The Engineering Properties of Mudrocks

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Q. J. eng. Geol. London, 1981 Vol. 14, pp. 325-346.

Printed in Northern Ireland

The engineering properties of mudrocks J. C. Cripps & R. K. Taylor* Department of Geology, University of Sheffield, Mappin Street, Sheffield, S1 3JD. *Engineering Geology Laboratories, University of Durham, South Road, Durham, DH1 3LE.

Summary The 'rock' and 'soil-like' properties of British mudrocks are shown to be influenced by: (a) their lithology; (b) their geological history of loading (especially during exhumation); (c) the type and method of testing; and (d) the degree of weathering. In particular, unloading and weathering leads ultimately to a normally-consolidated clay of much the same undrained shear strength, irrespective of age and origin of the parent material. For this reason, the engineering properties of the unweathered mudrocks are illustrated separately in terms of classification indices, undrained and effective shear strengths and deformability in relation to geological age.

Compaction and degradation of mudrocks Using the gravitational compaction model of Skempton (1964), with diagenetic bonding as postulated by Bjerrum (1967), the formation, and attributes, of 'clay shales' are clearly and simply described by Fleming et al. (1970). Due to the action of these burial-related processes many British sedimentary clays have attained the state of indurated rocks and display engineering characteristics which are a product of their composition, geological loading history and ultimately the degree of weathering. Laboratory consolidation studies indicate that with an increasing sedimentary overload the volume of voids will decrease as pore water is expelled. This is depicted schematically in Fig. 1A and D (a-c). The shear strength of the 'normally consolidated' clay is proportional to the existing overburden load as represented by points b and c in Fig. lB. The rapid decrease in porosity with depth of burial has been demonstrated by Skempton (1970a) amongst others. However, the abnormally high fluid pressures, much greater than hydrostatic, reported from borehole depths even in excess of 4877 m (see Weaver & Beck 1971) would significantly reduce the effective vertical stress. If, as a consequence of uplift and erosion, unloading takes place, the labormory analogy demonstrates that the sediment will become 'over-consolidated' (point d in Fig. 1). Although it is under the same effective pressure as the normally consolidated equivalent (point b) the water content of the over-consolidated

material is markedly less, so that the particles are in a denser state of packing and the shear strength is higher. Table 1 gives an indication of the preconsolidation loads determined in the laboratory and the estimated thicknesses of overburden subsequently removed from certain well-known over-consolidated clay horizons. Lateral variations in the estimates will occur in accordance with geological interpretations (e.g. the London Basin, Fookes 1966). Bjerrum (1967) postulated that at (say) point c the normally consolidated sediment might be subject to the same sedimentary overload for a considerable period of time. Diagenetic changes would be operative during this period so that bonds due to particle adhesion, particle recrystallization and cementation would be developed. The sediment would therefore become stronger and more brittle, with a further, although minor, secondary decrease in volume (to point c' in Fig. 1). It should be appreciated that diagenetic changes (including clay-mineral species changes) will also be governed by temperature increase as a function of depth of burial or heat flow rates. For example, Teichmiiller & Teichmiiller (1967) show that coal rank is a function of depth of burial. Very broadly the diagenesis of coal is paralleled by that of the associated sediments and rocks. Price (1966) recognized that the uniaxal strength of siltstones and sandstones in some coalfields was related to the ranks of the associated coal seam. Berkovitch et al. (1959) and Taylor & Spears (1970) drew attention to an incipient rank or induration factor in respect to the breakdown of mudrocks from the Coal Measures. From their study in the USA, Weaver & Beck (1971) have postulated that pre-middle Carboniferous shales have, in general, been subject to higher temperatures than most of the younger shales and clays in the geological column. According to Bjerrum, water uptake and the degree of swell on unloading will be dictated by the strength of the diagenetic bonds (e.g. Fig. 1A and B). Moreover, the less indurated clays will more readily release the strain energy stored during compaction (Brooker 1967). Because vertical expansion is less restricted than horizontal expansion the degree of

0481-2085/81/1000-0325 $02.00 t~) 1981 The Geological Society

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t~ased on Skempton (1961, 1964) and Bjerrum (1967).

vertical load slledding is greater than that in the horizontal direction. Consequently, the horizontal effective stresses would be smaller in the strongly bonded types (lpoint e, Fig. 1C) because the bonds would inhibit expansion. However, the importance of these stronger bonds is believed to be their ability for releasing strain energy on a time-dependent basis, thus leading to deformation and progressive failure in mudrocks and over-consolidated clays. In practice total rebound in thelse sediments is considered to be a combination of elastic and time-dependent rebound (see, for example, Nichols 1980). The ratio of horizon-

tal to vertical stresses (K0) is shown in Fig. 1E (for London Clay). It will be observed that the values vary from 1.46 at a depth of 33.53 m to 2.80 at 7 m below ground level. Computations by Bishop et al. (1965) suggest that the ratio might be even higher. Extensional failures, such as joints, are an expression of the elastic reponse of relaxation during uplift and denudation, whilst on a time-dependent basis, expansion promotes the opening of additional fractures and fissures. In the context of weathering (the converse of diagenesis) the disintegration of bonds and the opening of further fissures is accompanied by

ENGINEERING

PROPERTIES

327

OF MUDROCKS

TABLE 1. Laboratory determined preconsolidation loads with estimates of subsequently eroded cover rocks

Deposit London Clay London Clay London Clay Gault Clay Gault Clay (Albian) Fullers Earth (Aptian) Weald Clay (Barremian) Kimmeridge Clay (Kimmeridgian) Upper Oxford Clay Lower Oxford Clay (Oxfordian/Callovian) Fullers Earth (Bathonian) Upper Lias Clay (Toarcian)

Preconsolidation pressure kN/m2

Estimated depth of burial from the literature (m)

Reference

c. 1436 2145 4137 3430-7080 8346

152 152-213 366-396 -425-520

Skempton 1961 Skempton & Henkel 1957 Bishop et al. 1965 Samuels 1975 Smith 1978

7104

610-760

Smith 1978

Warnham, Sussex

13229

1220--1370

Smith 1978

Portland, Dorset

13229

1070-1220

Smith 1978

Location Bradwell Central London Ashford Common Ely-Ouse Water Tunnel Nr. Letchworth, Herts. Redhill, Surrey

1"Chickerell, Dorset ]Bletchley and Calvert, ~ Bucks. [Stewartby, Bucks. kWhittlesey, Hants. Calvert, Bucks.

Not Quoted Not Quoted

Jackson & Fookes 1974 Not Quoted Not Quoted 9583-14504

500 330 850-945

Smith 1978

9583

760-885

Smith 1978

14847

855-975

Smith 1978

Coombe Hay, Nr. Bath Empingham, Leics.

water entrainment and chemical degradation. The process is thus a progressive one, such that indurated mudrocks may once again ultimately attain the status of remoulded, normally consolidated clays. A classification scheme (with modifications when necessary) such as that recommended by the Engineering Group Working Party (Anon 1977) is appropriate for the description of the degradation stages involved. However, it is only within the last few years (or for specific projects) that weathered equivalents have been categorized as in Table 2. It is within this framework, ranging from competent, indurated rocks to soft, normally consolidated clays, that the engineering properties recorded in this paper are collated.

Variation in engineering properties The geological classification of mudrocks is based predominantly on grain size (see Taylor & Spears 1981), and although they contain a high percentage of silt and clay sized detritus,* the dominance of clay minerals is not a prerequisite. Any distinction between 'rock-like' and 'soil-like' properties is sensibly a reflection of the

*> 50% clastic grains of < 60 txm in size.

1560 "1 600

degree of induration (bonding and cementation) and subsequent rebound history. However, induration is by no means a systematic variation with age and depth of burial, even in unweathered mudrocks. In this paper the compressive strength criterion used for descriptive purposes is that of the Engineering Group Working Party (Anon 1977). This latter soft-rock criterion (UCS> 1.25 M N / m 2 - w e a k rock) is subtly different from that proposed by the Engineering Group Working Party (Anon 1972) and markedly different from the division advocated by Morgenstern & Eigenbrod (1974). On a strictly geological basis no distinction is drawn between mud 'rocks' and overconsolidated 'clays'. The present review of the literature has indicated that in terms of engineering properties the nomenclature problem is very complex indeed. Apart from the question of nomenclature, the magnitude of a particular parameter may be dependent on a number of factors which can be considered under the following headings: 1. lithology; 2. exhumation; 3. type and method of testing; 4. degree of weathering. In order to put the engineering properties into their proper context the effects of the above factors are first considered.

J. C. CRIPPS & R. K. TAYLOR

328

TABLE 2. Classification of weathered mudrocks Term

Grade

Description

Fresh

IA

No visible sign of weathering.

Faintly weathered

IB

Discolouration on major discontinuity surfaces.

Slightly weathered

II

Discolouration

Moderately weathered

III

Less than half of rock material decomposed.

Highly weathered

IV

More than half of rock material decomposed.

Completely weathered

Residual soil

V

All rock material decomposed; original mass structure still largely intact.

VI

All rock material converted to soil. Mass structure and material fabric are destroyed.

Lithology Lithogology is taken to be the combination of composition and degree of induration following compaction and diagenesis. In a simple sense most mudrocks are composed oi platy clay minerals and more equant minerals, of which quartz is the most important type. Other minerals, including feldspars, and diagenetic carbonates and uulphides are significant constituents of particular mudrock types and formations. Some effects on engineering behaviour of mineralogy and compaction were demoastrated by Price (1960), who showed that the compressive strength of a range of Coal Measures s a n d s ~ n e s and siltstones is proportional to their quartz contents and previous depth of burial. Although it would be imprudent to extrapolate these findings to all argillaceous rocks, corroborative evidence in the form of an incipient rank factor has already been referred to in the case of the Coal Measures succession. Regional studies have shown the influence of major component minerals on undrained shear strength. For example, Burne|t & Fookes (1974) found that in the London Basin an eastward reduction in undrained shear strength with increase in plasticity of the London Clay can be attributed to an increase in clay fraction in that direction. Russell & Parker (1979) related changes in undrained shear strength of Oxford Clay to the amount and species of clay mineral and the presence of cementimg agents. In particular, they obtained negative correlations between strength and the proportion of mixed layer clay plus montmorillonite (and illite shape factor) in the < 2 / z m fraction. Positive

correlations were obtained between shear strength and the diagenetic (cementing) minerals, calcite and pyrite. The residual shear strength (4)r') of mudrocks has been investigated by a number of workers in terms of composition, following Skempton (1964) who established an empirical relationship between (~/ and clay fraction (see Fig. 15). One way of demonstrating the influence of the clay fraction on residual shear strength is to determine the ratio of clay minerals to 'massive' minerals (detrital plus diagenetic minerals) by X-ray diffraction. Figure 2 shows that for many U.K. mudrocks 4 ) / c a n drop by 21.5 ~ as the ratio increases from 1.5 to 7.8. Since all these data were derived from the same equipment, it would appear that 4)~' depends on the degree of induration (or weathering) and the amount of clay present. The more indurated Carboniferous mudrocks have 4)/ values falling within a significantly higher range compared with overconsolidated clays. Tests on highly weathered Lias Clay, in which the imprint of over-consolidation has been lost, display a much lower range of 4)/- From Fig. 2 it is also clear that for a particular material type a lower ~ ' value is obtained when the proportion of clay minerals to massive minerals increases.

Exhumation The degradation of mudrocks commences with the removal of overburden which leads to associated fissuring, an increase in water content and softening. Many hundreds of vertical geotechnical profiles showing general reductions in plasticity and water content and increases in strength and elasticity modulus with

ENGINEERING

PROPERTIES

OF

329

MUDROCKS

529 Coal Measures (Spears & Taylor 1972) 0 Etruria Marl(Hutchinson et o._..~11973) Lias Clay(wthrd) 9 Kimmeridge Clay 1 Oxford Clay 0 Speeton Clay i - (Attewell 8t Taylor 1973) zx Ampthill Clay 9 Gault Clay J

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depth are illustrated in the literature. Such an example is the Oxford Clay profile from near Peterborough, shown in Fig. 3*. Significantly, the authors make reference to the fact that the average fissure spacing increases with depth. Similar observations to these have been made by Ward et al. (1965) for the London Clay at Ashford Common, and by Chandler (1972) for the Lias Clay shown in Fig. 4. Vertical profiles showing average undrained shear strengths are available for a number of unweathered over-consolidated clays and rates of increase with depth are shown in Table 3 for horizons of increasing geological age. With the exception of pressuremeter tests in the Keuper Marl, the shear strengths quoted were obtained from triaxial compression tests conducted on 3 8 r a m diameter specimens. The values imply that geological age (time period of diagenesis) is seemingly a significant factor, insofar as the individual rates of

increase in undrained shear strength with depth are concerned. From the results given below, which were obtained by Bishop et al. (1965) from the lower 23 m of the Ashford Common Shaft, it can be seen that the peak effective shear strength parameters increase with depth. However, the percentage change in cohesion is considerably larger than the change in the frictional component: Property Depth 23m 46m Undrained shear strength*, Su, kN/m 2 225 575 Effective cohesion, cp', kN/m 2 108 252 Effective angle of friction, ~bp' degrees 25 29 Thus, good evidence exists for increases in shear strength (and stiffness) with depth, but in terms of * su is equivalent to cu when ~bu = 0.

330

J . c . CRIPPS & R. K. TAYLOR

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absolute values the effect of fissuring, sample type/size and method of testing will all be shown to be important. Type

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the type of test performed and other details relating to the method of testing. In respect of the published data, processing necessitated a detailed consideration of the following factors in particular: (a) interpretation of 'effective strength' and 'apparent effective strength'; (b) effects of sample disturbance and anisotropy; (c) type of test--laboratory and in situ tests; (d) interpretation of residual shear strength.

C}

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14

FIG. 4. Geotechnlcal profile for Upper Lias Clay from Northhamptonshire (Chandler 1972).

Effective shear strength and apparent effective strength parameters Modification of mudrock behaviour as a consequence of fissures (and joints) is especially noticeable in the more indurated rocks. In these types the 'rocklike' character of unfissured, intact specimens tested in triaxial compression at a high confining pressure contrasts markedly with the 'soil-like' properties exhibited by fissured specimens in low pressure tests. The significance of this phenomenon is illustrated by the failure envelopes in Fig. 5 to which typical values of effective and apparent shear strength parameters have been appended. For a more comprehensive treatment of the shear strength characteristics of soils and rocks, reference should be made to Jaeger & Cook (1979). Suffice it to say that the intactness of the sample significantly influences its behaviour during triaxial testing. Tests by Carter & Mills (1976) on 38-mm diameter cores of intact Coal Measures m~adstone from the Kielder aqueduct together with results by Hobbs (1966, 1970)

ENGINEERING

PROPERTIES

OF

331

MUDROCKS

TABLE 3. Increase of undrained shear strength with depth Age

Formation

Palaeogene Cretaceous

London Clay . Gault Clay Kimmeridge Clay Oxford Clay Upper Lias Clay Keuper Marl

Jurassic Triassic

Rate of increase of s~ 6-10 kN/m2/m 7 kN/m2/m 15 kN/m2/m 28-30 kN/m2/m 37 kN/m~-/m 37.5 kN/mE/m

from various underground collieries, indicate the following apparent shear strength parameters: ca -- 5-7 MN/m 2

(ha -- 25-29 ~ Carter & Mills

ca = 2-13 MN/m 2

tba = 28-39 ~ Hobbs

Reference

Ward et al. (1965) Samuels (1975) Simm & Busbridge (1976) Burland et al. (1978) Chandler (1972) Leach et al. (1976)

of Keuper Marl in which it is particularly difficult to obtain undisturbed laboratory samples of the partially weathered Zone III material. O t h e r studies, such as those by Ward et al. (1965) on London Clay, and Samuels (1975) on Gault Clay have compared undrained shear strengths (su) of specimens obtained from boreholes with those from blocks. In the case of London Clay, the block samples which had suffered less disturbance than open-drive specimens were found to have Su values about 30% greater than the latter. Data given in Fig. 6 for Gault Clay indicate that

In contrast to these results, Spears & Taylor (1972), in back-saturated triaxial tests on 76-mm diameter borehole cores of fissured but largely unweathered Coal Measures shales and mudstones, obtained effective strength parameters of c ' - 0 - 1 3 1 k N / m z and 4)'= 32-45.5 ~ at low confining pressures. Sample disturbance and anisotropy

Undrained shear strength,Su(kN/m ) _o u

Excessive damage to specimens, so that their behaviour in tests is no longer representative of the in situ material, is most likely to occur in hard clays and weak rocks, and in mudrocks which contain structural and lithological strength inhomogeneities. Davis (1971) draws attention to this aspect of the behaviour

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100

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FIG. 6. Undrained shear strength of 38-mm diameter triaxial samples of Gault Clay from Ely-Ouse, Essex water tunnel (Samuels 1975).

332

J. C. CRIPPS & R. K. TAYLOR

Type of test--laboratory and in-situ tests

the specimens from blocks were 167% stronger than open-drive samples, and 28% stronger than those obtained by rotary coring. Since the removal of overburden is accompanied by vertical expansion, the preferential development of horizontal fissures may often impart an anisotropy to mudrocks which can enhance the effects of existing vertical inhomogeneity. This aspect of the behaviour of mudrocks has been considered in various investigations, including those conducted on the Oxford Clay by Parry (1972) and by Jackson & Fookes (1974). These authors found that in shear-box tests in which the laminations were parallel to the plane of shear, 4~' was reduced by approximately 10-12 ~ compared with tests conducted on specimens orientated with laminations at right angles to the shear plane. Specimen orietatation has also been found to have a significant effect on undrained shear strength. In tests on Gault Clay (see Fig. 6), Samuels (1975) found that the strength of horizontal specimens from blocks were on average 25% greater than vertical ones. Similar results were obtained by Ward et aI. (1965) in London Clay from Ashflord Common. Here the undrained shear strength anisotropy resulted in the horizontal specimens having su values about 46% greater than the vertical ones and 91% greater than samples inclined at 45 ~ Fkom research on Lower Lias Clay, Starzewski & Th~)mas (1977) report that in this material the horizontld elastic modulus is 4.7 times the vertical value.

Undrained triaxial testing of 38-mm diameter samples is a standard technique for assessing the strength behaviour of over-consolidated clays. In the last 10 years however, 100-mm diameter samples and in situ field tests h a v e become more commonplace. Typically, as shown in Fig. 6, the undrained shear strength values of 38-mm diameter samples plotted against depth are widely scattered about a mean line. Not only is it then necessary to conduct a large number of tests to arrive at a reliable average, but also features such as changes with depth become obscured. Importantly, because of fissuring, the true mass strength of mudrocks can be considerably less than the value obtained from tests on small samples. Bishop (1971) has investigated the effect on the measured undrained shear strength of sample size. In tests conducted on London Clay specimens of between 18- and 300-mm diameter, the su values ranged, respectively, from 217 kN/m 2 to 53 kN/m 2. London Clay is among the growing number of formations which have been the subject of research into the relationship between field and laboratory determined parameters (see Marsland 1973a, b,c; Marsland & Randolph 1978; Windle & Worth 1977). From data obtained by Marsland (1973a) in Fig. 7, it will be observed that the average undrained shear strength obtained from 865-mm diameter plate loading tests is approximately 28% less than that obtained

s u (kN/m 2) 100

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FIG. 7. 'Effect of type of test on the undrained shear strength of London Clay from Chelsea (Marsland 1973a).

ENGINEERING PROPERTIES OF MUDROCKS from triaxial compression tests (38- and 98-mm diameter specimens) and about 75% less than the value determined in 5.5-mm diameter penetrometer tests. If these latter results represent the unfissured strength of the London Clay, then dearly the presence of fissures has a highly significant influence on the strength of the clay en-masse. Although plate-bearing tests overcome many difficulties associated with sample size and disturbance, the effects of delays in carrying out the tests after site excavation having been shown by Ward et al. (1965) to cause a significant reduction in strength (presumably) due to rapid surface softening. Pressuremeter tests have also been employed to mitigate against the inherent draw-backs of triaxial testing of material sensitive to the effects of sample disturbance, such as Keuper Marl. Leach et al. (1976) report tests in this formation at Kilroot, Co. Antrim, in which the pressuremeter undrained shear strength results were on average 230% higher than the values obtained from triaxial tests for the same depths. In tests on London Clay at Hendon, Windle & Worth (1977) reported pressuremeter undrained shear strength values which averaged 60% higher than the equivalent triaxial tests. Precisely how these and other pressuremeter results relate to the mass strength of fissured clay is open to speculation since values obtained for a particular location in a borehole would generally be considerably higher than those derived from large diameter plate loading tests. Deformation moduli are probably even more sensitive to the effects of sample size, disturbance, and testing method than is undrained shear strength. The coefficient of volume compressibility determined in an oedometer on small (50- or 76-ram diameter samples) will typically be much lower than the equivalent value obtained for larger samples containing fissures tested in a Rowe cell (see Rowe 1972). Intuitively, it would be anticipated that the deformation modulus determined from triaxial tests on small samples would generally be greater than the value obtained by in situ plate loading or pressuremeter tests. The fact that it is not so may be due to the effects of stress relief adversely affecting laboratory samples. It is of interest to note that Simons & Som (1969) report that London Clay, re-consolidated to its original overburden pressure, exhibits increased modulus values compared with non-consolidated samples. In fact, the elasticity values in Fig. 8 which Marsland (1973c) obtained by large diameter plate tests in London Clay from Hendon are much closer to values derived from the analysis of settlement records, such as those of Hooper (1973) for Hyde Park, than they are to the equivalent laboratory test results. Part of the explanation for the discrepancy between deformation characteristics derived from settlement records and the predictions based on laboratory tests could rest with strain dependency effects. Simpson et al. (1979) have demonstrated that the ratio of elastic

333

Modulus of elasticity, E (MN/m 2) 00

40

80

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120

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38mm diometer samples 98ram diometer samples 865mm plate test, hand finished surfoce E=IO§ 5.2z . . . . .

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FIG. 8. Initial secant moduli of London Clay from Hendon (triaxial and plate loading tests; Marsland 1973c) and the elasticity (E) of London Clay from Hyde Park (analysis of settlement; Hooper 1973).

modulus (E) to undrained shear strength (su), that is E : Su, varies between 140 for the large strains (--~1%) used in laboratory experiments to approximately 1000 for the small ground strains ( < 0 . 5 % ) measured in practice. Residual shear strength

The measured value of residual shear strength depends primarily on test method and the effective normal pressure used. Unless the mudrock is slickensided, fissures should not influence this parameter although lithological lamination may result in anisotropic behaviour according to Jackson & Fookes (1974). The effects of varying both the type of test and the effective normal stress can be demonstrated by the results in Fig. 9 which are tests conducted by Chandler et al. (1973) on Upper Lias Clay from Northamptonshire. It will be noted that the residual shear strength is reduced as the effective normal stress is increased. Furthermore, compared with the reversing shear box,

334

J.C.

CRIPPS

& R. K. TAYLOR

20 t

Shear box peak strength 9 Shear box cut plane o Triaxial natural shear surface 9

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E 23o i kN/m2 t,,,,, L_ 0

/

f

co ~o

. ~ o - " ~ . ~

=o

._~=----~9_.------- r 8-5~ . C'' r= 2kN/m z I "~"" - I I I I _

c'r=TkN/m z

^

60

100

Effective

I 150

I 200

n o r m a l s t r e s s ( k N / m z)

FIG. 9. Effect of type of test and method on the residual shear strength of Upper Lias Clay from Northamptonshire (Chandler et al. 1973). the ring shear apparatus and the triaxial test approach in which the maximum resolved shear stress was coplanar with a natural shear surface, give respectively lower and higher values at a particular effective normal pressure. Research by Bishop et al. (1971) has produced similar results for London Clay (see Table 4). Degree

of weathering

The greatest variation found in the engineering properties of mudrocks can be attributed to the effects

of weathering. Ultimately, this process returns the material to a normally-consolidated, sensiblyremoulded condition, through the destruction of interparticle bonds by accommodation straining. This progressive softening and degrading is accompanied by reductions in strength and deformation moduli, with a general increase in plasticity and water content. Some measure of the effects of weathering on the shear strength of London Clay can be gauged by comparing determinations from blue, unweathered Zone I or II material, with its brown, weathered Zone III or IV equivalent (Table 5). Thus, for the cases cited the undrained shear strength is reduced by about 50% and the effective cohesion (c') can undergo a very large reduction. It is clear from the discussion regarding the removal of overburden that stress relief may be responsible for a large drop in the c' value possibly to about 35 kN/m 2. Nevertheless, weathering can reduce this further to a value close to, or equal to, zero. The effective angle of shearing resistance is also reduced and at ~b'= 20 ~ the value corresponds with a fully softened condition (Skempton 1970b). Some results from extensive research into weathering of Upper Lias Clay from Northamptonshire by Chandler (1972) are presented in Fig. 10. Since these curves are based on undrained triaxial tests on 38-mm samples, the individual values exhibit considerable scatter. However, considering only those results from which samples were not suspected of swelling, the average undrained shear strength of the clay is reduced from 2 0 0 k N / m 2 to 63 kN/m 2 as weathering proceeds from Zone I to Zone III material. The water content increases from approximately 16 to 30% during this process. As a further observation arising out of this work, Chandler contrasts the slow rate of oxidation found in Zone III Lias Clay with the more rapid rate apparent in London Clay. Weathering is responsible for dramatic changes in more indurated mudrocks such as those from the

TABLE 4. Residual shear strength of London Clay Method

Residual shear strength 4)'r

Effective normal stress

8.0-13.8 ~ 12.9-15.6 ~ 13.2-14.1 ~

250-7 kN/m 2 240- 60 kN/m z 257-126 kN/m 2

Ring shear Reversing shear box Co-planar triaxial

T.~BLE 5. Undrained and effective shear strength of weathered and unweathered London Clay Property

Brown

Blue

Location

Reference

s, kN/m 2 c' ItN/m2

100-175 0- 31

120-250 35-252

4)'degrees

20- 23

25- 29

Neasden Ashford Common,] Wraysbury and ~ Edgware .]

Sills et al. (1978) Bishop et al. (1965), Skempton & Hutchinson (1969), Bishop et al. (1971)

E N G I N E E R I N G P R O P E R T I E S OF MUDROCKS Weatherin(] zone x Landslip ] z~ Zonelll [ solid symbols denote Z .._ / samples which may o o n e llo I'hove swelled during o Lonella i sampling [] Zone I J

40 ~,~

SO

x

2 ~

Zonelll

~. o

UOC 25

~

o',~

.

~

ZoneUb ~

-"-.~osed on

"-"~.depth profiles

,=~,.~ w

9e-.

20

" ~

e

~

15

10

, l aZone . 9

o

50

11~0

I 150

o

200

Zone I

I 250

3;0

Su (kN/m 2)

FIG. 10. Effect of weathering on the undrained shear strength and natural water content of Upper Lias Clay from Northamptonshire (Chandler 1972).

Carboniferous period. Spears & Taylor (1972) attribute a reduction of 93% in effective cohesion and a drop in 4)' to 26 ~ (c'= 0) to this effect. Changes in the stress-strain behaviour may also be apparent. In the case of Keuper Marl, Chandler (1969b) describes the weathering related modifications to the stress-strain curve obtained during triaxial testing. For Zone I material the curve is of the type consistent with brittle failure. However, the curve for Zone IV material has no peak, the failure being entirely plastic. The suppression of the peak in the stress-strain curve is largely responsible for the reduction in the effective strength parameters recorded, viz:

Zone I III IV

c' kN/rn 2 28 17 17

r

degrees 40 ) 42-32 1 32-25

Reference Chandler (1969a)

Generally the modulus of elasticity is reduced as the degree of weathering becomes more advanced. In the case of Keuper Marl structural modifications bring about large changes in the deformation characteristics if a particular threshold pressure is exceeded (see Davis 1971). The value of this pressure is lower in more weathered marl.

Compilation of engineering properties and parameters of British mudrocks Having considered the major variations and difficulties, each one of which can have a blanketing effect on overall trends, it will be appreciated that a certain amount of in-built bias will also be inevitable. Thus, when viewing the data in the following section it must

335

be remembered that, although authors may well have related their experiences with problem mudrocks, similar difficulties will not always arise at other locations in the same horizon. Even more importantly, mudrocks not referred to in the literature are not necessarly without problems. In spite of the fact that the results presented herewith represent an extensive literature search, inevitably some important references will have been missed. No published data have been deliberately omitted in establishing the values presented, but the retrieval systems may have failed to locate them. For the purpose of the present exercise, the view has been taken that a resonable compilation of the engineering properties of mudrocks can be gained from a consideration of classification indices, strength and deformability. Hence, the engineering properties in terms of water content, liquid limit, plasticity index, porosity, clay fraction, undrained and effective shear strength, elasticity and consolidation characteristics are presented in Table 6. For data regarding the hydrological properties of mudrocks, reference should be made to Tellam & Lloyd (1981). The style of presentation adopted in Table 6 is described below for the water content of weathered London Clay: (a) 23-4

(b) 53, 55 (c) a

Range of values for the parameter. Code for the source of the data, indicated in the list of references. Code for footnote information regarding the type of test performed or sample used.

Because many of the engineering parameters of weathered mudrocks do not show very much variation throughout the geological column, values are given for the materials both in the weathered and unweathered states. Wherever possible this distinction has been based on the attributed weathering zone, but in other cases an appropriate description or depth below ground level has been used. In the latter case, a depth of approximately 7 m (Taylor & Spears 1981) has generally been assumed to mark the base of the weathered zone. Table 6 is a compilation of the maximum and minimum values of the engineering parameters as quoted in the references cited. In addition, in the case of undrained shear strength and coefficient of volume compressibility, average values are given. The magnitude of a particular engineering parameter is sensitive to the testing method used. Unless otherwise specified by a footnote, the values quoted were determined by the methods given in Table 7. For geological age (depth of burial) purposes, the engineering behaviour of the mudrocks is discussed with reference to a series of diagrams which show the values of the respective geotechnical parameters. On these diagrams, values for weathered materials are

336

J . c . CRIPPS & R. K. TAYLOR TABLE 6. Engineering properties of British mudrocks Formation

Code on graphs

,, I

Liquid limit

w, %

w. %

Weathered

Unweothered Weathered

6, 63

6

Paloeoge~ne Barton Cley

Water content

21-32

Plasticity index

Unweathered

beds

London Cloy

2 :5

45-82

beds

21-55

43

19-28

n,%

Weathered

50-105

40-65

42-67

Undrained shear strength $., kN/m 2 (average)

Unweathered Weathered Unweathered

68

6,63 25-70

68

20-210

50-350

40-72

68

68

40

112-150

43

43

118

15-27

-

Undrained shear strength $ . , k N / m 2 (renge)

66,89 Y 66-100

118 4

40

0.49-0"95

103 ,r

0"02

/03 "."

-44

44

54, 32

71

IO-IO0

0"42-0-67

I

,10 GN/m z {~,SGN/mz '- 2z-52 105

35

55

-

-

~r 3 3 3 - 5 0 0

0"008

30-40

103 -

18-30

-

200

83 5

19

56

59

0"25-2-7

_

-

83 II

103,58

103

4"5

105

24

>_ 4 0

62,74 0"75-95

500

J3-16

24 _> 3 0

113

28

27

2-80

370

0"5-60

/03 -

0"07 7

59

5,Z8

18-25

28! 5

17r

12

25,29

o-17

83 l 0"22

83

17"4

28,zz

e

74

74 0"2-2"0

_

O' 0 9

59

23-40

31

0 5/

25-141

103

0"007

59

19

65,115

-q

106

14

14"-23

IO

Unweathered

0.2-0-78

5

_

78

-

65

-

12-100

103

51

-

6-14

12

5

Weathered

115

0"04

11-20

14-67

Unweathered

lOG

/o/ rB-24

23-48

Weathered

10-35

r7-50

I/4

26

25-124

/0/ ~ 2J-34

Cv, 1112/ yr

MN/m t

;'6,118

0"02-0"06

30, 54

E,,=,

7

T0-065-0-5

t2

20-29

7

c::,o,::,;:,,,0,o

Modulus of elasticity

MN/m z

7 -

102,/3

17-25

olosticltr, E = i/m v

15

43 25

12

Equivalent modulus

68

27- 3 9

7 7 18-32

337

MUDROCKS

i Weathered

63,68

18-24

43

), 0 - 5 5

=heor strength

~..

OF

Coefficient of volume compression my, m Z / M N

Residual

Unweathered

68

PROPERTIES

100-1200

_

61 2"8

57 -

8.5-17-5

22

22,48

i

-

o-t~

-

/04

I

3r-3e

-

39

[email protected] z e - 3 9

I v Ca' 2-13 M N / m 2

/04

12-14

83 I

104,117 0-49

1o4 24-162 15 3 9

45,3

40

[ u%3Z t

9 22 !
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