The Engineering Properties of Mudrocks

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 Skemp- ton (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 at- tained 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 rep- resented 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

0481-2085/81/1000-0325 $02.00

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 accord- ance 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 adhe- sion, 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

t~) 1981 The Geological Society

326 J . c . C R I P P S & R. K. T A Y L O R

|

�9 (a) (a)

NOrmal ly consol idated

ition /Pre- ,

II x , . . . . ~ - ( I ! Overconsol idated -weak bonds . / - - - - Di . e . �9

- - - - - - - ~ ~ o (c)' ~' Overconsol idoted - s t rong bonds

(c)'

|

F(b> 5 " (a) [

I 1 [ Opening of fissures and fractures;

i _ I r "Shear failure

~(c),(c)'

I

(o) E f fec t i ve vert ical stress

P r e - consolidotion pressure

Diog en-'-e'es~

@ '

�9

Deposition

~ -

(o)

__ - " - - ~ - _ _-"

(b) (C), (C) l

_ _ _ Erosion

(d)

I0

15

2 o o_

25

3O

35 1.5 2.0 2.5 3 .0

Ko

L o n d o n C loy

~IG. 1. Schematic geological history of over-consolidated clay (after Fleming et al. 1970). 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 t ime-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 t ime-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 expres- sion of the elastic reponse of relaxation during uplift and denudation, whilst on a t ime-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

E N G I N E E R I N G P R O P E R T I E S OF M U D R O C K S 327

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

Estimated depth of burial from

Preconsolidation the literature Deposit Location pressure kN/m 2 (m) Reference

London Clay Bradwell c. 1436 152 Skempton 1961 London Clay Central London 2145 152-213 Skempton & Henkel 1957 London Clay Ashford Common 4137 366-396 Bishop et al. 1965 Gault Clay Ely-Ouse Water Tunnel 3430-7080 -- Samuels 1975 Gault Clay Nr. Letchworth, Herts. 8346 425-520 Smith 1978

(Albian) Fullers Earth Redhill, Surrey 7104 610-760 Smith 1978

(Aptian) Weald Clay Warnham, Sussex 13229 1220--1370 Smith 1978

(Barremian) Kimmeridge Clay Portland, Dorset 13229 1070-1220 Smith 1978

(Kimmeridgian) 1" Chickerell, Dorset Not Quoted 1560 "1 ]Bletchley and Calvert, Not Quoted 600

Upper Oxford Clay ~ Bucks. Jackson & Fookes 1974 [Stewartby, Bucks. Not Quoted 500 kWhittlesey, Hants. Not Quoted 330

Lower Oxford Clay Calvert, Bucks. 9583-14504 850-945 Smith 1978 (Oxfordian/Callovian)

Fullers Earth Coombe Hay, Nr. Bath 9583 760-885 Smith 1978 (Bathonian)

Upper Lias Clay Empingham, Leics. 14847 855-975 Smith 1978 (Toarcian)

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 En- gineering 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 pre- dominantly 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.

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 de- scriptive purposes is that of the Engineering Group Working Party (Anon 1977). This latter soft-rock criterion (UCS> 1.25 MN/m2-weak 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.

328 J. C. CRIPPS & R. K. TAYLOR

TABLE 2. Classification of weathered mudrocks

Term Grade Description

Fresh

Faintly weathered

Slightly weathered

Moderately weathered

Highly weathered

IA No visible sign of weathering.

IB Discolouration on major discontinuity surfaces.

II

III

Discolouration

Less than half of rock material decomposed.

IV More than half of rock material decomposed.

Completely weathered V

Residual soil VI

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

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 sands~nes 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 estab- lished 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) /can 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 Car- boniferous mudrocks have 4)/ values falling within a significantly higher range compared with over- consolidated 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

E N G I N E E R I N G P R O P E R T I E S O F M U D R O C K S 329

5 2 -

Z8 -

24

20

O'r 16 degrees

12

8

4

0

\

\ \ .

\

�9 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 -

zx Ampthill Clay �9 Gault Clay J

" ~ �9

�9 ,~. �9

" ~ �9 �9

' - ~ . , �9 2 0 [] �9

/x

(Attewell 8t Taylor 1973)

" ~ . ~ �9 ~ , ~ . . ....

D A Clay Band e in Coal

I rooge

1 I I ~ I , I 1 I I 2 3 4 5 6 7 8 9 10

Ratio clay minerals to detrital and diagenetic minerals

FIG. 2. Influence of clay minerals on residual shear strength. All mineralogical data from the same X.R.D. machine and ~b/values from the same reversing shear boxes.

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 38ram 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 2 3 m 4 6 m

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

* s u is equivalent to c u when ~b u = 0.

330 J . c . CRIPPS & R. K. TAYLOR

s u (kN/m 2) E(MN/m 2)

O0 400 800 1200 0 80 160 240 520 r o ' ' , , , , I ' ' ' " ' ' ' ' 1

. . . . . . . . . . t . . . . . . . . . . . c:"~

t X,~] �9 �9 XX .......

I- N . ~ o ~ l \ \ Lower Oxford-I

24I oX xX~x ~ I ~ x ~t" , .~ xe~ tangent 1

_ �9 Kellaways Sand_ 32 2]-' u

�9 Ketlawoys Clay

�9 BH4 �9 Esecant u BH7 x Ftangen t x B l o c k s a m p l e s

FIG. 3. Undrained shear strength and modulus of elasticity of Oxford Clay from near Peterborough (Burland et al., 1978).

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

T y p e a n d m e t h o d o f t e s t i n g

Geotechnical test results must always be interpreted in the light of the characteristics of the material tested,

Water Content % s u ( kN /m 2)

60 0 100 200 I I I I I I 1 l I I ]

.• 6

C}

oi T , 2o, . 2 oilder Clay ~-~ . . . . .

A e # ~ Zonellb / ,

i,

6

14

I

I

'1 =

\

3 0 0

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

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 tes t -- laboratory and in situ tests; (d) interpretation of residual shear strength.

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 'rock- like' 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 signifi- cance 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)


TABLE 3. Increase of undrained shear strength with depth

Age Formation Rate of increase of s~ Reference

Palaeogene London Clay Cretaceous . Gault Clay

Kimmeridge Clay Jurassic Oxford Clay

Upper Lias Clay Triassic Keuper Marl

6-10 kN/m2/m Ward et al. (1965) 7 kN/m2/m Samuels (1975)

15 kN/m2/m Simm & Busbridge (1976) 28-30 kN/m2/m Burland et al. (1978)

37 kN/m~-/m Chandler (1972) 37.5 kN/mE/m Leach et al. (1976)

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

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

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

-=d

. P

r Nk-

LId

/ I

I / / / / I

J Co= 3 0 M N / m 2 / ~ / -

0 0 = 1 5 0 - 3 0 0 1 1 " . ~ I n t o c t rock \ s , . 1 1 J " f IntOct rOcK

/ .~. ,-~ ~ n ~ = 5 M N / m 2

/ / ~ i rock

/ c ' , - 5_?OkN!_~ 2 0'= 3 5 0 - 4 5 0

Effective normal stress FIG. 5. Effective shear strength parameters and apparent effective shear strength parameters of fissured and intact rock.

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

Undrained shear strength,Su(kN/m ) _o 200 400 600 8oo lOOO ~aoo u i | i i i

Tests by Le Grand Adsco Ltd. Tests by 'B,R.S.

x Ist series of t es t s } Rotary cores @ Rotary core o 2nd series z~ Open-dr ive samples

10 �9 Open-drive samples �9 Horizontal(block) I t I

Vertical block samples (range and mean)

20 x x x

o OD o o I I 0 ( 0 I

~o~ ~ x x

x ~D 0

r o o x XxX

x

o o

o C o

X 0 0 0

0 0 0 A.~, ,. ' A

6 0 o co �9 (10

~xo �9 t I t �9 x o 8 ,~ ,',,,

8 0 x ~ x x o o o

9 0 x I

x x

100 I I 1 t I I

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

4O A

E v t -

r 5O

E3

1400


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.

Type of test--laboratory and in-situ tests

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 have 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

E .c: 4 . , -

C3

5 -

10-

15-

s u (kN/m 2)

100 200 300 400 500 I I I I I

�9 865mm plate tests

38mm triaxial [] 98rnm triaxial

~ ac~o ~ o o Penetration tests

~ ~ ; o ~ , m m penetration tests

~ \ Oo ,~\ [] . ~ o

I ~ ',58end 98mm 865mm o\ triaxials plate test, - ~ ~ [] z~ � 9 �9

ooo \ ~ 0 0

6OO 1

sand and gravel

soft brown clay

brown and clay mottled clay

stiff grey clay

0

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

ENGINEERING PROPERTIES OF MUDROCKS 333

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 dear ly 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 carry- ing 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

10 A

E N

20

25

00

Modulus of elasticity, E (MN/m 2) 40 80 120

I I I I I

38mm diometer samples 98ram diometer samples 865mm plate test, hand finished surfoce

E=IO§ 5.2z . . . . .

\ "\ I

\ i \ i " \ :

"k.

I \ i \ \

�9 �9 i ' \

\

I \ �9 "" �9 i " \

i �9 . ~1 ~ "g

i i

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 slicken- sided, 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 Northampton- shire. 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 . C R I P P S & R . K . T A Y L O R

20

15

1o

o

~..~1oo

E

t,,,,,

L_ 0

co ~ o

t Shear box peak strength �9 e'N~ Shear box cut plane o " X ~ Triaxial natural shear surface �9

I I I I I fi0 100 150 2 0 0

23 o i kN/m2

/ _ . ~ o - " ~ . ~ =o c'r=TkN/m z f ._~=----~9_.------- r 8-5 ~ ^

. C'r = 2- kN/m z "~"" - I I I ' I I I I 60 100 150 200

E f f e c t i v e 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 co- planar 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).

D e g r e e of w e a t h e r i n g

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, sensibly- remoulded 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 oxida- tion 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

Ring shear 8.0-13.8 ~ 250-7 kN/m 2 Reversing shear box 12.9-15.6 ~ 240- 60 kN/m z Co-planar triaxial 13.2-14.1 ~ 257-126 kN/m 2

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

Property Brown Blue Location Reference

s, kN/m 2 100-175 120-250 Neasden Sills et al. (1978) c' ItN/m 2 0- 31 35-252 Ashford Common,] Bishop et al. (1965), Skempton

Wraysbury and ~ & Hutchinson (1969), Bishop 4)'degrees 20- 23 25- 29 Edgware .] et al. (1971)


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

~ ,~ x

SO 2 ~ Zonelll ~.

o o',~

U OC 25 ~ . ~ Zone Ub ~ -"-.~osed on

, =~ , . ~ "- depth profiles �9 e- . w "~.

20 " ~ e ~ Zone , l a . �9

15 o o Zone I

10 50 11~0 I I 150 200 250 3 ; 0

S u (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) attri- bute 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 sup- pression of the peak in the stress-strain curve is largely responsible for the reduction in the effective strength parameters recorded, viz:

Zone c' kN/rn 2 r degrees Reference I 28 40 )

III 17 42-32 1 Chandler (1969a) IV 17 32-25

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

be remembered that, although authors may well have related their experiences with problem mudrocks, similar difficulties will not always arise at other loca- tions 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 7m (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


TABLE 6. Engineering properties of British mudrocks

Water content Liquid limit Code

Formation on w, % w. % graphs

Weathered Unweothered Weathered Unweathered ,,

Paloeoge~ne 6, 63 Barton Cley I 21-32

43 Brack lesham beds 2 5 --3,55 7 19-26

52,38 London Cloy :5 <~ 2 3 - 4 9 19-28

Woalwieh and 118 Read ing beds 4 - 15-27

CretaceoUs

Cha lk forimotian 5

Goult Cloy 6

Ather f ield and Weald City 7

Weald Clqjy 8

Fullers Fm-th 9

Speeton Clay I0

Plasticity Cloy index fraction Porosity i e ,% <2F% n ,%

6 6 6,63 45-82 21-55 25-70

43 43

66-100 50-105 40 -65 40-72 ~ 37-59

118 118 4 2 - 6 7 20-37

30 32~42

106 27-46

I01, 99 25 -34

Undrained shear Undrained shear strength $ . , kN /m 2 s t rength $ . , kN /m 2

( renge ) (average)

Weathered Unweathered Weathered Unweathered

68 68 68 68 20-210 50 -350 40 112-150

43 66,89 Y 143 66,115 89 66'115

40-190 80 -800 I00 - f75 e 100-400

73 73 - 34-814 ~ 400

114 114- 114 21 58 38

86 ~o 54 1o3,3o 3o ~4,3o 30 18-30 70-92 U~60-120 27-80 58-62 ~J 31-48 17-76

106 I. 90~110 106 106 106 - 41-90 17-58 17-71 v 2 0 - 8 5

103 8 8 8 8,12 103,106 5 42-82 55 28-32 20 -74 r t5 -68

103 103 103 46 - 69 57

5 5 - 50 - 28 - -

.Jurassic 90 103, 90 103,90i 103 103

Kimmeridlge Clay II 18-22 - 70-Sl 24 -59 57 35 5 5 5

Ampthill (]lay 12 23-88 79 - 49 - 19 59,103 19 59

84 2 0 - 2 8 5[3-76 31-40 35 -70 Middle Ox|ordClay 15 2 0 - 5 3 19 59 19t78 59 19,7~103 Lower OxfDrd Cloy f4 15-25 - 45-75 2 8 - 5 0 3 0 - 7 0 o )30-54

31 103 36 103 31,103 36,103 103 Fullers Eolr th 15 26-41 35 41-77 I 0 0 20 -59 38 -68 48

26 38 36 38 38 36 103t28 Upper Lias Clay 16 2 0 - 3 8 I 1-25 5 6 - 6 8 5 3 - 7 0 2 0 - 3 9 5 5 - 6 5 r 5 2 - 4 8

28 105 56 ,28 105 105,28 56 28,105 Lower Lies Clay 17 29 t6 -22 o 5 6 - 6 2 55-63 52-57 o 5 0 - 5 6 r 37 -44

Triassic 24, 34 24 24 24 24 24 24,49 Keuper Ml r l 18 12-40 5-15 p 2 5 - 6 0 2 5 - 3 5 10-35 10-50 ~/ 10-50

30, 87 30 87 56-1280 ,X. 60 3 0 0 - 5 5 0

Carboniferous 57 57 57 57 57 57 57

Etruria Marl 19 17-44 9 - 2 2 43 -79 5 5 - 5 2 8-:52 12-25 r 21-35

Coal meaeures 104 104 I 104

- Mudstane 20 6 - 8 8 5 9 - 4 9

/04 104 104 - Shale 21 9-14 9 42 -45

104 104 - Seo tea r th 22 I I 33 -34

18,39 18 86,108 - Undivi~led 25 3 - 3 2 3 2 7 - 7 2

45, 82 82 45, 82 Limestone series 24 15-88 36 40-106

88 88 Culm meaeures 25 2-5 - 39

Remainin~ Lower 50 50 Palaeozoip rocks 26 5 -28 - 2 5 - 3 5

Symbols an g raphs �9 �9 �9

90 7 0 - 5 0 0

19 4 5 - 4 9 0

78 78#9 52 -95 96-1500

31 10-120 -

26 26, 38 20-180 4 0 - 1 2 0 0

1 7 44 o 28-57 J:~'5-45 MN/m 2

o95 -179 a

67 61,49

9O - 130-470

19 110-560

19 -31 360-1100

50 26 26

30-150 110-240

67 67 7 0 - 2 0 0 130-2800 100-150 300-1200

5 z ! 5 z 4 0 - 2 4 0 120-620 -

104 104108 ,104 104 46 46 42 9-19 2 4 - 5 3 " 2 - 2 5 } 8 3 0 9 " 1 0 3 } 83 u 100-180 MN/m

104 104 108 104 MN/m~8 44-51 12-19 37 -87 ~ 3 -50 u 29 MN/m z _

104 104 108 104 70 30-35 13-41 35-77 W 3 - 2 8 - u 12-14

39 108, 39 83 M N/m = - 26-3 '# 24-7<. 15-335 - - -

82 82 82 82 82 2 8 - 4 5 20-74 - v 16-88 > 2 0 0 v 3 2 - 5 4

88 41 41 33 - 16-17 21-52 qs I0 140 - -

50 - I I - 21 - -

�9 �9 �9 O 0

a, From 'medium hard lumps';/3, large specimen; % state of weathering not known; ~, high value of water content for mudflow; e, depth up to approximately 30 m; ~, depth of 50-70 m; rl, higher value at 46 m depth; 0, from back analysis; L, borehole sample; K, swell pressure plus 100kN/m2; )t, 100-mm diameter samples; /z, Upper Gault w 1 = 60-620%, Lower Gault w 1 -- 90-110% ; v, shear vane test results; ~, Atherfield Clay; o, undivided Leas shales; 7r, deep borehole sample; p, may not be plastic; tr, pressuremeter test; -r, rockfill-Silurian mudstone; v, tests on

indicated by circles and those for unweathered materials by squares. In addition, ranges of extreme values are distinguished from average values by the use of solid and open symbols, respectively.

Water content and porosity

So far as poss ib le , the moi s ture content s for the m u d r o c k s p r e s e n t e d in Fig. 11 s h o w the range o f va lues for unwemthered mater ia l and the m a x i m u m value for the w e a t h e r e d state. U n w e a t h e r e d U . K . m u d r o c k s have water contents in the range 3 - 3 0 % with

only the Fuller's Earth horizons and 'Calp' shale from the Limestone Series of Co. Fermanagh having higher values. A general reduction in water content value with age, and previous overburden is just discernable. Unfortunately, this trend is partially obscured in Fig. 11 by differences of sampling depth between the formations considered. Thus, the high values for Middle Oxford Clay and Lower Leas Clay are for shallow samples, whereas the lower than expected values found in Weald Clay and Woolwich and Reading Beds were obtained from borehole material. The maximum values for the weathered material have been added in


E f f e c t i v e cohesion c ' , kN/m 2

Weathered Unweathered

68 63,68 7-11 a 8 - 2 4

7 43 ), 0 - 5 5 34

97,/2 102,/3 12-18 31-252

/ / 4 7 30, 54

2 5 - 1 2 4

/0/ ~ 2J-34

_

51 14-67

5 2 3 - 4 8

5 9

78 IO 19 0 - 2 0 10-216

0 5/

28,zz o-17

2 8 ! 5

61 24 2 - 8 0 _> 30

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

- i o - t ~ / 0 4

104,117 [ u%3Z 40 0 - 4 9 t

- �9 2 2 1o4 ! <,c iz .N / ,%=

24-162 ' ,w~ i o-16 22,40

15 3 9 r~,ca ; ' -32 M N / m 2 L 0-131 108

45,3 2 4 - 4 6 0

88 II

Effect ive onole of Residual f r i c t ion , r o =heor

strength Weathered Unweathered ~ . .

68 63,68 68 18-24 e 27- 39 15

7 43 7 7 18-32 25 - T0 -065 -0 -5

12 102,/3 t2 74 17-25 2 0 - 2 9 10.5-22 0-05-0"18

/ o / rB-24

Coefficient of Equivalent modulus volume compression olosticltr, E = i/m v Modulus of e last ic i ty

my, m Z / M N M N / m z E , , = , M N / m t i

W e a t h e r e d Unweathered W e a t h e r e d Unweathered Weathered Unweathered

7

-~qR,52 ), 2-15 - - - 74 ~2, 98 65 65,115 0-02 -0 -12 0 - 5 - 2 0 8 - 5 0 10-35 -q 25-141

118, 76 ;'6,118 113 0 " 0 2 - 0 " 0 6 r7 -50 - e 3 7 0

5 14"-23

5 17-32

114 26 30,54

19-53 5 4

12-19 O i l 2

lOG 6-14

12 11-20

5 14

I/4 115 K 0 "04 - K 25 1

5 8 5,8 58 56 581 58,87 0 . 0 1 - 0 - 0 8 8 12-100 I0 2 0 - 1 6 2

/ 0 3 I / 0 3 0"002 i - 500

103 103 0"007 - t 143

_

78 21"5-28

31 17"4 25,29

18-25 29

2 7

59 31

59 2 3 - 4 0

5 10-18

5 10-14

13-17 3/

12 5,Z8

9-13"5 28

J3-16

24 24 2 5 - 4 2 >_ 4 0 18-30

83 l 103 83 103 0 " 2 2 17r 0 " 0 0 2 4 "5 vr 5 0 0

} - 83 19 83 O' 09 0 "07 7 I I

59 /03,/9 59 /9,/03 0 " 0 1 7 - 0 . 0 2 3 7 r0 -003-0-03 4 3 - 5 9 Tr 3 3 - 3 3 3

103 103 ~r 0 " 0 0 5 - T 2 0 0

83 105 83 103 0"2 , r0 .002-0 .003 5 ~r 3 3 3 - 5 0 0

55 0"008

19 19 5 6 - 3 0 - 4 0

19 - 4 0 - 1 4 0

-44 44 - ,10 GN/m z {~,SGN/mz

'- 2z-52 105

35 54, 32 71 I "2 IO-IO0 100-1200

57 - 8.5-17-5 -

22,48 o@. z e - 3 9 39 83

- I 3 r - 3 e / 04 12-14 0.1 I

104,108 r 40 117 83 i 2 9 - 2 9 i. u~'~ 12 16 0 " 0 8 I -

- i 104 r u r i 6 40 85

31-39 [ 32-3"# 104 13-26 39 i u@a 12"3940'22 86,107

21 45.S / 0 4 9 - 3 6 - - 4 5 , 3 4 5 , 3

2 2 - 2 6 8 -19 - - 88 88

31 - 2 3 - 3 2 - i -

1at I I I0, # r 4 0 - 5 Q ' $ - " 2 6 " 5 - 4 0 - -

�9 �9 I �9

i083

83 12

2 2 , 4 8 { u2-49GN/m 2

- 12-97 104 48

/04 ~ ~,lo aN/m 2 9 - 4 9 ~ Z5 /04

22,70 -- ~" u2-2i GN/m z

72 ~- 3z-13e 104 o- I I - 161

5O o- io7-26oo I

c::,o,::, ;:,,,0,o Cv, 1112/ yr

W e a t h e r e d U n w e a t h e r e d

74 74 0 " 2 - 2 " 0 0 " 5 - 6 0

I 62,74 0 " 7 5 - 9 5

] / / 5 , 4 " 9

30 87,58 1 '0 -8"9 0 -09 -1 -4

106 0 . 2 - 0 - 7 8

103 25"1

103 0 ' 0 2

103,58 - 0 " 2 5 - 2 - 7

_

/ 9 - 0 ' 0 7 6

59 103,19 > 4 0 0 . 4 9 - 0 " 9 5

103 - , r 0 " 0 2

/03 "." 0 " 4 2 - 0 - 6 7

I _

6 1 2 " 8

intact rock cores (s, =~o-c); X, higher value from field shear box test; r higher value determined from published values of bulk density, water content and specific gravity; oJ, both values determined from published values of bulk density, water content and specific gravity.

ca, ha, Apparent shear strength parameters. Figures in italic refer to references.

order to demonstrate the extent to which the water content can vary in particular mudrocks.

The values of porosity (n) quoted in Table 6 have been obtained either directly from the literature or by calculation from published bulk density, specific gravity of soil particles and water content data. In general, the upper and lower limits of the range apply respectively to weathered and unweathered materials. When plotted in Fig. 11, these data suggest that expected reductions in the porosity of mudrocks with increasing age and depth of overburden are confirmed. In fact, remarks similar to those regarding the variation in the

natural water content apply to the changes in porosity value.

Plasticity and clay fraction

Plasticity is defined in terms of liquid limit in Fig. 12 and the plasticity ranges of U.K. mudrocks are illustrated in terms of the Engineering Group Working Party's (Anon 1972) classification. Also displayed on this diagram are the clay size functions for these materials, where the actual values lie between the maximum and minimum limits indicated. So for


Natural ~ater content, w%, Porosity, n% 0 2 0 4 0 6 0 8 0 1 0 0

I I I

Numerical codes given in table 6

Polaeogene

Cretaceous

Jurassic

Triassic

Carboniferous

L. Pal.

4

5

/5. ,(-----~-

11

20

L____ 23

: 24 �9

26 N a t u r a l [ Wea the red = s �9 - woter - t

; :: 26 _ contentL Unweathered - -

Porosity

FIG. 11. Natural water content and porosity of U.K. mudrocks.

instance, weathered Barton Clay has been found to consist of between 25 and 69% clay size fraction.

The plasticity of U.K. mudrocks ranges from low in Keuper Marl and some Coal Measures rocks to extra high in the (fault Clay. In their weathered state, Fullers Earth, clays from the Limestone series and London Clay may also exhibit extra high plasticity characteristics. Apar t from the Gault Clay whose extra high plasticity is attributed to a large expandable clay mineral contenlt (Perrin 1971) and the low value for Weald clay due to a small clay fraction, there is a general trend ~ reduced plasticity with age and depth of burial. The parallel trend found in the clay size fraction may be due either to a genuine reduction in the clay size c~mponent or to an increase degree of clay mineral aggregation in the older rocks.

Undrained shear strength

The undrained shear strength data are plotted on a logarithmic scale in Fig. 13. Generally, the range of values for the unweathered, the maximum value for the unweathered, and the minimum value for the weathered materials are displayed on this diagram. When other data are available and space permits, these are also included.

The results presented in Fig. 13 demonstrate con- cisely how weathering reduces all the mudrocks to materials with much the same properties. For the unweathered material the undrained shear strengths range from firm clays to very strong rocks (Anon 1977). A general increase in su with age is just discernable, although differences in geological loading history, lithology and thickness of present overburden impose variations which mask this trend. As already explained, the undrained shear strength generally increases with depth in any one horizon, and profiles showing the extent of this variation for the Oxford, Lias, Gault and London Clays are given in Figs 3, 4, 6 and 7, respectively. In the case of London Clay practically the whole of the variation found in the average values can be accounted for by the increase of strength with depth. Some high average Su values (su = 575 kN/m 2) are quoted by Ward et al. (1965) for samples obtained from a depth of 46 m in the Ashford Common shaft. These, and similar values obtained by Dumbleton et al. (1978) for material from depths of up to 60 m, are not included in the averages given in Table 6 or Fig. 13, which are based on results for depths between 14 m and 30 m. From Fig. 3 it can be seen that the low strength values for the Middle Oxford Clay are a reflection of the sampling depth (maximum 12 m). The su values for the Kimmeridge Clay are also unexpectedly low. This is not surprising, in view of its complex loading history in which re- loading with Recent deposits has followed the removal of the original over-burden. The comparatively high water contents of the Fullers Earth and Upper Lias Clay would appear to account for their low strength. In both instances the data apply to shallow depths and additionally, in the case of the Fullers Earth, there is a high percentage of expandable clay minerals.

Lower Lias calcareous mudstone from Hinkley Point, Somerset is the youngest mudrock in the stratigraphic column to have become sufficiently indurated to be described as strong rock. In this, and older mudrocks, compressive strengths of small intact specimens give equivalent su values far in excess of the values from undrained triaxial tests on fissured samples. As already mentioned, in the less strong rocks sample disturbance can have a significant effect on the strength determined in laboratory tests. A wide scatter of results due to this effect was found by Leach et al. (1976) in triaxial tests on Keuper Marl from Kilroot, Co. Antrim. Although the laboratory results covered

E N G I N E E R I N G P R O P E R T I E S O F M U D R O C K S

TABLE 7. Methods used for the determination of parameters

3 3 9

Clay fraction, < 2/xm

Undrained shear strength, su

Effective shear strength parameters, C', t~'

Residual shear strength, qS/

Apparent effective shear strength parameters, Ca, 4~a

Coefficient of volume compression, m v Coefficient of consolidation, Cv

Modulus of elasticity, Ese c

Sedimentation test.

Undrained triaxial compression tests on 38-ram diameter specimens.

Consolidated drained, or undrained with pore water pressure measurement triaxial compression tests on 38- mm or 100-mm diameter samples. Or direct shear tests on 60-ram square samples.

Consolidated drained reversing direct shear tests on 60-mm square samples.

Triaxial compression tests on small cores of dry rock.

Oedometer consolidation tests on 76-mm diameter samples. m v value given for a pressure increment of 100 kN/m 2 above the effective overburden pressure.

Secant modulus for the initial stress-strain curve at 50% ultimate load in undrained triaxiai compression tests.

Liquid limit, w~ %, Clay fraction,< 2pro% 0 20 40 60 80 100

! , i i | 1 Numerical codes given in table 6

�9 i

Poloeogene

' f 4 ' Z: Cretaceous .,. �9 ~ 8 ~ 1 12

14 Jurass ic 15

___Zwo I . . . . .

23 �9 I

24 �9 �9

25 ~ : Weathered ] �9 t LIqmd B

limit _ 26 _ = Unweathered

L.Pal. ~ Cloy f ract ion

P L A S T t C l T Y ] low {mediate. high

F]o. 12. Plasticity and clay size fraction of U.K. mudrocks (Anon 1972).

practically the same range as p re s su reme te r results, the mean value for the fo rmer was approximate ly 44% of the in situ value for the same range of depths . The re fo re , in the case of K e u p e r Marl , the averages quo ted in Table 6 and plot ted in Fig. 13 are pres- su reme te r values.

It is apparen t from Fig. 13 and ear l ier discussion that the previous dep th of burial, fissuring and presen t ove rbu rden depth exer t a ma jo r control on the un- dra ined shear s t rength of mudrocks in the unwea th - e red state. If the wea the r ed condi t ion is also consi- de red , the range of su values ob ta ined for a par t icular hor izon can be very large indeed. Moreove r , the un- d ra ined shear s t rengths of w e a t h e r e d m u d r o c k s are very much the same notwi ths tanding the o ther causes of variat ion. Previous commen t s regard ing the importance of fissures are re inforced in Fig. 13, par t icular ly with re fe rence to the more indura ted L o w e r Lias mudrocks or even Lower Oxford Clay, and o lder mudrocks .

Effective and apparent effective shear strength parameters

It is conven ien t to consider both effective s t rength (c', d/) and apparen t effective s t rength pa rame te r s (ca, ~b,) unde r the same head ing since, as expla ined with re fe rence to Fig. 5, the dist inction be tween them in rocks is dic ta ted by the process of degrada t ion . If, as depic ted in Fig. 14, these pa rame te r s are p lo t ted with c' or ca on a logar i thmic scale against 4~' or 4~a, then the results can be separa ted into the fol lowing groups:

i. intact rocks; ii. jo in ted and fissured rocks and n o n - e x h u m e d

over -conso l ida ted clays.


Palaeogene

Cretaceous

Jurassic

Triassic

Carboniferous

Undrained shear strength, su(kN/rn 2) 10 1 0 0 1 0 0 0 1 0 0 0 0 1 0 0 0 0 0

! i i i

. . . . . l c ~ ~ . - . a -- -- Weathered ]- Range values _2 = -- Unweathered BE

o - - - o Weathered " 1 . 3 P Average values

- - - - o o - - - - n o ~ a Unweathered J

4 �9 o �9 Numerical codes given in table 6

_ _ 6 D- . . .o D - e

7

~ 11

13 ~ . - c 3 . - i i

14 - -- . . . . . o o �9

15 �9 z c

16

17

18

19

2 4

S O I L S

R O C K S

very w e o k ' ~ ~ moderately weak T ~ v e r y strong

: : ; ; ~ , a r d FIG. 13. Undrained shear strength of U.K. mudrocks

iii. exhumed over-consolidated clays and degraded rocks.

In Fig. 14, groups (ii) and (iii) represent the end members of ranges in that, as already demonstrated, due to the weakening of interparticle bonds, stress relief causes a large reduction in c', with a smaller reduction in 4", It is not surprising therefore, to find that some results for partially weathered Coal Meas- ures mudstone and Keuper Marl exhibit behaviour intermediate b~tween groups (ii) and (iii). Pertinent to the degradation history of certain mudrocks are the parameters obtained for seatearths tested by Spears & Taylor (1972) which fall within group ii in Fig. 14. These materials, can become softened at unusually great depths as a function of groundwater circulation in the immediately overlying coal seam. Hence, they can assume the character of stiff over-consolidated clays. Unfortunately, there are insufficient effective strength data for mudrocks to detect any trend which may be related to age or previous depth of overburden.

Residual shear strength

The high values of maximum residual shear strength (~b'r = 26.5-36 ~ given in Table 6 for indurated mudrocks of Keuper Marl, Coal Measures, Culm Measures and Lower Palaeozoic age contrast markedly with the

lower values (~br'=5-24 ~ ) obtained for over- consolidated clays.

It is useful to compare these data with empirical relationships obtained between residual shear strength and other geotechnical parameters. For example, Skempton (1964) related ~br' to the clay fraction by the envelopes shown in Fig. 15. Superimposed on these envelopes in Fig. 15 are residual shear strength and clay fraction data from Table 6. For each formation the minimum ~br' value is assumed to correspond with the maximum clay fraction value, and vice-versa. Hence, the line for a particular formation represents ranges of points that could theoretically lie anywhere within the rectangle of which the line is a diagonal. In certain cases (Etruria Marl and Weald and Atherfield Clay) the residual shear strength is lower than expected. Furthermore, some Coal Measures mudrocks have slightly higher 4)/values than their clay contents suggest is appropriate.

Instead of plotting clay fraction against residual shear strength, Voight (1973) and others have drawn a plasticity index (Ip) versus tb/ diagram. Data from Table 6 have been added to Voight's plot in Fig. 16 where the same method of plotting has been used as in Fig. 15. Of the mudrocks for which the variation in both Iv and tb~' is known, Jurassic Fullers Earth, Lias Clay, Etruria Marl, Coal Measures mudstone and


100000

10000

1000

, I C or Ca ( k N / m Z )

100

I B2~" ~" ~'- ~ (i) Intact rocks

\ / \ /

\ / \ /

N\ ao// ~-

�9 Weathered �9 Unweathered

Codes denoting mudrock formations given in Table 6

k / •2 • ' . • (ii)Fissured rocks end ~" ~.~ ~.overconsolidoted cleys

e21

/.1, ~.... / 3 7eiii2 012 ~, w20

/6 �9 181r (iii} Overconselideted cloys ;2o~ e24 e14 21e 111 I ond weothered rocks

\ 17e

\ / /

\~ 018 ~' /

0' or 0o (degrees) FIG. ]4 . Effective shear s t reng th and appa ren t effective shear s t rength pa rame te rs o f U . K . mudrocks .

�9 Goult Cloy - ~ -~- ~ ~ 0 Speeton Cloy

~ ~ �9 Kimmeridge Cloy o ~ ~ '~ Ampthill Cloy Erem Atle

-~ \ \ \ . \ o O~for~ Cley - Sk:~:on X ~ \ ~ ~ * Lies Cloy ona t, nen,

- - \ ~ \ . %. \ �9 Cool Measures " \ . ,,,,m,.ioo,

\'X~\ " \ N N

X * \ O'r 20

degrees

Atlewell & Former(1976), )ton (1964~,

Chendler (1969),

12 \ 4 ~ - 15 [] t ~ " ~ ~ -~ \~ ~ - - ' ~ Envelope from

t \ ~ ' - ~ 114 - ' ~ \ ~ - Skempton (1964)

"o 1~ ~'o 3'o ;o 5'o 6'o ~o 8'o ;0 1~o Cloy f r o c t i o n , < 2 p

FIG. 15. Residual shear strength and clay size fraction of U.K. mudrocks.


36 \ 1 ~ "~176 c'~ I \ / II Gault Clay /

52 ~ ~ O Atherfield Cloy | ~, ~ . w,old Clay I-~on~ v0io.,(1973~

I\ ~ * Lios Cloys / [\~ | O Keuper Marl /

; k % / Numerical codes given in table 6

degrees ]

10 I ~0 "

'i f:%.\ % ,g 2'0 3'0 go ,'o ~'o ,'o ;o ,%

Plasticity index%

F~c. 16. Residual shear strength and plasticity index values of U.K. mudrocks.

shale, Atherfield and Weald Clay are among the formations that appear to have residual shear strength Values which are lower than expected.

Deformat ion parameters

The average values of the consolidation parameters and elastic moduli are presented in Table 6 for a number of U.K. mudrocks. Since these are averages, values which are either lower or higher can be found in the literature,. In the case of London Clay, Butler (1974) quotes a minimum coefficient of volume compressibility of 0 .002m2/MN which compares wi th-a mean value of 0.02 m2/MN. Where both coefficient of volume compressibility and modulus of elasticity data are available for the same formation, then a comparison between bor(h parameters has been made on the assumption that E~--1/my, In Table 6 it can be seen that these parameters do not relate particularly well to each other.

Although the paucity of the information precludes correlations being made between deformation characteristics and age or depth of burial of mudrock formations, a slight trend of increasing modulus and decreasing coefficient of volume compression is discernable. The effects of induration are dramatically demonstrated b F the comparatively high deformation modulus found in the Lower Carboniferous and the remaining Lower Palaeozoic mudrocks.

Conclusions In terms of their general geotechnical properties, mudrocks have characteristics intermediate between those of soils and rocks. The more indurated varieties can exhibit uniaxial compressive strengths (or undrained shear strengths) and elasticities which are commensurate with very strong rocks. Even many of the over- consolidated clay horizons range in strength up to weak rocks. The engineering properties they display are a product of a number of genetic factors, including composition, degree of over-consolidation and diagenetic changes. A retrogression of these properties on exhumation results in the loss of over-consolidation through associated softening and breakdown of interparticle bonds. This degradation process is continued during weathering and ultimately leads to a normally consolidated soft to very soft clay, notwithstanding the parent material.

Although variations in many factors preclude any rigorous demonstration of the causes of variation in just one factor, changes in composition, in particular reductions in the clay size fraction or amount of expandable clay mineral in proportion to the other- more equant constituents, can ' improve' the engineering properties of mudrocks. These improvements, which are largely commensurate with age and depth of burial, may be brought about by a reduction in the clay size fraction or enhanced diagenetic clay particle aggregation and species changes.


The residual shear strength (4~r') of a mudrock is a geotechnical parameter which is particularly sensitive to the clay size fraction present. It is noticeable that more indurated mudrocks generally have high 4~'r values compared with those of over-consolidated clays.

Perhaps the most important genetic feature of mudrocks that significantly influences their engineering behaviour is the removal of overburden (uplift and erosion). This process is accompanied by a dominantly vertical expansion of the formation which involves the development of joints and fissures together with softening. These changes are accompanied by reductions in strength and increase in deformability, water content and plasticity. It is significant that the actual rate of increase in undrained shear strength with depth is more rapid in more heavily over-consolidated mudrocks. The process of fissuring has many important ramifications in that these structural weaknesses affect mass behaviour. The effect of fissuring is particularly significant in the older, more undurated mudrocks where the 'rock-like' shear strength characteristics of the non-fissured and jointed rock are replaced by the 'soil-like' characteristics of the fissured material. Thus, the cohesion intercept can undergo a reduction of several orders of magnitude, with the angle of internal friction also being reduced, but to a lesser extent.

The presence of fissures can cause difficulties in establishing engineering parameters in that sample disturbance and size have significant effects on the results of tests. Typically, the lowest strength values are obtained in large diameter (865-mm) plate loading tests. Providing that sample disturbance has not been excessive, then undrained triaxial compression tests on 100-mm diameter specimens yield results which are intermediate between plate tests and the value obtained for small (38-mm diameter) intact specimens. In fact, plate loading tests may yield conservatively low values, since rapid surface softening can occur once the test site has been excavated. Compared with values obtained by back analysis, stress relief effects appear to cause significant reductions in the modulus of elasticity values obtained in laboratory tests. Part of the difference may be due to strain dependency effects.

Comparisons between residual shear strengths obtained from tests using a ring shear apparatus, the reversing shear box and co-planar triaxial tests indicate that the first gives the lowest values with triaxially determined values being the highest. Generally the reversing shear box produces results which are not greatly different in magnitude from those obtained from the back analysis of slope failures. The strong dependence of the value of residual shear strength on the value of effective stress used in the tests is another important behavioural aspect.

Weathering is accompanied by increase in water content, plasticity and compressibility with reductions in strength. Large decreases in effective (stress) cohe-

sion to a value near, or equal to zero and smaller percentage reductions in the angle of internal friction occur. This latter parameter can be reduced to the 'fully softened' value through the development of progressive small-scale failures in the weathering mudrock mass. The weathering zonation is a useful framework for the classification of weathered mudrocks. Unfortunately, since partially weathered mudrocks tend to be heterogeneous, only in certain cases are there sufficient data to define the zones according to their engineering properties. In general the engineering behaviour of more fully weathered mudrocks is a function of their composition rather than other genetic features.

ACKNOWLEDGEMENT. JCC acknowledges the financial sup- port provided by the University of Sheffield Research Fund.

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The Engineering Properties of Mudrocks