the engineering geology of clay minerals:...

Clay Minerals (1986) 21, 235-260

This and the following four papers were presented at a joint meeting of the Engineering Group of the Geological Society and the Clay Minerals Group of the Mineralogical Society entitled 'Clay Minerals in Engineering Geology--The Geotechnical Propertie s of Clays' held on 12 March 1985.

THE E N G I N E E R I N G G E O L O G Y OF C L A Y M I N E R A L S : S W E L L I N G , S H R I N K I N G A N D

M U D R O C K B R E A K D O W N

R. K. T A Y L O R AND T. J. S M I T H *

Department of Engineering (Engineering Geology), University of Durham, South Road, Durham DH1 3LE

(Received 4 July 1985; revised 22 August 1985)

A B S T RA C T: Swelling, shrinking and physical breakdown processes are reviewed with reference to well-known mudrock and overconsolidated clay formations in the UK and USA. Swelling results from two processes: the equilibration of depressed porewater pressures following stress relief, and the physico-chemical (osmotic) response of component clay minerals. Expansion in Na-smectite, and to a lesser extent Ca-smectite, clays is governed by double-layer swelling, whereas in kaolinites it is purely a mechanical unloading phenomenon; illites show an intermediate response. Intraparticle swelling in mudrocks older than the Silurian in the UK, or Upper Mississippian in the USA, can be expected to be reduced because of the removal of expandable layers by burial diagenesis. Shrinkage, like mudrock breakdown, is restricted to the partly saturated zone. Suction pressure-moisture content curves of indurated mudrocks are shown to be different from mudrocks and clays with high proportions of expandable clay minerals. Classification of expansion potential based on activity ratio poses problems with indurated types, but with some modification of method reasonable predictions can be made. Controls on physical disintegration are identified as: (i) incidence of sedimentary structures and discontinuities, (ii) slaking (air breakage), (iii) expandable clay mineral content, especially smectite, and (iv) clay mineral fabric orientation. Exceptionally high exchangeable sodium percentages have been measured in Coal Measures rocks susceptible to breakdown.

Clay soils susceptible to significant volume change occupy about 20% of the U S A land surface and the annuat damage to proper ty is regarded by some as being of naturaI disaster proport ions. Indeed the bill for man-made structures was est imated by Holtz (1983) to be about $6 billion per annum at 1982 prices. Driscoll (1983) considered that the scale of the problem is less in the U K because of the mild, damp climate. Nevertheless, the shrinkage of heavy clays (from Lias to London Clay) in east and southeast England is part icular ly severe in drought years. These ground movements are influenced by the t ranspirat ion of broadleaf trees and between 1971-80 the damage cost to dwellings resulting from this type o f ' subsidence ' was est imated by Reece (1980) to be about s million. Mos t of this cost can be attr ibuted to the drought of 1975-6.

Present address: NCB Opencast Executive, Southern Area, Framwellgate Moor, Durham City DH1 5ER

1986 The Mineralogical Society

236 R. K. Taylor and T. J. Smith

TABLe 1. Average mineral composition ofmudrocks (world-wide).

Yaalon Shaw & Weaver Pettijohn (1962) (1965) (1975)

Clay minerals 59 66.9 58 Quartz 20 36.8 28 Feldspar 8 4.5 6 Carbonates 7 3.6 5 Iron oxide 3 0.5 2 Organic carbon - - 1.0 - - Miscellaneous - - 0.2 - -

Mineralogy of mudrocks and clays in Fig. 2

North American United Kingdom (Tertiary-Upper Cretaceous) Smectite 141 301 Mixed-layer clay 14 3

lllite 23 73 8 Kaolinite 21 19 Chlorite 1 0 Quartz 20 32 Feldspar 1 3 Carbonate 4 3 Pyrite 2 tr Organic carbon I 1

60

tr = trace

On a world-wide basis argillaceous sediments make up about 60% of the stratigraphical column, with clay minerals accounting for up to two-thirds of the constituents (Table 1).

Part of the volume change behaviour of clays and mudrocks is controlled by the physico-chemical properties of constituent clay minerals. In this respect it is the more common expandable minerals which are of greatest importance--smectites and mixed- layer illite-smectite clays (see e.g. Grim, 1962; Taylor & Cripps, 1984). These 'active' clay minerals are reduced quantitatively by burial diagenesis, the depth profiles (Fig. 1) being remarkably similar in both the UK and the USA. Post-burial modifications are controlled by: (i) temperature or heat flow rates as a function of burial depth, (ii) porewater chemistry, and (iii) (geological) time. Progressive iUitization of smectite below ~500 m increases the mechanical stability of the rock by removing expandable layers from the clay mineral structure. Kaolinite decomposes with depth, whereas the content of neoformed chlorite increases at high diagenetic levels. Relative abundances shown in Fig. 1 are indicative of the above changes.

The dominance of a geotechnically more inert clay mineral assemblage in older rocks means that intraparticle swelling due to expandable types is reduced. Fig. 1 suggests that rocks older than Silurian age in the UK, or Upper Mississippian age in the USA, will fall within this category. Universal mechanisms which promote the swelling and shrinking of clays and mudrocks are considered in this paper. Moreover, since the combination of shrinking and swelling leads to the disintegration of mudrocks under climatic fluctuations

Swelling, shrinking and mudroek breakdown 237

after Shawp 1981)

PLEISTOCENE

TERTIARY

U.CRETACEOUS L. CRETACEOUS U,JURASSIC MoJURASSIC L.JURASSIC TRIASSIC PERMIAN Uo CARBONIFEROUS

L, CARBONIFEROUS

DEVONIAN SILURIAN ORDOVICIAN CAMBRIAN PRE- CAMBRIAN

UNITED KINGDOM

% CLAY MIN. TYPES 20

/ / / / / / ' J I I : : ,. '. �9 %.

Y////I/'/'///I r.. :. "" I

NO DATA GIVEN

EXPANDABLE CLAY MINS. ~ KAOLINITE

I I ILLITE ~ CHLORITE

UNITED STATES OF AMERICA

% CLAY MIN, TYPES (after Weaver 11967)

NO DATA GIVEN PLEISTOCENE

~ / / / / M MIOCENE

/111 r!i..;;i:.. ' U.CRETACEOUS . . . . . Lo CRETACEOUS

.;.:. ! JURASSIC

P ' i / / / A I PERMIAN ; / ' / / ~ ~" ~" ;~;. PENNSYLVANIAN r / ' / / I / /~ t: " - U.MISSISSIPPIAN

I , .~ . L. MISSISSIPPIAN .~ . DEVONIAN -- SILURIAN

~ ] ~--. ORDOVICIAN " CAMBRIAN

~ . ~ PRE-CAMBRIAN

ATTAPULGITE

FIG. 1. Changes in clay minerals with geological age. (Note: U. Carboniferous not exactly equivalent to Pennsylvanian.)

which promote desiccation followed by saturation, factors controlling physical breakdown are also discussed.

Attention must always be paid to volume changes which result from chemical weathering and other processes. For example, pyrite oxidation and sulphate formation (Moum & Rosenqvist, 1959; Penner et al., 1973), excess porewater pressures in foundations (Penman, 1978, fig. 7.2) and the effects of frost and permafrost (Andersland & Anderson, 1978) can give rise to severe ground movements. These processes are not considered in the present paper.

S A M P L E S

Mainly original data are used in the paper to demonstrate the swelling, shrinking and breakdown characteristics of British and North American mudrocks and overconsolidated clays. Information on expansive properties of British clays is supplemented by data from Driscoll (1984) and on Coal Measures mudrocks by Taylor & Spears (1970) and Taylor (1984). The semi-quantitative clay mineralogy of those samples which are new to the literature is illustrated in Fig. 2 in (relative) terms of a three-component system: 7/~ clay (kaolinite plus minor chlorite), 10 /~ clay (illite), and expandable clay (which includes smectite and mixed-layer illite-smectite). Details of the X-ray diffraction techniques used in the analyses are given by Smith (1978).

The UK sediments range from London Clay (Lower Eocene) to roof and floor rocks of coal-seams, of Coal Measures age. North American examples are all from Tertiary or


Upper Cretaceous formations. It will be observed that the centre of gravity of the latter samples lies towards the right-hand side of Fig. 2, whereas British samples are grouped more centrally. However, both the Cretaceous (FE23) and Jurassic (FE19) fuUer's earths from the UK are Ca-smectites, whilst the Fox Hills Shale, Pierre Shale (Dakota) and Dawson Shale from the USA contain 56%, 56% and 40% expandable clay minerals (absolute). Shown separately in Fig. 2 is the clay mineralogy of Coal Measures rocks

7/~(KAOLINITE PLUS MINOR (~HLORITE)

~IR'- Wy rley / \Yard tonstein

WRo ~K6

:)KM

// L36 .~Ip. Harvey \ P a r k ; LIO

u O %NIJ 2 ..~ High Hazels

GC4 00C44 0 FTR oOC I0

oLCI 4 oKCIO SWR 0LC37 0 +Brooch �9

o F T S ~,YC j~ '~tN3 "qtPStafford ~

tonstein

DS PSD �9 FOX ~FEI9

OFE23 I0/~ (ILLITE) EXPANDABLE CLAY

( Smectite+ Mixed-layer ) O + UNITED KINGDOM

�9 NORTH AMERICA FIG. 2. Three-component clay mineral variations in United Kingdom and North American argillaceous rocks. United Kingdom: LC14, LC37--London Clay, Regent's Park, London. GC4--Gault Clay, Letchworth, Herts. FE23--Cretaceous fuller's earth, Redhill, Surrey. WC3--Weald Clay, Warnham, Sussex. KC10--Kimmeridge Clay, Portland Naval Base, Dorset. OC10, O C 4 4 ~ x f o r d Clay, Calvert, Bucks. FE19--Jurassic fuller's earth, Coombe Hay, Bath, Avon. L10, L36--Lias Clay, Empingham, Leics. KM--Keuper Marl, Cardiff Docks. SWR--Swallow Wood seam roof shale, Scalm Park, Selby, Yorks. FTR, FTS- - Flockton Thin seam roof and floor, Selby Common, Yorks. WR--Widdrington roof shale, Chesterhouse Farm, Northumberland. Other Coal Measures rocks: Brooch and Park seatearths--Littleton Colliery. High Hazels roof shale--Warsop Colliery. Harvey seatearth-- Fishburn Colliery. Stafford tonstein--Stafford Colliery. Wyrley Yard tonstein--Hilton Main Colliery. North America: YC--Yazoo Clay, Jackson Formation, Jackson, Miss. K6, KS--Kincaid Shale, Midway Formation, Cooper Dam site, Texas. N1, N2, N3--Lr. Nacimiento Shale, Puerco Unit, San Juan, New Mexico. FOX--Fox Hills Formation, Lontree Damsite, N. Dakota. D5--Palaeocene Dawson mudstone, Chatfleld Damsite, Colorado. PSD--Pierre Shale (Campanian), Oahe Dam, S. Dakota. PSC--Pierre Shale, Fort Carson,

Colorado.


described by Taylor & Spears (1970) and Taylor (1984). Four of these are susceptible to water and pose problems on coal-faces in UK mines: Harvey, Brooch and Park seatearths and the High Hazels roof shale. Another Coal Measures mudstone which is literally explosive in water is the Stafford tonstein (an 11 A mixed-layer clay) derived from volcanic fall-out. In contrast, the Wyrley Yard tonstein is kaolinite-rich. The Keuper Marl sample from Cardiff Docks (Fig. 2) contains 6% swelling chlorite and the Weald Clay from near Horsham is strictly a softened siltstone from the near-surface zone (55% quartz). The Kincaid Shale specimens from the Cooper Dam in Texas are also high in quartz. Kaolinite contents of the Kincaid specimens are also relatively high, as is that of the Widdrington shale (55%) from the Northumberland Coal Measures. Kaolinite is abundant in mining wastes from the National Coal Board's North East and Scotland Areas (Taylor & Spears, 1970). Most of the well-known British overconsolidated clay and mudrock horizons form the central grouping in Fig. 2, together with three North American samples.

S W E L L I N G M E C H A N I S M S

Bolt (1956) conveniently sub-divided swelling processes into (i) mechanical, and (ii) physico-chemical. In general, the larger void spaces in a clay or mudrock are involved in mechanical responses such as capillarity (Terzaghi & Peck, 1948), whereas smaller voids and pores are important in 'clay mineral' (physico-chemical) swelling. Pore-size ranges of UK clays and mudrocks determined by mercury injection are given in Table 2. The grouping together of the two indurated mudrocks of Lias and Carboniferous ages, and the significant difference in the size distribution of the fuller's earth, with voids larger than 1 gm, are notable features.

The place of expandable clay minerals in the volume change processes has already been commented upon generally. Free swell data given in Table 2 indicate that smectites imbibe water to give the greatest (physico-chemical) expansion, whereas illite and kaolinite show

TABLE 2. Pore-size distributions by mercury injection (as percentage of total porosity). Tests by C. H. Chuay, Durham University.

Diameters less than (nm) Sorting

d2o dso dgo Skewness index

1 London Clay 18-0 (Warden Bay, Isle of Sheppey)

2 London Clay 18.4 (City)

3 Fuller's earth 9.9 (Woburn, Bedfordshire)

4 Oxford Clay 18.6 (Oxford City)

5 Lias clay 10.4 (Near Minehead, Somerset)

6 Coal Measures mudstone 10.8 (Nun Monkton, North Yorkshire)

62.2 206.0 0-958 3.38

61.2 185.2 0.910 3.17

27.6 1136.0 14.600 10.70

82.0 312.0 0.863 4.10

19.2 50.0 1-410 2.19

198 60.0 1.650 2.36

Sorting index = (dso/d2o) ~ Skewness = (dso. d2o)/(dso) 2


TABLE 3. Clay mineral expansion (after Shamburger et aL, 1975).

Clay Average free-swell (%) Range (%)

Na-smeetite 1500 1400-1600 Ca-smeetite 102 65-145 Illite .89 60-120 Kaolinite 28 5-60

progressively less susceptibility. Vermiculite and swelling chlorites do not usually exhibit volume changes to the same extent as smectites, and halloysite expansion may be of the same order as illite. Table 3 illustrates the importance of the Na-cation insofar as smectites are concerned, and this is considered again later in relation to deeply buried British Coal Measures rocks from the zone of chloride-rich groundwaters.

The size disparity between larger, inter-aggregate voids and smaller intra-aggregate voids within clay mineral domains and between clay minerals themselves (Delage & Lefebvre, 1984), helps to establish the two-component swelling model for precompressed natural clays and mudrocks.

Mechanical swelling

Mechanical swelling occurs in response to elastic and time-dependent stress unloading, which in practice can be brought about by man in digging excavations, or by nature in tectonic uplift and erosion. Relaxation of the mean stress,/~, in a clay sets up a negative (suction) pressure, u s, in the porewater of about the same magnitude as the mean stress, viz:

Us ~ p = 1/3(a'v + 2a~) (1) where:

o" is the vertical effective stress, ol o{ is the horizontal effective stress, o; I see Fig. 4 o~ is assumed to be equal to o~.

In stiff overconsolidated clays and mudrocks the in situ horizontal stresses have been found greatly to exceed the vertical stresses in the near-surface zone (see Fig. 3). The values which are shown in the figure as a stress ratio K0(= o~/a') have been extracted from a large number of literature references.

It is generally accepted that the horizontal stresses develop during unloading because the clay is free to expand in the vertical direction but not in the horizontal direction*. In other words, vertical load shedding is accomplished more readily than the release of the horizontal effective stress (Bjerrum, 1967).

Equation 1 can thus be modified to allow for K0, viz, ifKo = a~/o':

u s ~/3 = 1/3(1 + 2K0) a" (2)

* The higher values Of Ko approach Kp, the passive earth pressure coefficient. Some authorities conclude that the resultant shear failure is evidenced by slickensides found in the near-surface zone.

0

IG E

.J W.l > uJ

"~ 15

Z

o r r

?~ 2O o w o0

-r" I , -

a. 25 e~

Swelling, shrinking and mudrock breakdown

LATERAL EARTH PRESSURE RATIO, K o

1 2 3 4

0 0 0 0

0 0 0 0 - 0 0 0

| 174 tD 0 OD 0 0 O 0

�9 0 � 9 0 O 0 0 t ~ O 0

- 0 0 0

0 �9 0 0 O 0 0 �9

0 0 0 0 0

-- 0 0

�9 � 9

0

0

O

IN SITU MEASUREMENTS

LONDON CLAY O 3C KEUPER MARL �9

FIG. 3. Horizontal stresses near to the ground surface. Data from large number of references. Shown as stress ratio K o (horizontal/vertical effective stress).

1

241

Chenevert (1970) demonstrated that the same general principles apply to mudrocks. Equation 2 is used in the example shown in Fig. 4 to calculate the likely value of suction

in a specimen taken from 8 m below the watertable. This sample would imbibe water and swell in a consolidation cell if the total overburden pressure (in this case, a = 200 kN/m 2) were applied to it in the laboratory, i.e. the calculated swell pressure would be equal to 80 kN/m 2.

Inspection of equation 2 shows that the porewater pressure deficit is sensitive to the K o value. However, mechanical swelling is dependent on stress release, and in this respect Driscoll (1984) cites heave-resistant foundations being unnecessarily designed on the basis of laboratory swelling tests for conditions such as those illustrated in Fig. 4.

The equilibration of depressed pore pressures was assumed by Vaughan & Walbancke (1973) to obey conventional Terzaghi consolidation theory. Lutton & Banks (1970) also

242 R . K . Taylor and T..1. Smith

EXAMPLE: SUCTION DEVELOPED IN SPECIMEN FROM 1Om

L E T : K o a t l O m � 9 Unit weight of clay = 2 O k N / m 3 Unit weight water ~___lOkN/m 3

O';= O ' - U , where: (3" is the total overburden stress U is the pore water pressure

At m depth

(3" = 1 0 = 2 0 = 2 0 0 k N / m 2 U = 8 = 1 0 = 8 0 k N / m 2 O~= 2 O O - 8 0 = 1 2 0 k N / m 2 U I . ~ 1/3(1+ 2Ko~O~ where U s is the suction Pressure

+ 6) 1 2 o 4"- 8 0 k N / m 2

m

CLAY lOm 8m "o-

" I l E

6

FIG. 4. To show estimation of porewater pressure deficit (suction) in specimen retrieved from below watertable.

showed that the rate of swelling of Cucaracha Shale along the Panama Canal was in line with laboratory test predictions. In the former case, delayed failure in London Clay cuttings was attributed to slow pore-pressure equilibration. The delay would seem to be up to 50 years in 7-10 m deep cuttings and over 10 years in cuttings of less than 4.5 m depth. In the case of the Panama Canal, reduced porewater pressures were recorded in strata below canal level some 55 years after it was opened to shipping.

Equilibration time is dependent on field permeabilities as recognized by Vaughan & Walbancke (1973). More recent work by Mesri et al. (1978) has shown that in samples remoulded to exclude discontinuities and any preferred flow channels, the Terzaghi one-dimensional theory correctly predicts the shape of the percent swell-log time curve only up to 60% primary swell. Observed excess negative pore pressures equilibrated faster than predicted by the Terzaghi theory and, in tests on reconstituted Cucaracha and Bearpaw shales, 50% of the excess negative pore pressure dissipated 30 times faster. In fact, in the latter specimens pore pressures eventually became positive, which Mesri et al. (1978) interpreted as being shear induced.


Physico-chemical swelling

In standard consolidation tests the gradient of the void ratio-log effective pressure curve is defined as the compression index, C c. The gradient of the corresponding rebound curve is the swelling index, C s. Bolt (1956) pointed out that the ratio Cs/C ~ for pure clays and other colloids is close to unity, but that in natural clay soils values are much lower. For the UK clays in Fig. 2 (London to Lias Clay) the average ratio is 0.330 + 0-065, whilst for USA clays and mudrocks in the same figure it is 0.347 + 0.069 within the vertical loading range 1868-35025 kN/m 2. Thus, even in a general way this ratio is not really indicative of the relative importance of physico-chemical forces in the swelling of natural clays.

Terzaghi's (1931) view was that physico-chemical activity only occurred after water had entered the system in response to mechanical causes. Nevertheless, it was Terzaghi's principle of effective stress which Lambe (1960) extended to provide the link between all the forces which exist in a clay-electrolyte system subject to unloading. Sridharan (1968) examined Lambe's equation and concluded that the average intergranular (effective) stress, c, may be formulated as follows:

= ~a m = rr - t/w - fia - R + A (3)

where

b = effective contact stress b = mineral to mineral contact stress a m = fraction of the total interparticle contact area that is mineral to mineral contact rr = externally applied pressure on unit area (total pressure) h W = effective porewater pressure h a = effective pore air pressure R = total interparticle repulsion divided by total interparticle area A = total interparticle attraction divided by total interparticle area. If the clay is fully saturated then equation 3 may be expressed as:

= rr' + a " (4)

where a ' is the conventional effective stress = a - u and a " is the nett electrical attractive force (or intrinsic effective stress) = A - R.

Physico-chemical swelling is governed by repulsive forces, R, between clay minerals, since attractive forces, A, are small by comparison within the range of external loadings customarily involved in, for instance, ground engineering.

The interaction between clay mineral double layers (Fig. 5) is the primary generator of repulsive pressures. In the double-layer model there is a balance between coulombic electrical forces and thermal diffusion which largely excludes exchangeable cations from the inner, tightly-bonded water layers which total about 1 nm in width (Fig. 5). The cations which are attracted to negatively charged external surfaces of clay minerals and to internal surfaces of expandable minerals are exchangeable, so clay mineral properties can be changed. Importantly, the cations of the diffuse part of the double layer in Fig. 5 originate in the free porewater which is outside the influence of clay minerals. The order of replacement ability of the principal cations found in nature is the same as their abundance (Ca 2+ > Mg 2+ >> K + > Na,+).

At spacings below 1-5 nm there is a nett attraction between small particles due mainly to van der Waals' forces. The cations in this case do not form separate double layers but are


oT E : I- z uJ

"Free" porewater 0

o DISTANCE

DIFFUSE DOUBL E LAYER

--->1

j j

C UP TO ABOUT

4 0 n m

OVERLAPPING DIFFUSE DOUBLE

t~ LAYERS r ~

. , .J sg~ V' b water N I

O , l e e i ~ negatively

/ j char9 ed c lay platelet

Excess cation x concentration ~t due to overlap i

.=J = v ,

DISTANCE 1.5nm

Q Dipolar water molecule

Hydrated cation - - Anion

Note: strongly bonded water layer--c. lnm

FIG. 5. Model of double-layer (osmotic) swelling of two clay mineral platelets. Double-layer formed by negatively charged mineral surfaces attracting cations and polar water.

uniformly distributed between any two clay platelets. When the spacing is greater than 1.5 nm, which is beyond the influence of van der Waals' and other surface forces, separate diffuse ion layers form and the minerals will exhibit a nett repulsion.

In Fig. 5 it can be seen that the double layers overlap so that there is an excess cation concentration between the parallel platelets (= x). Consequently, equilibrium is restored by (free) water being drawn into the system from adjacent voids and capillaries. Geological unloading from high overburden pressures will undoubtedly promote this type of osmotic swelling since an 'out of balance' cation condition will be created and water will be drawn into the system to restore equilibrium.

Swelling pressures can be calculated from double-layer theory if a number of simplifying assumptions are made (see e.g. Bolt, 1956; Sridharan & Jayadeva, 1982). Calculations are based on the Van't Hoff equation and a parallel plate model illustrated in Fig. 6; osmotic pressures generated in randomly orientated clays are generally lower (Yong & Warkentin, 1975). The swelling pressure in the parallel plate configuration is essentially equal to the electrical repulsive pressure (R in equation 3) since no allowance is made for any attractive pressure in the calculations. The total fluid pressure at the mide-plane between the platelets is therefore the sum of the swelling pressure (-"-R) and the porewater pressure, u w, as measured by the piezometer in the larger voids and capillaries of Fig. 6.

Electrolyte concentration, cation valency, temperature and dielectric constant all play their part in physico-chemical swelling. In practice, cations of low concentration generate

Swelling, shrinking and mudrock breakdown 245

l UW= Pore water pressure

at B Overlapping double layers /

' ' , . V ' " 1 4 - 6 �88 # i , I - -

";F-I: i - -------- :--I e" porewater

Negatively charged clay mineral platelet

Osmotic pressure at A= p = RTC B (CA + C..~B - 2 ) c B c A

Where : R = gas constant T = temperature in kelvins Ca--concentration of cations in porewater C A=concen t ra t ion of cations in central plane d . .ha l f distance between parallel clay platelets

FIG. 6. Parallel plate model to show total fluid pressure.

the most extended diffuse layers and therefore greater swelling pressures. In contrast, an increase in valency and/or concentration will reduce swelling--see e.g. fig. 2.21 in Yong & Warkentin (1975). Cation exchange has been adopted in a number of locations in an attempt to stabilize expandable clays and mudrocks. For example, a blanket of lime was used to try to combat slips in mudrocks containing Na-clays at some embankment locations on the Panama Canal. At the other extreme, increased swelling has been promoted by the repeated watering of saline soils with solute-free water (Richards, 1967).

Not all clay minerals are subject to physico-chemical (osmotic) swelling, particularly those of larger grain size. Mitchell (1960), who performed compression tests on Na-saturated kaolinite, illite and smectite, concluded that double-layer theory is not relevant to clay mineral sizes greater than 0.2-1.0 pm diameter. Kaolinite (Olson & Mesri, 1970; Sridharan & Venkatappa Rao, 1973), like granular materials (Lambe & Whitman, 1969), is dominated by the mechanical concept of rebound, with exchangeable cations in aqueous solution having little or no effect. According to Olson & Mesri (1970), the behaviour of illite is dependent on the cation present. The Na-form is predominantly controlled by physico-chemical swelling, whereas the Ca-form shows that both types of mechanism are involved.

Consolidation tests carried out in 'non-wetting' fluids of low dielectric constant will suppress expansion of smectites. Sridharan & Venkatappa Rao (1973) found that smectite


and kaolinite showed very nearly the same magnitude of rebound under these conditions. Although this effect demonstrates that smectite expansion is predominantly a physico- chemical phenomenon, only Na-smectites expand according to predicted Gouy-Chapman double-layer theory. The reason for this can be explained by the observation that Ca-smectite crystallites compact to form irreversible domains (Aylmore & Quirk, 1959) which cannot expand to spacings greater that 19-20 A. 'Dead volume' corrections (Blackmore & Miller, 1961) are therefore necessary. MacEwan (1948, 1954) and Norrish & Quirk (1954) concluded that the weaker electrostatic forces in monovalent Na-smectites (as opposed to stronger forces in Ca-smectites) allow the double layer to have a greater effect.

Reasonable agreements between measured and predicted swelfing pressures developed by pure clays were reported by Verwey & Overbeck as long ago as 1948. More recently, Madsen (1979) reported compatibility in Alpine mudrocks.

C L A S S I F I C A T I O N OF E X P A N S I V E C L A Y S

Skempton's (1953) activity concept has been used for many years as a means of assessing the expansiveness of clay soils. The classification chart shown in Fig. 7 is after Williams and Donaldson (1980). In order to accommodate UK overconsolidated clays and mudrocks it has been extrapolated, because many of the well-known clays lie to the right of the conventional chart.

On the chart are shown UK clays and mudrocks from Fig. 2, including some of the problematic Coal Measures types depicted by their own symbol in the latter figure. Unfortunately, basic data were not available for all of the latter specimens. However, the number of points on the chart have been extended by including Driscoll's (1984, table 10.3) data for well-known clay beds.

Not all specimens in Fig. 7 would appear to show their maximum expansive potential and this is especially so for a number of the indurated Coal Measures mudrocks. There is always a problem in determining clay sizes (<2/~m) in these materials, although for the samples shown ultrasonic methods were used to improve disaggregation. In the present context, however, a larger error concerns the Atterberg limits from which the plasticity index is calculated. The standard tests (BS 1377: 1975) are conducted on the less than 425 #m size fraction. A more satisfactory prediction of expansiveness is obtained if the limits are determined on sediment washed through a BS 200 sieve (<75 #m). In this way sand-sized aggregated particles are excluded and the limits are a better reflection of the fundamental particles in the rock.

The expansiveness of the Park and Brooch seatearths are almost certainly underestimated in Fig. 7 (see uplift pressures in Table 6; also Taylor & Spears, 1970). Unfortunately, these seatearths were investigated many years ago when only standard-size material was used for Atterberg limit determinations. However, the change in class illustrated in Fig. 7 by the vertical arrows and blocked-in symbols is striking for a number of mudrocks and clays for which modified limits were obtained. Kimmeridge, Oxford, Lias clays and Carboniferous mudrocks are seen to move up one class and in some cases, two. The kaolinitic Wyerley Yard tonstein, and broadly, the Weald Clay (softened siltstone) are in their correct class (low expansion). Driscoll (1984) shows that the London Clay specimen in the 'low expansion' class of Fig. 7 is strictly a 'highly expansive' type when

8 0

7 0

6 0

5O tll r~ z

>- 40

~, 3o Q,

2 0

10

Swelling, shrinking and mudrock breakdown

9

6'

R"

247

O �9 Overconsofidated clays El �9 Carboniferous mudrocks

. ~ Car boniterous mudrocks subject to swelling tests

OGC

r ~-. FE19 J VERY HIGH ~FE23 / I EXPANSION 0 GC ~ .O/STAFFOR D - - (~ FE23 / TONSTEIN

LC _OKC / / I 0 "~ / ; 0 ULC14 . / .~'t ~

/ I llOClO / -" .t\q ~' / ; T , o , o Gc4 ' / . I / _ ~ O '

/ ' / ~ O ,,,~L1/~ ~,, ,"-- / ! / 4_ WC~ v / .|.,,BROOCH "~

I , A IPoc I / ~ " / ' / ,c %y,, .T , , / .-

,,4 l / F,S K C , O 0 / e , L36 . .. M l i ~, / / & , o

I ~SWR ! / J L o / ',FEX;;"s,o 3I /, i / 0c44(~,~Ii FIR /

/ !I,,,EO,U'., i I T .... / ; I EXPANSION I / ~ ]_ ~ PARK ~ 1

/ �9 - - - - ~ . . , , . e,,R &WR ~-YA.O

[ LOW EXPANSION I

' ' ' ' ' ' r 8'o ' ,o =o ~o , o ,o ~o 90 PERCENTAGE CLAY CONTENT

FIG. 7. Classification of UK overconso]Jdated clays and mudrocks using (extended) chart of Williams & Donaldson (1980). Arrows indicate change in activity when modified limits procedure used. Sample identification as in Fig. 2. Results for specimens with no depth

indication taken from Driscoll (! 984),

classified according to the Building Research Establishment (1980) 'shrinkage potential' table.

The shrinkage potential classification as a measure of expansiveness is shown at the head of Table 4. Also classified in the table are specimens from Fig. 7, together with the Tertiary and Upper Cretaceous mudrocks from N. America (see Fig. 2).

For most UK clays and mudrocks this classification would seem to provide reasonable predictions using standard data. Although the Park seatearth's expansiveness may still be underestimated, both it and the Brooch seatearth are at least raised to a higher class more in accord with their heave potential (Table 6). The modified limits procedure tends to specify the maximum class within the range indicated by the standard test. However, in the case of certain N. American shales (Dawson and Pierre Shale, Dakota) a higher class is signified. Indurated kaolinitic mudrocks with high clay size contents would appear to be somewhat anomalous. For example, the Widdrington roof shale and the Wyrley Yard tonstein both indicate a range from 'low' to 'high' potential using standard data. Modified procedure moves the Widdrington shale into the high shrinkage category. Their true expansiveness is more correctly represented in Fig. 7 (low expansion).


TABLE 4. BRE clay shrinkage potential applied to mudrocks from the UK and USA (sample details given in Fig. 2).

BRE (1980) CLASSIFICATION Plasticity index (P) Clay fraction (CP)

% % Shrinkage potential

>35 >95 Very high (VH) 22-48 60-95 High (H) 12-32 30-60 Medium (M) < 18 <30 Low (L)

Plasticity Plasticity Clay index

index fraction Shrinkage (slaked Shrinkage (BS tests) (<2 gm) potential 75 gm) potential

UNITED KINGDOM London Clay (LC 14) 53 65 H-VH NSC - - London Clay (LC37) 40 53 M-H NSC - - Gault Clay (GC4) 46 60 H-VH NSC - - Fuller's earth (FE23) (Redhill) 56 69 H-VH 63 H-VH Weald Clay (WC 3) 12 32 M NSC - - Kimmeridge Clay (KC 10) 36 57 M-H 46 M-H Oxford Clay (OC 10) 31 37 M NSC - - Oxford Clay (OC44) 22 46 M 29 M Fuller's earth (FE 19) (Bath) 63 68 H-VH NSC - - Lias Clay (L19) 32 62 H 45 H Lias Clay (L36) 20 65 M-H 37 H Swallow Wood seam roof (SWR) 13 35 M 32 M Flockton Thin seam roof (FTR) 11 48 L-M 23 M Flockton Thin seam floor (FTS) 14 42 M 35 M-H Widdrington roof (WR) 10 64 L-H 22 H Brooch seatearth 41 77 H - - - - Park seatearth 16 60 M - - - - Wyrley Yard tonstein 13 77 L--H - - - -

NORTH AMERICAN Yazoo Clay (YC) 64 85 H-VH 85 H-VH Kincaid Shale (K6) 41 68 L--M 37 L-M Kincaid Shale (K8) 25 16 L-M 37 L-M Nacimiento Shale (N 1) 11 21 L NSC - - Nacimiento Shale (N2) 21 54 M NSC - - Nacimiento Shale (N3) 34 34 M-H 61 M-VH Fox Hills Shale (FOX) 20 61 M-H NSC - - Dawson Shale (DS) 26 63 H 51 H-VH Pierre Shale (PSD) (Dakota) 79 71 H-VH 106 H-VH Pierre Shale (PSC) (Colorado) 33 47 M-H 45 H

NSC = No significant change

P r e s e n t conc lu s i ons a re t h a t B R E ' s (1980) c lass i f ica t ion p rov ides s a t i s f a c t o r y

p r ed i c t i ons for o v e r c o n s o l i d a t e d c lays a n d m o s t m u d r o c k s . F o r m u d r o c k s wi th a p las t i c i ty

index o f less t h a n 18 a n d a h igh c lay size c o n t e n t , Fig. 7 is poss ib ly m o r e mean ingfu l .


S H R I N K A G E

The combination of heavily overconsolidated clays and high soil moisture deficts in eastern and southeastern England leads to desiccation and high suctions in the surface layers. Discontinuities as well as macro- and micro-voids are involved, so there is a capillary component as well as an osmotic component in shrinkage phenomena.

It is customary to measure suction on a logarithmic pF scale, where pF 1 ~ 1 kN/m 2, pF 3 (the negative equivalent of atmospheric pressure) _~ 100 kN/m 2 and pF 7 is the equivalent of oven-drying (~ 105 kN/m2).

Suction pressures in the range pF 0 to 3 which are plotted against moisture content in Figs 8 and 9 were measured with the suction plate apparatus (Croney et al., 1952), whilst pressures between pF 2 to 5 were obtained by the pressure-membrane method (Croney et al., 1958). It was necessary to use a fresh undisturbed specimen for every determination, and test discs of indurated and brittle rocks prone to slaking had to be retained peripherally.

Indurated mudrocks with a small content of expandable clay minerals have suction-moisture content curves which are very different from those of clays and mudrocks with a high proportion of expandable minerals. The curves of indurated mudrocks shown in Fig. 8 have characteristics of both incompressible granular soils and compressible clays. Both the wetting and drying curves of mudrocks in Fig. 8 are sigmoidal in shape. This can be accounted for by the pores being non-uniform in size and shape (Childs, 1969). Those with large channels will empty at low suctions, whilst those with narrower channels of entry will not empty until larger suctions are applied. Similarly, the marked hysteresis of wetting and drying curves depends on the irregularity in shape of porespaces which may be viewed as comprising larger voids connected by narrower channels. The greater the disparity between the size of the void and that of the channel, the more marked is the difference between suctions of emptying and refilling (e.g. greater hysteresis shown by High Hazels shale compared with Flockton Thin roof (shaded)).

The vertical section of the drying curves shows that quite large suctions can be applied to the porewater without any change in moisture content. This means that one can usually expect fine-grained mudrocks at outcrop to retain water and remain saturated. The difference between the chalk specimen and the mudrocks in Fig. 8 is that once drainage and air entry commences in the former case it drains very rapidly. Although the siltstone specimen empties at around pF 5 its shape is more like the roof shales in which the shape is controlled by shrinkage and air entry at higher values of suction. Hence, the chalk specimen has the characteristic shape of an incompressible material (cf. fine sand in Fig. 9), whereas the mudrocks from pF 2 and upwards are more akin to compressible clays.

When the UK and North American clays and mudrocks are plotted together (Fig. 9, wetting curves only), the indurated mudrocks and quartz-rich Weald Clay group together in the low-moisture content area of the figure. At the other extreme are the Stafford tonstein and Pierre shale (Dakota) with high expandable clay contents and saturation moisture contents >>80%. They have no vertical stem and display the characteristics of compressible materials with shrinkage and air entry occurring over the entire moisture content range.

A general assessment of desiccation potential can be gained from Fig. 9. It has already been shown (Fig. 8) that changes in moisture content start to occur at suctions >pF 2. This 'indurated mudrock' standard can be applied as a lower limit to all rocks and clays in Fig.


I'ION CURVES |UDROCKS --

LOCKTON THIN 5 :IOOF SHALE

I,i,. o. HIGH HAZELS UPPER

J ROOF (TAYLOR 1984) r 4 t/) t,/)

"v@' . . . ~ J HARD CHALK ~" 3 ~. ,~ .. ~ ~" '~ ; \ (LEWIS AND CRONEY _o m85) t- O ~... ,re-\ i �9

" k \

I "

I

, i ' i

I ~ I I I J ' I * i

0 2 8 10 14

MOISTURE CONTENT, %

FIG. 8. Suction curves of indurated mudrocks and chalk. (Note: rising arrow = drying curve; falling arrow = wetting curve.)

9. If the moisture content change (and hence volume change) is noticeable at a higher suction then desiccation potential may be deemed to be significant. For low-rise structures (conventional houses) the moisture content change between pF 2-3 is probably appropriate, since a suction release equivalent to pF 3 (100 kN/m 2) would cause the structure to heave. Fig. 9 indicates that within this range the moisture content change (and therefore shrinkage potential) generally increases from left to right across the diagram.

M U D R O C K B R E A K D O W N

Cycles of desiccation followed by saturation from precipitation lead to shrinking and swelling to give mudrock breakdown in the near-surface zone. In other words, weathering processes start at the ground surface. The resultant physical disintegration of mudrock is


4

w

~3

z 0 I - g2 03

WETTING CURVES FOR BRITISH AND NORTH AMERICAN MUDROCKS AND CLAYS

10 2 0

)C44

LC3

30

FTR - FLOCKTON THIN ROOF FTS- FLOCKTON THIN SEATEARTH

WC - WEALD CLAY (SILTSTONE)

HH -HIGH HAZELS ROOF I I

DS - DAWSON SHALE K6 - KINCAID SHALE L36- LIAS CLAY

N2 - NACIMIENTO SHALE

LC 37 LONDON CLAY

FOX- FOXHILLS SHALE GC - GAULT CLAY

LIO " LIAS CLAY LCI4" LONDON CLAY YC - YAZO0 CLAY

EEl9- FULLERS EARTH (JURASSIC)

K6 �9 KINCAiD SHALE ST" STAFFORD TONSTEIN

0C44" OXFORD CLAY RSD" PIERRE SHALE ~ (DAKOTA)

4 0 5 0 6 0 70 8 0 9 0

MOISTURE CONTENT %

FIG. 9. Suction pressure (pF scale) versus moisture content curves for undisturbed mudrocks and clays from the United Kingdom and North America (wetting curves: mainly new data).

rapid and, by virtue of increasing the available surface area, physical breakdown can be viewed as a control on chemical decomposition (Taylor & Spears, 1970). Indurated mudrocks undergo a much greater degree of disintegration, to produce a wider range of particle sizes than clays with their weaker diagenetic bonds.

A higher degree of diagenesis, as measured by the rank of the associated coal seam, has been found to restrict the breakdown of Coal Measures rocks in 'end-over-end' slaking tests (see e.g. Berkovitch et al., 1959; Taylor & Spears, 1970). Similarly, disintegration of seven 'ranked' mudrock discards from underground mining activities was shown by Ratsey (1973) to be greatest in those associated with the coals of lowest (diagenetic) rank, i.e. high code numbers 700, 802 and 902.

The physical disintegration of Coal Measures mudrocks has been studied for over 30 years in the UK in relation to the coal washing process. Following the flowslide at Aberfan in 1966 which killed 144 persons, the stability of mudrocks in spoil heaps has also been intensively researched. Relevant literature includes: Badger et al. (1956), Beckett et al. (1958), Berkovitch et al. (1959), Horton et al. (1964), Taylor & Spears (1970) and Taylor (1984).

Arising from these investigations three major controls on breakdown are apparent: (i) the incidence of sedimentary structures and discontinuities, (ii) slaking by air breakage, (iii) swelling of expandable clay minerals, especially when sodium is a significant exchangeable cation. These controls would seem to apply in general to other mudrocks and clays.

Discontinuities in overconsolidated stiff clays fall into two groups, (i) bedding plane surfaces of depositional or diagenetic origin and (ii) structural dislocations (Skempton & Petley, 1967). The latter dislocations comprise systematic joints, non-systematic 'fissures', minor shears and principal displacement shears. Bedding plane separation as a consequence of rebound in mudrocks also produces additional extensional fractures and fissures near to the ground surface (Nichols, 1980).


Varve-like units consisting of light- and dark-grey laminations are also significant contributors to breakdown (Taylor & Spears, 1970), especially in marine strata (Spears, 1969). The floc-type fabric of dark laminations is prone to swelling so that the rock splits along the interface with the adjacent lighter-grey units. Spears (1980) concluded that the fissility found at outcrop in stress-relieved shales is the surface expression of laminations recorded in borehole cores at depth. Since a slow rate of deposition, in the absence of bioturbation, leads to the formation of very thin laminae symptomatic of marine strata, there will be an in-built tendency towards fissility in this environment. More importantly, from an engineering viewpoint, mudrocks are predominantly marine on a world-wide basis.

Slaking has been succinctly described by a number of authors (e.g. Terzaghi & Peck, 1948) and the term 'air breakage' is used for the same phenomenon in the mining industry (Badger et aL, 1956). Referring back to the shrinkage of clays and mudrocks in the previous section, it is evident that as high suctions are developed during dry periods conducive to desiccation, the outer macro-voids and discontinuities in the material will be filled with air. Subsequent saturation causes this air to become pressurized as water is drawn in by capillarity and failure can follow rapidly along a predisposed plane of weakness (invariably a discontinuity). Laboratory tests show that disintegration by air breakage is important in some mudrocks and clays. For example, Taylor & Spears (1970) demonstrated that breakdown can be restricted in some mudrocks if slaking tests are conducted entirely in vacuo. Air may also be expelled explosively in rapid slaking types--fuller's earths, Nacimiento shale (N3) Fig. i0; Stafford tonstein, Fig. 2, Table 6.

Van Eeckhout (1976) observed that humidity fluctuations cause swelling and shrinking on a more restrained scale which lengthen internal cracking; the increased moisture content which is then permitted by newly formed discontinuities helps to reduce the fracture energy required to fail the rock. Compared with measurements at constant humidity, changes in relative humidity had a significant effect in increasing unconfined swelling strain rates in Ordovician borehole cores monitored by Harper et al. (1979).

The 'end-over-end' (dynamic) slaking tests conducted by NCB scientists (e.g. Berkovitch et aL, 1959) did not conform to a first-order decay law, which implies that more than one breakdown mechanism is probably involved. Similar conclusions were drawn by Smith (1978) who used a modified version of the Public Roads Alternative Slaking Test procedure (Hogentogler, 1937) in which the rate of disintegration through a BS No. 36 sieve (425/~m) is measured. By reference to the (almost) linear relationship observed over the initial stages of disintegration in this type of slaking test, both the UK and North American materials shown in Fig. 2 can be divided into three groups:

1. Rapid slaking rate >3.5%/rain 2. Fast-medium slaking rate 3.5-0.5%/min 3. Slow slaking rate <0.5%/min

These boundaries are shown in Fig. 10 where the ultimate breakdown percentages are plotted against the rate of disintegration of the linear section. Apart from the Kincaid Shale (K6) all the specimens in the 'rapid slaking' (group 1) part of the diagram contain high amounts of expandable clay (40-100%), principally smectite. Orientation ratios determined by X-ray diffraction (Smith, 1978) indicated that all the specimens in this group, with the exception of the Yazoo Clay, had a randomly orientated clay mineral fabric. White (1961) considered that such a fabric is conducive to breakdown and work by Spears


(I 969) has already been referred to earlier. The high degree of preferred orientation parallel to the bedding in the Yazoo Clay had little influence on its slaking behaviour which would appear to be overriden by intraparticle swelling of the very high smectite component (49% by weight). The mudrocks and clays in the 'rapid slaking' group disintegrated completely under vacuum, which again points to expandable clay being a major breakdown control in these sediments. Of the unstable Coal Measures rocks shown in Fig. 2, de-airing only partly restricted the breakdown of the Park and Brooch seatearths; it was completely ineffective in the case of the Stafford tonstein (maximum 78% mixed-layer illite-smectite (Taylor, 1984)). Here again, intraparticle swelling of expandable clay minerals was concluded to be the principal control of the breakdown process.

Within the 'fast-medium' slaking group of Fig. 10, breakdown was observed to occur in a variety of ways. At the faster end of the scale there was a general loss of structure and up to 90-100% of the material passed through the mesh. The other extreme was typified by splitting along laminations, followed by a slow dislodgement of particles. Specimens in the fast-medium group contained 9-27% expandable clay minerals but there did not appear to be any direct relationship with disintegration rate. Higher smectite contents, however, generally signify the higher final breakdown percentages in Fig. 10. The tendency for a more randomly orientated fabric to be associated with faster slaking was noted but in this group the clay minerals were better oriented than in group 1.

Those rocks falling within the 'slow' slaking category (group 3) exhibited a wide range of behaviour. Lias Clay (L36), Oxford Clay (OC44), Swallow Wood roof (SWR), Flockton Thin roof (FTR) and the Widdrington roof shale (WR) all have a high degree of preferred orientation parallel to the bedding and expandable clay contents of 9-18%. Splitting along planes of weakness did occur but there was little or no tendency for particles to slake to less than 425 /~m mesh size. The Flockton Thin seatearth (FTS), which contains 45% expandable mixed-layer clay and has a randomly oriented fabric, behaved differently. It disintegrated rapidly (within 15 min) into chips and flakes which were generally >425/tm. However, these remained hard and intact for the remainder of the test.

The Weald Clay (siltstone) behaviour was not unexpected, but that of the two Nacimiento Shales was. Both Nacimiento samples had an expandable clay content of 15-17% but the clay fabric of N2 was somewhat more randomly oriented than N1 (orientation ratios*: N2 = 0.82, N 1 = 0.57). At the end of the test 82% of N2 had passed through the mesh compared with only 22% for N 1.

It is concluded that samples in group 3 of Fig. 10 are probably slower to slake because of their generally higher level of clay mineral orientation and diagenetic bonding/ cementation (induration). The mesh size used in the test is of course arbitrary, although it was purposely adopted to retain compatibility with Atterberg limits (e.g. plasticity index in Fig. 7 and Table 4).

The exchangeable sodium percentage (ESP)'i" has been used as a possible guide to the breakdown of mudrocks. For example, Sherard et al. (1972) found it to be directly related to the amount of dispersion in shales, and earlier work suggested that it could be used as a measure of complete dispersion in water of Ca,Na-smectites and Ca,Na-iUites. Emerson (1967) suggested that an ESP value greater than 12-13 is appropriate for dispersion and agricultural scientists generally regard an ESP of > 15 as being of significance.

* Random = 1.0, parallel orientation = 0. ~" ESP = (exchangeable Na/cation exchange capacity) • 100%.


04

(.,'3

0 --" ~"

/ I P:~C , PSD

~ 4 0 10

N 3

,og,, I LFTS I

--'0~4' J . l , I , , , , , , , , I F l n FTR 2 4 6 8 10 12 14 16 18 SWR

L

0

I 0

N 3

T

20 22 24

RATE OF DISINTEGRATION

( % / / m i n )

FIG. 10. Relationship between percentage breakdown and disintegration rate using modified Public Roads Alternative Slaking Test procedure. United Kingdom: LC--London Clay, GC--Gault Clay, FE23--Cretaceous fuller's earth, WC--Weald Clay, KC--Kimmeridge Clay, OC--Oxford Clay, FE19--Jurassic fuUer's earth, L---Lias Clay, SWR--Swallow Wood roof shale, FTR--Flockton Thin roof shale, FTS--Flockton Thin seatearth, WR--Widdrington roof shale. North America: YC--Yazoo Clay, FOX--Fox Hills Shale, D--Dawson Shale, K--Kincald Shale, N--Nacimiento Shale, PSD--Pierre Shale (S. Dakota), PSC--Pierre Shale

(Colorado). (Note: Fuller sample details in Fig. 2.)

Swelling, shrinking and mudroek breakdown 25 5

Exchangeable cations were determined for all specimens shown in Fig, 2, using the ammonium acetate method of Chapman (1965). A disadvantage of the method is that the presence of calcium minerals and fossil debris will commonly enhance the 'exchangeable' Ca figure. A correction has therefore been made for this effect in UK types shown in Table 5, in that the CEC was also determined using methylene blue. Total CEC values were not determined by an alternative method for North American samples so those specimens with abnormally high exchangeable Ca values have been excluded from Table 5.

It will be observed from the table that the two highest ESPs (Gault Clay and Pierre Shale, S. Dakota) are only marginal in terms of the dispersion values mentioned above. Compared with the estimated ESP of the Wyoming bentonite with an elevated exchangeable Na value (Table 5), the Pierre Shale, Dakota is predominantly a Ca-rich type (Na + = 6.2 mEq/100 g; Ca 2+ = 30.6 mEq/100 g).

In contrast to the data in Table 5 it is evident that exchangeable Na is high in the Coal Measures specimens identified as being troublesome or unstable on underground coal faces (see Table 6). Moreover, uplift pressures generated during expansion are seen to increase in line with exchangeable sodium content.

Many Coal Measures mudrocks have high exchangeable sodium ion contents, as well as abnormally high ESPs. Specimens depicted with an asterisk in Table 7 include those used for the uplift pressure measurements reported in Table 6. Referring back to the reference clay minerals of Carroll & Starkey (1958) it is pertinent to note that there are two ESP

TABLE 5. Exchangeable cations in reference clays and exchangeable sodium percentage (ESP) in sediments from UK and USA (all new data except where specified).

Reference clay minerals* Na K Mg Ca (mEq/100 g) ESP

1 Smectite, Wyoming, API 25b 54 ND 15 11 67.5 2 Illite, Fithian, API 35 0 ND 3 17 0 3 Kaolinite, Bath 0 ND 0.4 0.5 0

ESP for United Kingdom sediments, excluding 4 London Clay (LC37) 2.0 5 Gault Clay (GC4) 11.0 6 Fuller's earth (FE23) 1.1 7 Kimmeridge Clay (KC10) 4.0 8 Oxford Clay (OC10) 7.4 9 Oxford Clay (OC44) 4.3

10 Fuller's earth (FE19) 0-6 11 Lias Clay (LI0) 6.0 12 Lias Clay (L36) 5.1

Coal Measures (sample details in Fig. 2).

ESP for selected mudrocks from USA (sample details in Fig. 2). 13 Kincaid Shale (K8) 2.7 14 Nacimiento Shale (N2) 3.6 15 Fox Hills Shale (FOX) 7.6 16 Pierre Shale (Colorado, PSC) 2.1 17 Pierre Shale (S. Dakota, PSD) 13.5

* Carroll & Starkey (1958) ND = not determined


TABLE 6. Uplift pressures in Coal Measures mudrocks (data from Taylor, 1984: see also Fig. 2).

Exchangeable Na Uplift pressure Rock mEq/100 g MN/m 2

1 Wyrley Yard tonstein (kaolinite) 0.0 2 Harvey seatearth 5.3 3 High Hazels immediate roof shale 9.6 4 Park seatearth 12.0 5 Brooch seatearth 10.1 6 Stafford tonstein (11 A clay, mudstone) 14.2

0.008 2.603 4.192 4.451 6.601 9.880

Tests conducted on air-dry rock in small oedometer

values in the Coal Measures group which are higher than that of the Wyoming bentonite, and one (the Brooch) which is not far below. Moreover, the immediate roof shale of the High Hazels seam (High Hazels 1) and the Flockton Thin seatearth both have ESPs of over 50; three other ESP values are between 20 and 30.

With the exception of the Widdrington roof shale there is a major difference in sampling depth between the Coal Measures specimens of Table 7 and the other formations considered in this paper. The former specimens come from deep UK mines where strata waters (brines) are not uncommonly high in sodium (and C1) (Chamberlain & Glover, 1976; Taylor, 1984). The boundary between the groundwater sulphate zone and the underlying chloride zone varies in elevation across UK coalfields. The chloride ion content of strata waters can vary with depth within any one mine, as well as from mine to mine. This may well be the reason why the Little Smeaton mudrock in Table 7 is low in Na + and ESP (albeit a marine shale). The Wyrley Yard tonstein is rich in kaolinite so a low value is to be expected (cf. Bath kaolinite, Table 5). A similar argument can be advanced for the Widdrington shale, but this sample is from relatively shallow depth and may have resided in the sulphate-rich groundwater zone for a considerable period of time.

TABLE 7. Exchangeable sodium percentage (ESP) for Coal Measures mudrocks.

Type ESP

1 Wyrley Yard tonstein (kaolinite) (Hilton Main Colliery) 0 2 Widdrington roof shale (WR) (from 63 m; borehole, Northumberland) 5.6 3 Marine shale (Spears, 1973) (from 294 m; Little Smeaton borehole) 5.2 4 Swallow Wood roof shale (SWR) (from 317 m; Selby borehole) 24.5 5 Flockton Thin roof shale (FTR) (from 593 m; Selby borehole) 22.5 6 Flockton Thin seatearth (FTS) (from 595 m; Selby borehole) 56.7 7 Harvey roof shale (Fishburn Colliery, Co. Durham) 29.1 8 Harvey seatearth* (Fishburn Colliery, Co. Durham) 40.2 9 High Hazels immediate roof shale* (Warsop Colliery) 53.0

10 Brooch seatearth* (Littleton Colliery) 60.8 11 Park seatearth* (Littleton Colliery) 70.6 12 Stafford tonstein* (Stafford Colliery) 51.3 13 Ryder roof shale* (Badger et al., 1956) 75.0

* Rapid slaking in water or troublesome underground


The study of exchangeable cations and associated anions in mining wastes is still in progress and the full implications may not have been resolved. It is clear that ESPs can be spectacularly high in these mudrocks but this does not necessarily appear to be matched by exceptional breakdown behaviour.

C O N C L U S I O N S

Expansion in overconsolidated clays and mudrocks is a function of mechanical unloading and the physico-chemical (osmotic) response of the porewater and the smaller clay mineral components. The bigger inter-aggregate voids and capillaries (Table 3) are largely involved in mechanical swelling, whereas smaller intra-aggregate void spaces (albeit > 1.5/~m size) between clay minerals and within domains are concerned with double-layer swelling. Both processes are governed by effective stress principles; the mechanical swelling by conventional effective stress and physico-chemical swelling by the intrinsic effective stress (essentially the repulsive pressure, R in equation 3).

The porewater pressure deficit (suction pressure) induced by unloading can be increased by the influence of high horizontal stresses measured in overconsolidated clays and mudrocks near to the ground surface. Pore-pressure equilibration time (overall) is probably faster than that predicted by the Terzaghi one-dimensional theory.

Clay mineral swelling is dependent on clay mineral type, the electrolyte concentration and the nature of the cations. A number of workers have shown that the swelling of kaolinite (a large clay mineral) is a mechanical rebound response irrespective of the associated cations. At the other size extreme the enhanced swelling of Na-smectite is primarily physico-chemical in nature. Burial diagenesis removes expandable layers in the clay mineral structure such that inter- and intralayer swelling of expandable clay mineral types can be expected to be at a minimum in rocks older than the Silurian in the UK or Upper Mississippian age in the USA.

Shrinkage and physical breakdown processes are a feature of the partly saturated zone near to the ground surface. Indurated mudrocks with small expandable clay mineral contents require the application of suctions of over 10 kN/m 2 before air entry commences. Their suction pressure/moisture content curves are very different from those of clays and mudrocks with high expandable clay mineral contents, where air entry and shrinkage occurs over the entire suction range (from full saturation to the oven dry condition).

Expansiveness and shrinkage classification systems based on clay size and plasticity index ('activity') pose some problems with indurated mudrocks, in that the predicted behaviour is that of a disaggregated specimen. The rate and degree to which a mudrock disaggregates in nature is dependent on factors such as diagenetic bonding, cementation, fabric orientation, in addition to clay mineral type and content. Slaking test predictions suffer from similar problems since material is customarily slaked through a mesh of fixed size. In the case of volume change predictions for mudrocks, the (South African) chart of Williams & Donaldson (1980) used in combination with the Building Research Establishment's (1980) classification, would appear to give sensible assessments, provided that clay mineralogy is used as a guide. For overconsolidated UK clays the Building Research Establishment's System is appropriate.

The degree of physical breakdown which occurs during cycles of desiccation and saturation (shrinking and swelling) is largely governed by: (i) the incidence of sedimentary structures and discontinuities, (ii) slaking or air breakage, (iii) swelling of expandable clay


minerals, and (iv) clay mineral fabric orientation. Expandable clay minerals, in particular

smectites, can completely dominate breakdown, especially when sodium is the exchange-

able cation. A number of the Coal Measures rocks used in this paper to illustrate the latter

aspect of breakdown behaviour are from coal faces within the chloride-rich groundwater

zone. Consequently, the exchangeable sodium percentage (ESP) is exceedingly high.

However, the resultant breakdown does not appear to be affected disproportionately by this factor. The majority of clays and mudrocks considered in the text were sampled in the

near-surface zone which is of more general interest to ground engineers and engineering geologists.

ACKNOWLEDGMENTS

We are grateful to the following who provided many of the samples used in the investigations: The Fuller's Earth Union, Dr A. B. Hawkins, Messrs T & C Hawksley, London Brick Company, National Coal Board, Waterways Experiment Station, Vicksburg, US Bureau of Reclamation.

REFERENCES

ANDERSLAND O.B. & ANDERSON D.M. (1978) Geotechnical Engineering for Cold Regions. McGraw-Hill Book Co., London.

AYLMORE L.A.C. & QUIRK J.P. (1959) Swelling of clay-water systems. Nature 183, 1752-1753. BADGER C.W., CUMMINGS A.D. & WrnTMORE P.L. (1956) The disintegration of shales in water, J. Inst. Fuel

29, 417-423. BECKETT P.J., CUMMINGS A.D. & WHITMORE R.L. (1958) Interfacial properties of coal measure shales in

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