, valency and other things being equal, tend to have the greatest replacing power in exchange reactions and to be less readily displaced once adsorbed at exchange sites on clay mineral surfaces.

The understanding of the structure of water and ionic solutions owes much to analysis by X-ray diffraction and neutron diffraction. Recently, improved information regarding ion-ion and ion-hydration interaction has been obtained from the neutron diffraction patterns of solutions containing different isotopes of the same compounds (Soper et al., 1977). It has also been found that interlayer water in montmorillonite is not highly structured, that only about half of the water in the first monolayer is associated with the silicate structure of the surface, that counter ions have well-defined hydration shells and that the structure of interstitial water is similar to that of water in bulk (Hall et al., 1979; Hawkins & Egelstaff, 1980).

C L A Y S A N D M O I S T U R E I N T E R A C T I O N IN G E O T E C H N I C A L E N G I N E E R I N G

The behaviour of soil as a foundation material is affected both by the total amount of water and by the energy with which it is retained. Consistency, strength and density are affected by total water content, and volume change characteristics and moisture migration are affected by energy considerations. Gradients give rise to moisture movements and typical effects are shrinkage on drying and swelling on moisture uptake. Heave may result from this cause, from ice-lens growth and from other causes such as biogeochemical oxidation of sulphides (Gillott et al., 1974; Caldwell et al., 1984). To overcome problems caused by moisture movements, footings and foundations may be placed below the level affected by seasonal fluctuations of moisture content, the soil may be replaced by a fill less affected by

Clay-related problems in engineering geology 263

moisture movements, soil stabilization procedures may be adopted, or stiffened slab or raft foundations may be used. Economics is a major factor in determining which of these options is adopted.

During the Quaternary, about 97% of Canada was glaciated and areas to the south were affected by the climatic extremes associated with the four major glacial advances. The Wisconsin ice sheet began its retreat about 18 000 years BP and by about 6000 years BP ice had withdrawn from mainland North America except at high elevations. The present soil cover in Canada is therefore mainly composed of glacial debris. Frequently, the hard, consolidated basal till is overlain by soft 'soils' which were deposited in proglacial or post-glacial lakes or seas which existed during the retreat of the continental ice sheet. These bodies of water owed their existence to the isostatic depression of the crust by the continental ice cap, to the release of large volumes of freshwater in the continental interior and to the corresponding eustatic rise of sea level caused by the melting of the ice and to the blockage of present drainage channels by ice during its retreat.

Sediments were deposited under marine conditions in coastal regions of eastern Canada, in the Champlain Sea which occupied the Ottawa and St. Lawrence Valleys from 12 500 to 10 000 years BP, in the Tyrrell Sea which existed in areas adjoining Hudson Bay, in other coastal regions of Arctic Canada, and in a few localities on the west coast near Vancouver and on Vancouver Island. In some instances, as in parts of the Champlain Sea, marine sediments overlie earlier deposits of lacustrine origin. The freshwater regime is believed to have been terminated when an ice dam was breached and seawater entered formerly protected lakes.

Sediments of lacustrine origin are extensive. These accumulated in eastern and central Canada in ice-dammed bodies of freshwater such as Lake Vermont, Lake Barlow-Ojibway farther north and in Lake Algonquin, a precursor and extension of the modern Great Lakes. On the Prairies, Lake Agassiz was the largest lake (Teller & Fenton, 1980; Baracos, 1977) but ice-dammed lakes covered much of western Canada during this period. The position and extent of these lakes changed and migrated northwards and northeastwards with the retreat of the ice so that the area occupied by lacustrine soils is greater than that of the lakes at any one time. Glacial deposits on the Prairies average about 200 ft thick but in places are known to exceed 1000 ft. In addition to the hard glacial till and soft marine and lacustrine clays there are extensive deposits of loess in regions marginal to those which were glaciated. The wind-blown silt was derived from glacial flood plains.

Marine sediments consist of sands, gravels and clays which sometimes include ice-rafted stones and boulders so that they resemble till. Clay is flocculated in sea-water and sometimes digested and pelletized by marine organisms. In Champlain Sea sediments, rock flour, consisting of primary minerals of clay size, is an important component (Gillott. 1971). Clay minerals, mainly illite but with chlorite and sometimes smectite, are also present together with carbonates and amorphous minerals. Lacustrine sediments overlie till or ice-scoured bedrock and consist of soft waterlaid till, mudflow deposits, turbidity current deposits and varves. Deposits formed in large lakes are often overlain by homogeneous brown or grey post-glacial clays. Interestingly, primary carbonates formed in lakes in humid regions are mainly inorganic chemical precipitates (Kelts & Hsu, 1978). Lake deposits in western Canada are frequently clayey and smectites, derived from underlying argillaceous formations, are an important component in the soils. The possibility that authigenic silicates form under lacustrine conditions has been discussed by a number of authors but the question remains open (Johnson, 1984).

264 J. E. Gillott

Some of the more serious geotechnical problems due to moisture interaction with soft soils result from slope instability, heave and settlement. Slope instability may lead to abrupt failure but slow plastic creep deformation may also prove costly (Gillott, 1982). Movements result from gravitational forces, frost-action, expansion and contraction due to heating and cooling, moisture migration and the action of living organisms including animals and plants. Disturbance of equilibrium is caused by increase of shear stress or decrease in shear strength. Stress increase on slopes results from steepening, loading at the head, and undermining at the foot by excavation or erosion. Strength decrease results from increase in porewater pressure, moisture uptake, change in porewater composition, leaching ofintergranular cement and frost-action.

A typical example of a problem due to slow but continuous movement occurred in the Swan Hills of north-central Alberta, about 140 miles northwest of Edmonton, where creep affected the structural integrity of an oil pipeline. The pipeline, about 20 years old, had been draped into a trench approximately 6 ft deep. Downhill movements due to creep and solifluction had given the pipeline a 15 ~ bend and sideways warp of ~6 ft. Slides 20-30 ft deep were also present on either side of the pipeline (Beddoes, personal communication). In the Swan Hills area the Paleocene Paskapoo formation is overlain unconformably by sands dissected by stream erosion into discontinuous patches; these deposits are overlain by Pleistocene till of low permeability (Vonhof, 1969). At the site of the pipeline the soil consisted of a Silt containing s clay composed mainly of montmoriUonite with small amounts of illite and chlorite. The fabric studies showed that, although the equiaxed primary minerals were relatively angular, there was a significant amount of silt-sized mica which would tend to reduce shearing resistance. The primary minerals were embedded in a clay matrix composed mainly of smectites occupying the porespaces (Fig. 1). The high moisture content of the soil, clay mineral composition and location in pores, where it would trap moisture and decrease permeability, together with phyllosilicates in the silt-size range appear to have been major factors contributing to slope instability of the material.

The Champlain Sea sediments are commonly sensitive and are associated with slope failure due to slides of flow type. Sensitive soils from different localities have features in common both with respect to composition, which includes a significant proportion of inactive minerals, and fabric which is commonly open (Gillott, 1979). High values of sensitivity have been explained as due to a tendency to change from an open fabric formed on deposition to a more closely-packed arrangement. As voids are full of water an abrupt decrease in voids ratio causes the soil to become oversaturated and it may flow like a viscous liquid. Both physical and chemical processes have been invoked to account for the fabric change. The initial open fabric has been attributed to clay flocculation by salt in the marine environment, to an open arrangement of agglomerates linked by connector assemblages and to an open arrangement of primary mineral particles cemented at junctions by carbonates or by amorphous materials. The tendency for a more close-packed arrangement to develop was attributed to salt leaching by Rosenqvist (1960) and Bjerrum (1957). An open, flocculated clay fabric formed under marine conditions was considered to be unstable in the fresh-water regime of the present. S6derblom (1966) suggested that the fabric change resulted from introduction or formation of dispersing agents and Smalley (1978) considered that structural collapse resulted from rupture of cemented junctions between primary mineral particles. Thixotropy, ion exchange, weathering and diagenetic reactions including sulphide-sulphate and organic compound redox equilibria have also been proposed as factors of importance (Donovan & Lajoie, 1979).

Clay-related problems in engineering geology 265

A. Silt: mainly equiaxed particles in clay matrix.

B. Silt: mainly layer-silicates in clay matrix.

C. Equiaxed silt in clay matrix. D. Clay matrix: fabric detail.

FIG. 1. Scanning electron micrographs of silt, Swan Hills, Alberta.

Volume increase or decrease of soils due to changes in moisture content causes damage considered to exceed that due to the combined effect of earthquakes, tornadoes, hurricanes and floods; property damage has been estimated at $6000 million annually in 1982 dollars (Holtz, 1983). Moisture uptake commonly causes volume increase and the most serious effects are often found in soils containing significant amounts of smectite. Such soils may often be recognized in the field since they are sticky when wet, hard and cracked when dry and form a very fine powder when crushed. Mound-depression features termed gilgai structure are sometimes present in field exposure. Soils containing significant amounts of smectite are common in the Prairie Provinces of Canada and in parts of the United States.

The magnitude of the volume change on moisture uptake is affected not only by the mineralogy but also by the moisture content, the nature and concentration of the pore solutions, the fabric, and the extent of interparticle cementation. Rate of volume change depends on permeability and in this respect the larger pores are of most importance. It is probable that fabric has more effect on permeability and moisture relations than any other soil property. An interesting soil in which this factor is very apparent was found near the Mt. Norquay overpass in Banff National Park. The moisture content of this silt was found to be high, 55%, but visual estimates by different observers indicated that it was about 5-10%. The discrepancy was explained when scanning electron micrographs showed a high intraparticle porosity in addition to the interparticle porosity (Fig. 2) (Clark & Gillott, 1985).

266 J. E. Gillott

A. Interparticle porosity.

B. Intraparticle porosity.

F[~. 2. Scanning electron micrographs showing interparticle and intraparticle pores in silt, Banff, Alberta.

Volume decrease results from consolidation due to expulsion of water under load, due to moisture loss on drying or transpiration by plants, and sometimes due to fabric change on moisture uptake. The latter effect is relatively common, but interesting since moisture uptake is generally associated with heave. It is often referred to as soil collapse and occurs because the fabric elements take up a more close-packed arrangement. It has been described in loess, some residual and alluvial deposits and fill-materials. A microstructural classification in which 12 types of loess are recognized has been proposed by Lin & Liang (1982). Typically a collapsing soil has an open fabric of sand, silt or clay platelets or agglomerations. Weak bonds between particles result from surface tension forces, clay buttresses or coatings, or cementitious precipitates (Fig. 3). These bonds are weakened or removed when water fills the void-spaces due to rise of groundwater table because of natural or artificial causes. Under normal or increased load the voids ratio decreases and the soil collapses.

In the Kamloops area of British Columbia there are extensive deposits of silt of this sort which have caused serious foundation problems (Clark & Gillott, 1985). Tests have shown that under light loads (e.g. 48 kPa--corresponding to the footing load of a normal house) collapse occurs on admission of water and the resulting decrease in voids ratio is apparent

Clay-related problems in engineering geology 267

A. Clay envelope surrounding silt. B. Clay envelope surrounding silt: detail.

C. Fine silt with clay. D. Clay at junction of silt particles.

E. Silt with precipitate? F. Detail of precipitate?

FIG. 3. Scanning electron micrographs of contact relations in loess, Kamloops, British Columbia.

on microscopic examination (Fig. 4a,b). In more quantitative terms the strain was found to vary from about 3-6% for different samples from the same site; the significant difference obtained with closely-spaced samples indicates the potential for differential settlement. An example of related behaviour in fill materials has been described by Dusseault et al. (1985). Differential settlement occurs in fill material placed after strip-mining of Prairie coal. The fill is composed of smectitic clay shale lumps and when placed has relatively high macroporosity. The clay lumps initially have quite high strengths due to high suctions associated with the unsaturated smectite. Although the permeability is low, water is slowly

268 J. E. Gillott

FIG. 4a. Mosaic of scanning electron micrographs of 'undisturbed' loess, Kamloops, British Columbia.

imbibed with decrease in suction and strength loss. Deformation of the lumps leads to decrease in macroporosity and subsidence.

C L A Y A N D M O I S T U R E I N T E R A C T I O N IN B U I L D I N G M A T E R I A L S

Concrete is the most widely used building material in the developed world and aggregates form the largest component in concrete. Many properties of concrete are affected by the aggregates, such as fire protection, thermal insulation and strength, but appearance and durability both depend heavily on the interaction of aggregates with moisture. Appearance may be adversely affected by dark or reddish-brown stains on exposed surfaces. Such stains often originate in particles of aggregate and the source is most obvious in precast panels of exposed aggregate concrete. Oxidation of iron minerals in the aggregate leads to formation of soluble products which migrate in percolating water and are precipitated on outer surfaces.

Durability of concrete is adversely affected by aggregates which readily absorb water, have a high moisture content when saturated and a high capacity for moisture retention. Ease of saturation results from high permeability, and a high capacity for both moisture uptake and moisture retention results from the presence of fine-grained constituents, particularly clay. Changes in moisture content in such aggregates often leads to excessive volume changes and the generation of high swelling pressures, causing pop-outs, cracking

Clay-related problems in engineering geology 269

FIG. 4b. Mosaic of scanning electron micrographs of 'collapsed' loess, Kamloops, British Columbia.

and other forms of deterioration. For this reason the amount of fines, argillaceous rocks and clay lumps is restricted in most standards specifications.

The presence of clays sometimes goes undetected or is disregarded in the conventional examination of aggregates so the potentially harmful effects on concrete durability are missed. Igneous rocks often make excellent aggregates but clay minerals are sometimes present due to the alteration of primary minerals by weathering or hydrothermal action. A typical example was described by Cole & Beresford (1980) who concluded that secondary clay minerals in some basalts near Melbourne, Australia, caused dimensional instability with potentially serious effects on long-term durability of concrete. Similar problems have been reported with Scottish dolerites. All clay minerals when dry have a capacity for moisture uptake but the effect is most marked when swelling clay minerals, particularly smectites, and some very poorly crystalline minerals such as allophane are present.

Clay minerals are very often present as an integral constituent of sedimentary rocks and when physically sound rocks of that sort are used as concrete aggregates the clays commonly cause little or no problems. This proves not to be the case with certain argillaceous dolomitic limestones. That variety of rock has a fight fabric, is physically sound and is composed of small dolomite rhombs (<70 am) in a matrix of irregular grains of calcite (~4/~m) and acid-insoluble minerals including clay (Fig. 5). The clay consists mainly of illite and chlorite and swelling clay minerals are generally absent. In a moist environment, concrete made with high-alkali cement and that rock as aggregate expands and often cracks within about a couple of months. The behaviour results from the alkali-carbonate reaction which is a variety of alkali-aggregate reaction.

270 J. E. Gillott

A. Clay at calcite grain-boundaries. B. Dolomite rhombs.

C. Dolomite rhombs with clay at interstices.

D. Detail of clay at interstices between dolomite.

E. Dolomite, calcite and clay. F. Dolomite, calcite, clay: detail.

FIG. 5. Scanning electron micrographs of dolomitic limestone, Kingston, Ontario.

The key to understanding the mechanism was provided by results of dimensional change tests obtained with a variety of dilatometer in combination with studies of the petrography and mineralogy of the rock (Gillott, 1963a,b). Data from X-ray powder diffraction experiments showed that dolomite was attacked by strong alkali, similar to that present in the pores of hydrating portland cement paste. Calculation indicated, however, that the solid volume of the reaction products was less than that of the starting material so the dedolomitization reaction, by itself, could not account for the expansion. Nonetheless it was believed to be a factor.

Clay-related problems in engineering geology 271

In the dilatometer, powdered slurries were placed under constant pressure and allowed to consolidate to equilibrium in contact with water. Alkali was then admitted and expansion data recorded as a function of time. Results showed that, contrary to expectations, the amount of expansion decreased as the particle size of the samples decreased. It was reasoned that it was possible to account for this observation if expansion resulted from moisture pick-up by dry clay. Finer grinding exposed more and more clay which was pre-wetted before alkali was admitted to the system. Expansion which was detected resulted from moisture pick-up by dry clay released, from within what remained of the original tight fabric, by alkaline attack on the dolomite. Expansion of the rock in alkali therefore depends upon moisture pick-up by dry clay exposed by dedolomitization.

This explanation accounts for the petrographic observations that expansive rocks invariably contain clay, are physically sound and have a tight fabric. Conversely, it explains why little or no expansion is shown by similar, but physically unsound rocks, since in that case pre-wetting of the clay is to be anticipated. It also becomes clear why the size of the dolomite crystals is important. If they are too coarse the rate at which they are attacked by the alkali will become very slow. In this case the clay will be released only very slowly and rate of expansion will correspondingly diminish. A somewhat unpalatable corollary for petrographers is that expansive and non-expansive rocks may not be distinguished with complete certainty since the moisture state of the clay may not be established by microscopic methods; additional techniques are required.

The question as to whether non-swelling clay could account for the magnitude of observed expansion by moisture uptake was answered by other experiments. Compacts of Fithian illite and of acid-insoluble residue separated from the expansive limestones were exposed to controlled relative humidities and dimensional change data was recorded using an optical extensometer. Results showed that expansions were of an order which could account for those observed in the alkali-carbonate reaction (Swenson & Gillott, 1967).

Aggregates sometimes have a detrimental effect on the durability of sulphur concretes in the presence of moisture. This drawback has been found often to result from the presence of clay (Gillott et al., 1980). The type of clay mineral present has a significant effect and as little as 1% montmorillonite causes large expansions and cracking in water. Significant expansions are also caused by the non-swelling clay minerals illite and kaolinite though larger amounts of the clay are tolerated (Fig. 6).

Sulphur-bonded composites of clays and other phyllosilicates (in a proportion of 70 : 30 sulphur :phyllosilicate) show varying changes in dimensions in water. When made with montmorillonite the composite rapidly breaks up and disintegrates. With illite and kaolinite expansions exceed 0.3% and 0.15% respectively after 120 days in water, cracks being visible in both types of specimens. Mica and chlorite, ground to pass a 325 mesh sieve, showed changes in dimensions of +0.04% and -0 .02% respectively after 75 days of exposure (Fig. 7).

Sulphur-bound materials are normally made by mixing heated aggregate or filler with molten sulphur so the method of fabrication ensures that any clay present is dry. Hence, its affinity for moisture is at a maximum. Expansion occurs on moisture uptake with detrimental or disastrous effects to durability. Expansion in water of sulphur-concrete containing different clays may be significantly reduced by use of suitable admixtures (Fig. 8) (Gillott et al., 1980).

Sulphur-concrete gains strength rapidly on solidification and hardening of the molten sulphur. Strength is comparable to that of portland cement concrete, rate of strength gain is

272

o~ "N

<3

J. E. Gillott

0 0 4 4 0 �9 BENTONITE ( 1% TOTAL AGGREGATE) / /

- - �9 tLLITE 3 / / TO 0 .20% AT x KAOLINITE / 5 % OF / / 211DAYS

0 0 3 6 0 - - o CHLORITE (<325MESH) ~" TOTAL /

0 0 2 8 0 - �9 MICA ( < 3 2 5 MESH) J A G G R E G A /

/ 0 . 0 2 0 0 -- ~ � 9 �9 - - ~ �9

0 0120 - - ~ , ~ x ~ ~ x

0 0 0 4 0 - - " / ~ J ' " , ~ ~' . . Z - ~ " ' ~ ~ - o

w.. . ~ . x

o o ~ t Z ~ o . . . . _ _ o ~ - 0 0 0 4 0 - -

J I ] I ] ] t [ ] / 1 I t ] i I I - 0 Or20 0 8 16 24 32 40 48 56 64 72

AGE ( D A Y S )

FIo. 6. Dimensional change of sulphur-concrete prisms (3 x 3 x 11 in) made with limestone aggregate plus different phyllosilicates (in water).

O,g4 - - �9 ILLITE �9

- - x KAOLINITE ~'~ 0 CHLORITE (,=325 MESH) ,~.,? :I~5)~:'~-/" /

0.20 - - �9 MICA (<325 MESH)

0.12 - - x . ' ' - �9 ~'

o~ x

"" 0.08 <3

0 .04

0.0

I - o o 4 i ! k l ~ [ i J ,L I ! I t I I I I I

0 (0 20 30 40 50 60 70 80 90

AGE ( D A Y S )

FIG. 7. Dimensional change of sulphur-composite prisms (1 x 1 • l l in) made with different phyllosilicates (in water)

Clay-related problems in engineering geology

0,0440 --

0.0360 - -

0 . 0280 - -

l 0 .0200

0.0120

%

�9 BENTONITE (3%TOTAL AGGREGATE) �9 ILLITE } x KAOLINITE 5 % OF 0 CHLORITE ( , :325MESH) TOTAL �9 MICA (~:325 MESH) AGGREGATE

TO 03B ~ AT 0'21~ DAY~

_ � 9 1 4 9

f _ _ � 9

0.0040 ~ ~--~

-o.oo o - o ,o o o',_L% o

- o . o , 2 o i I l t t I i 1 i I I I I l l i I I I o a ,B 24 32 4o 4B 56 64 22

AGE (DAYS)

FIG. 8. Dimensional change of sulphur-concrete prisms (3 x 3 x 11 in) made with limestone aggregate plus different phyllosilicates treated with admixtures (in water).

273

much more rapid and fatigue resistance is excellent. Peak strengths of 30 MPa within 24 h and of over 50 MPa within 3-4 days of casting may be achieved. The rapid strength gain gives sulphur-concrete potential advantages in precast plants, in cast-in-place structures, in low-temperature concreting and in military applications. When raw sulphur is used as the binding agent, sulphur concrete is more brittle than portland cement concrete. Its strain capacity, however, may be greatly increased and varied in a controlled manner by use of suitable admixtures or by treatment of the sulphur with 'plasticizing' agents, though often with loss of peak strength (Jordaan et al., 1978; Pickard, 1981; Loov et al., 1983). Sulphur-concrete, when made with suitable materials, has better resistance to acidic and saline conditions of exposure than portland cement concrete. Hence, it is finding application as a flooring material in industrial plants and elsewhere where acidic and saline conditions are encountered.

CLAYS AND M O I S T U R E I N T E R A C T I O N IN P E T R O L E U M G E O L O G Y

In petroleum geology it is recognized that the amount and type of clay significantly affects reservoir quality. Clay minerals also have been been shown to catalyse formation of hydrocarbons, to increase hydrocarbon retention, to influence hydrocarbon composition in pyrolytic experiments and, by inference, are thought to affect the type of hydrocarbons found in natural deposits. Sedimentology, structure and mineralogy are often interrelated variables which control hydrocarbon distribution (Shimoyama & Johns, 1971; Johns &

274 J. E. Gillott

Shimoyama, 1972; Espitali6 et al., 1980; Horsfield & Douglas, 1980; Tilley & Longstaffe, 1984).

Water is also important. As sediments are buried by younger strata there is a reduction in voids ratio and more and more load is transferred from the porewater to the points of contact between the mineral particles where the stress increases to support the overburden pressure. The pressure acting on the pore-fluids is referred to as the formation pressure and normal formation pressure is taken as the hydrostatic pressure, but a variety of factors may lead to abnormal formation pressures. When permeability is low and rates of loading are high, excess pore pressure may develop because part of the overburden load is supported by the pore-fluids rather than by grain-to-grain contacts. Other factors which may lead to abnormal formation pressures include high shale:sand ratios, disposition of groundwater table, structure, tectonic movements, and biochemical, geochemical and diagenetic changes.

Clay minerals show a general improvement in crystallinity with increase in depth of burial. In mixed-layer illite-smectite the proportion of illite layers has been shown to increase from <20% to ~80% in the depth interval from 2000 to 3700 m (Hower et aL, 1976). Burst (1969) suggested that smectite dehydration occurred in distinctive stages. Dehydration of clays, particularly the transformation of smectite to illite, has been linked with petroleum formation and migration and significant relationships to hydrocarbon production have been claimed particularly in the Gulf coast region. In the North West Territory of Canada, Foscolos & Powell (1980) have shown that very complex mineralogical transformations may involve amorphous constituents as well as clay minerals; they also cast doubt on a straightforward relationship between petroleum migration and water released on clay dehydration.

Much of the clay in sediments is known to be detrital but it is now recognized that authigenic clay minerals are present in many sediments. These minerals are readily recognized under the scanning electron microscope by their delicate appearance, position and characteristic morphology (Wilson & Pittman, 1977). They occur as coatings on primary grains and in pore-lining, pore-bridging and pore-filling relationships. Illite often has a fibrous appearance, chlorite is common as rosettes, kaolinite occurs as pseudohexagonal plates with book- or fan-like form (but may be elongate or vermiform) and smectite often appears as very thin crumpled sheets or foils (Fig. 9). Many of these minerals have high microporosity often associated with large irreducible water-saturations.

In reservoir engineering the permeability close to a well needs to be as high as possible and acids are often introduced at a pressure greater than the formation pressure. Acids used include HC1, HF, HNO3, H2SO4, formic, acetic and various mixtures. The object is to clean the pores and increase the permeability particularly near the well, because, in recovery of oil and gas, fluids from further afield are migrating to this zone. High rates of flow are also important when fluids are injected into formations in secondary oil recovery and waste disposal operations. The acid treatment produces soluble salts such as NaCI, CaCI 2, Na2SO 4 and NaF, and may leave unspent acid in pore solutions; these compounds may subsequently corrode the borehole casing or attack the oilwell cement used in well finishing and completion operations.

While the object of acidization is to improve permeability, some reactions degrade reservoir quality. Fluid sensitivity of this sort is particularly common in rocks containing Fe-bearing minerals. Problems arise due to the formation of Fe-hydroxy gels which precipitate and block pore-throats. The iron may come from any Fe-rich mineral, siderite

Clay-related problems in engineering geology 275

A. Pore-bridging smectite, Bow island fmt., B. Pore-gridging smectite, Bow island fmt., cret., S.W. Alta. cret., S.W. Alta.

C. Pore-filling kaolinite, bluesky fmt., cret., D. Pore-filling kaolinite, blueskyfmt., W. Alta. W. Alta.

E. Grain-coating-chlorite, jurassic, Sable Is., F. Grain-coating-chlorite, jurassic, Sable Is., E. Canada. E. Canada.

FIG. 9. Scanning electron micrographs of authigenic clay minerals.

probably being one of the most common sources, but it may originate from pyrite, chlorite or other Fe-rich clay minerals. To help overcome the problem, compounds sometimes termed ion scavengers, such as EDTA (ethylenediaminetetraacetic acid), are used which tend to keep the iron in solution and inhibit Fe-gel precipitation.

Mineralogical transformations may also result from secondary recovery operations or from in situ recovery of heavy oil. The more promising techniques under consideration

276 J. E. Gillott

include cyclic steam stimulation, steam drive and forward combustion. The viscosity of the heavy oil is reduced by the heat and by the formation of an oil-and-water emulsion so that it is able to flow when the pressure is reduced. These procedures are under consideration for the extraction of oil from the early Cretaceous age Alberta Oil Sands which are estimated to contain of the order of 249 x 109 m 3 (1572 billion barrels) of heavy crude bitumen (Energy Resources Conservation Board, 1983).

Fluid flow sometimes leads to formation damage due to dispersion and migration of clay minerals and permeability reductions of 50x have been reported in laboratory tests of cores containing only about 2% montmorillonite (Waldorf, 1965). In the Alberta Oil Sands the amount of montmorillonite is generally small but hydrothermal studies have shown that montmorillonite forms readily from kaolinite, quartz and dolomite (Perry & Gillott, 1979). These minerals are commonly present and thermal methods of in situ recovery are likely to generate the aqueous solutions at the temperatures required (250-300~ for these transformations. At higher temperatures (500-600~ which may be expected in the vicinity of the fire zone, metakaolin may form. Laboratory studies showed that when this mineral, rather than kaolinite, was present the rate of montmorillonite formation was approximately 2-3 times faster.

The maximum temperature reached is an important parameter in evaluating methods of in situ recovery of heavy oil. In laboratory studies temperatures may be monitored by thermocouples but in the field it is not always possible to foresee optimum locations of observation wells and their numbers are limited by economic considerations. Cores are frequently taken after tests to determine residual oil saturations and sweep efficiencies. Thus a method of temperature estimation based on mineralogical transformations may be of considerable practical significance. Not all mineralogical transformations are suitable and there are a number of factors which require consideration. Among these are reaction rate, reversibility of reaction, partial pressure of gases, pH of the system, ease of detection and hydrothermal transformations. Some mineralogical changes of use in this regard involve clay minerals (Table 1).

In general, the overall mineralogy has to be considered and Perry & Gillott (1982) found that in the range 350-700~ temperatures could be estimated with an accuracy of about +50~ Other workers have made use of transformations of authigenic minerals in paleotemperature analysis (Aoyagi & Asakawa, 1984). Clay minerals, zeolites and silica

TABLE 1. Mineral transformation temperatures.

Temperature Mineral Change o C Comments Suitability

Kaolinite Decomposition 500-600 PH20 x/ I l l i t e Dehydroxylation 500-600 XRD unchanged x

Decomposition 750-950 x/ Chlori te Dehydroxylatiod 250-600 x/

of brucite sheet Dehydroxylation 450-800 ~/ and decomposition

Smectite Irreversible collapse 300-700 x/ Dehydroxylation 500-750 XRD unchanged • Decomposition 650-950 x/

Degraded illite Irreversible collapse 250-300 x/

Clay-related problems in engineering geology 277

minera ls were used in establ ishing a geo the rma l t empe ra tu r e gradient as a funct ion o f depth

o f burial. Pe t ro l eum geologists have an interes t in condi t ions o f bur ia l because t empera tu re

and pressure are ma jo r fac tors affect ing the type o f h y d r o c a r b o n s found in reservoi r rocks

(Tissot & Welte , 1978; Hunt , 1979).

C O N C L U S I O N S

C l a y minera ls are so c o m m o n l y found in ear th mater ia ls tha t an unders tand ing of their

na ture is essential in all aspects o f engineer ing geology. W a t e r and solutions are a lmos t

invar iably present in engineer ing opera t ions . A clear unders tand ing o f the in te rac t ion

be tween these two classes o f mater ia ls well r epays the effort.

A C K N O W L E D G M E N T S

I wish to express sincere thanks to a number of engineers and scientists for helpful discussions and for sample material; in particular, Jack Clark of C-Core, and Mark Thomas of Shell Canada Research. This paper would not be possible without the conscientious technical support of Mrs Lucie Jermy, the secretarial help of Carolyn Macarthur and financial support from the Natural Sciences and Engineering Research Council of Canada to whom I extend my grateful thanks.

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