1985 Residual Strength of Clays in Landslides Skempton GE350101

S-ON,

A. W. (198.5). G&technique 35, No. 1. 3-18

Residual strength of clays in landslides, folded strata and the laboratory* A. W. SKEMIlONt

The post-peak drop in drained shear strength of an overconsolidated clay may be considered as taking place in two stages. First, at relatively small displacements, the strength decreases to the ‘fully softened’ or ‘critical state’ value, owing to an increase in water content (dilatancy). Second, after much larger displacements, the strength falls to the residual value, owing to reorientation of platy clay minerals parallel to the direction of shearing. If the clay fraction is less than about 25% the second stage scarcely comes into operation; the clay behaves much like a sand or silt with angles of residual shearing resistance typically greater than 20”. Conversely, when the clay fraction is about SO%, residual strength is controlled almost entirely by sliding friction of the clay minerals, and further increase in clay fraction has little effect. The angles of residual shearing resistance of the three most commonly occurring clay minerals are approximately 15” for kaolinite, 10” for illite or clay mica and 5” for montmorillonite. When the clay fraction lies between 25% and 50% there is a ‘transitional’ type of behaviour, residual strength being dependent on the percentage of clay particles as well as on their nature. The post-peak drop in strength of a normallyconsolidated clay is due only to particle reorientation. Measurements of strength on natural shear surfaces agree, within practical limits of variation, with values derived from back analysis of reactivated landslides. This ‘field residual’ strength can be recovered by multiple reversal shear box tests on cut-plane samples, but in high clay fraction materials it is typically somewhat higher than the strength measured in ring shear tests. Residual strength is little affected by variation in the slow rates of displacement encountered in reactivated landslides and in the usual laboratory tests, but at rates faster than about lOOmm/min qualitative changes take place in the pattern of behaviour. A substantial gain in strength is followed, with increasing displacement, by a fall to a minimum value. In clays and low clay fraction silts this minimum is not less than the ‘slow’ or ‘static’ residual, but in clayey silts (with clay fractions around 15-25% according to tests currently in progress) the minimum can be as low as one-half of the static value.

d’abord, pour des d&placements relativement petits, la resistance decroit jusqu’a la valeur correspondant a I’Ctat critique, a cause d’une augmentation de la teneur en eau (dilatance). Puis, apres des deplacements beaucoup plus considtrables, la resistance tombe a la valeur residuehe, a cause de la reorientation des mineraux d’argile en forme de feuillets paralleles a la direction du cisaillement. Si la fraction d’argile est inftrieure a environ de 25% la deuxieme &ape apparait rarement et I’argile se comPorte a peu prts comme du sable ou du limon avec des angles de resistance rtsiduelle au cisaillement typiquement suptrieurs B 20”. Inversement, avec une fraction d’argile d’environ 50% la resistance rtsiduelle est rtgie presqu’entierement par le frottement glissant des mintraux argileux et une augmentation ulterieure de la fraction d’argile n’a que trts peu d’effet. Les angles de resistance rtsiduelle au cisaillement des trois mineraux argileux les plus souvent trouves sont approximativement 15” pour la kaolinite, 10” pour l’illite ou I’argile mica&e et 5” pour le montmorillonite. Lorsque la fraction d’argile est comprise entre 25% et 50% il y a un type pour ainsi dire transitoire de comportement, puisque la resistance residuelle depend du pourcentage de particules d’argile aussi bien que de leur nature. La chute de resistance qui suit la valeur de pit est due exclusivement 9 la reorientation des particules. Dans les limites pratiques de variation les mesures de la resistance effect&es sur des surfaces naturelles de cisaillement s’accordent avec les valeurs obtenues a partir de l’analyse a posteriori de glissements de terrains reactives. Cette resistance residuelle in situ peut &tre retrouvee par des essais de bone de cisaillement alternatifs multiples effect&s sur des Cchantillons a plans coupes; mais dans des mattriaux ayant une grande fraction d’argile elle est typiquement un peu superieure a la resistance mesurte a l’aide d’appareils de cisaillement circulaire par torsion. La resistance rdsiduelle n’est que legbrement affect&e par des variations dans les vitesses lentes de dtplacement qu’on trouve dans les glissements de terrains reactives et dans les essais habituels de laboratoire, mais a des a environ lOOmm/min des vitesses superieures changements qualitatifs ont lieu dans la forme du comportement. Un gain appreciable de resistance est suivi, au fur et a mesure que le d&placement augmente, par une chute a la valeur minimale. Dans les argiles et les limons a basse fraction d’argile ce minimum n’est pas inferieur a la valeur residuelle lente ou statique, mais dans les limons argileux, avec des fractions d’argile d’environ 15-25% selon des essais en cours actuellement Ie minimum peut etre aussi bas que la moitie de la valeur statique.

On peut admettre que la chute qui suit la valeur de pit dans la resistance au cisaillement dans l’etat drain& d’une a&e surconsolidee a lieu en deux &apes. Tout * Special lecture given to the British Geotechnical Society, at the Institution of Civil Engineers, on 6 June 1984. t Imperial College of Science and Technology. 3

S-ON

Residual

Low (e g. <

INIRODUCIION In the Rankine Lecture of 1964 the Author drew attention to the nature and engineering significance of residual strength. Much has been learnt during the past 20 years, and the present lecture is an attempt to summarize our knowledge of this subject. Residual strength is the minimum constant value attained (at slow rates of shearing) at large displacements. The displacements necessary to cause a drop in strength to the residual value are usually far greater than those corresponding to the development of peak strength and the fully softened (critical state) strength in overconsolidated Consequently, residual clays. strength is generally not relevant to first-time slides and other stability problems in previously unsheared clays and clay fills, but the strength of a clay will be at or close to the residual on slip surfaces in old landslides or soliflucted slopes, in bedding shears in folded strata, in sheared joints or faults and after an embankment failue. Therefore, whenever such pre-existing shear surfaces occur the residual strength must be known, as it will exert a controlling influence on engineering design. DEVELOPMENT OF RESIDUAL STRENGTH The post-peak drop in drained strength of an intact overconsolidated clay may be considered as being due, firstly, to an increase in water content (dilatancy) and, secondly, to reorientation of clay particles parallel to the direction of shearing. At the end of the first stage the ‘fully softened’ or ‘critical state’ strength is reached. At larger displacements, when reorientation is

20%)

z

clay

N-C

peak

fraction

complete, the strength falls to and remains constant at the residual value (Fig. l(a)). In normally consolidated clays, which consolidate when sheared (to displacements a little beyond the peak) the post-peak drop in strength is due entirely to particle reorientation. The effects of particle reorientation are felt, to any appreciable extent, only in clays containing platy clay minerals and having a clay fraction (percentage by weight of particles smaller than 0.002 mm) exceeding about 20-25%. Silt and sandy clays with lower clay fractions exhibit nearly the classical ‘critical state’ type of behaviour in which, even at large displacements, the strength is scarcely less than the normally consolidated peak value, and the post-peak drop in strength of overconsolidated material of this kind is due almost entirely to water content increase (Fig. l(b)). The change from ‘sand’ to ‘clay’ type of behaviour is clearly demonstrated by a series of ring shear tests on sand-bentonite mixtures (Fig. 2). As will be seen later, the same pattern is found in natural clays. There is ample evidence from the field, as well as the laboratory, for an increased water content in sheared overconsolidated clays. London Clay, for example, has a water content of about 34 at and near slip surfaces, compared with 30 in neighbouring unsheared material (Skempton, 1964). A still larger increase has recently been observed in the heavily overconsolidated Siwalik strata at the Kalabagh Dam site where water contents in tectonically sheared claystone are around 23 in contrast with values of about 15 in unsheared material having the same clay fraction of anoroximatelv 60%. 1I

RFSIDUAL.

STFtF.NGTH

Orientation of platy clay minerals in shear zones and on slip surfaces has been observed under the microscope in samples from the field, as at Walton’s Wood (Fig. 3, from Skempton & Petley, 1967a) and several other landslides (Morgenstern & Tchalenko, 1967), and also in laboratory shear tests (Lupini, Skinner & Vaughan, 1981). Plasticity

index

PI

critical

E u zoEC

state)

-----e-o

_J

100 Clay Normally

fraction

consolidated PVCF

CF. % at o’ =

=

350

kPa

1.55

2. Ring shear tests on sand-bentonite (after Lupini, Skinner & Vaughan, 1981)

Fig.

mixtures

A

I

1

pellet . organic

0

,\”

Clay

lncluslon

Z

Partlcle

OF CLAYS

Displacements at various stages of shearing Peak strengths are attained at small strains corresponding to displacements of the order 1 mm in shear box or ring shear tests on overconsolidated clays, and after rather more movement for normally consolidated clays: see Table 1. Water content changes (softening in overconsolidated and consolidation in normally consolidated clays) seem to be essentially complete at displacements generally smaller than 10 mm; often about 5 mm is sufficient (Petley, 1966). Ring shear tests at normal effective pressures up to about 600 kPa indicate that displacements usually exceeding 100 mm, and in some cases exceeding 500 mm, are necessary before the strength of an intact clay falls to a final steady residual value, represented by an angle of shearing resistance & However, strengths approaching close to this final value, for example to a strength represented by &+ l”, are reached at displacements ranging from about 20% to 50% of those required for the full drop to the residual (see Fig. 4 and data given by Lupini, 1980). At higher pressures it would be expected that particle orientation, and therefore the fall to residual strength, is completed at smaller displacements. This idea receives support from tests on a clay shale by Sinclair & Brooker (1967). With cr’ = 100 kPa the strength was still falling after displacements of 6Omm, but when cr’ = 2000 kPa the residual was reached at about 25 mm. Less information is available on the strength characteristics of structural discontinuities in clays, such as joints and bedding planes, which have not been sheared in nature. Tests on joint surfaces in the S. Barbara Clay (of Pliocene age, near Florence) show a reduced peak strength compared with that of the intact clay, and the residual is attained at displacements of 3040 mm (Fig. 5). In tests on London Clay joint surfaces all the cohesion had been lost and the angle of shearing resistance was within 3” of the residual after 8 mm displacement (Skempton & Table 1. Typical displacements shear in clays having CF>30%

Stage

orlentatlon

Peak Rate of volume change approximately zero At &,+1” Residual 6, Fig. 3. Fabric of shear zone and slip surface at Wafton’s Wood

5

Intact clays, with a’

Upper Carboniferous

29 21

Bury Hill Various M4, near Swindon Sevenoaks bypass various

Etnria Marl Upper Lias Gault Athetfield London Clay

30 29 36 35 34

PL

60 64 64 75 80

Marl. Investigations made in 1968 (Hutchinson, Somerville & Petley, 1973) enabled the slip surface and piezometric levels to be determined, and four sets of slip surface tests were carried out. The results showed some scatter, but three of the four samples gave reasonably consistent strengths corresponding to an angle of shearing resistance of about 13-6” at the average normal effective pressure of 97 kPa acting on the slip surface. This result has to be compared with 12.0” as the best estimate from back analysis, but there are difficulties in figuring the piezometric levels at the time of the 1960 failure, and the material is variable. The difference, of about 12%, is therefore considered not to be of great significance. In Table 2, summarizing data on field residual strength, the angle of residual shearing resistance deduced from this case record is taken as 12.5” at 100 kPa with a curvature of the envelope as given by the slip surface tests. London Clay The first line relating field residual strength and normal effective pressure for London Clay

27 28 29 29 29

&=

tan

’ (s/u)at the following cr’ values: deg

CF

PIICF

150 kPa

70 36

0.4 0.6

12.8

52 52 47 58 55

0.6 0.7 0.8 0.8 0.9

12.1 9.9 11.1 11.8

was based on slip surface tests from sites at Guildford and Dedham, and on a single back analysis of a reactivated landslide in a railway cutting at Sudbury Hill (Skempton & Petley, 1967a). However, at the small average pressure in this slip (30 kPa) a considerable percentage difference existed between back analysis and the test results. Nine years later Hutchinson & Gostelow (1976) presented data from analysis of slips in an abandoned London Clay cliff at Hadleigh which confirmed the Sudbury Hill result and extended the range of back analysis to 50 kPa. An improved field residual envelope could then be drawn, much as in Fig. 11, but still with only the few low pressure Guildford slip surface tests affording a (poor) comparison with back analysis strengths. However, the situation greatly improved in 1978 when Bromhead published analyses of several rather deep-seated slips at Herne Bay, with normal effective pressures of lOO150 kPa (Bromhead, 1978). As will be seen, these new results strongly support the best-fit line drawn through the slip surface test points and despite the scatter (to be expected with tests

10

SKEMITON

London

LL =

Clay o

loo-

Tests on SllP surface

80

PL =

CF

29

=

55

GuIldford

D Dedham v

Walthamstow

0

Warden

Back analysis

Point

.

Sudbury

HalI

l Hadleigh

.

Herne

Bay

M Wraysbury

100 Normal

150 effectw

Fig. 11. Field residual strength for London

from different sites) there can be little doubt that the tests and back analysis are measuring essentially the same strength. Summary of the comparisons A statistical summary of the comparisons between back analysis and slip surface test results is given in Table 3. This shows that while there is a tendency for the tests to give slightly higher strengths, on average by about 0.5” in the angle of shearing resistance, the difference is within the limits of variation. Thus the conclusion is reached that back analysis of reactivated landslides and slip surface tests (at the relevant effective pressure) both give the field residual strength. It also follows from the statistics in Table 3 that, even in the almost ideal conditions of these case records, where pore pressures are known with reasonable certainty and problems such as the effects of progressive failure are absent, stability analysis and laboratory tests cannot be expected to yield results with an accuracy better than about &lo%. Table 3. Comparison between back analysis of reactivated landslides and slip surface test results (14 case recolds)

Parameter

Angle of shearing resistance: deg

Mean 4 from analysis Mean 4 from tests Mean A+ Standard deviation in A+ Maximum A+ Minimum A&

12.8 13.4 +0.6 Zt1.2 +2.5 -2.2

A&l&: %

200 stress

u’

kPa

Clay

Other clays Granted the above conclusion, it is possible to collect values of field residual strength from several other investigations. Three will be mentioned here; a unique set of results from the Siwalik claystones is separately discussed. One of the earliest examples of back analysis of a reactivated landslide, at Jackfield, was published by Henkel & Skempton in 1955, before the subject of residual strength was understood. However, the analysis is sound and provides data on a clay having a smaller clay fraction than is common in landslide studies. Slip surface tests on Atherfield Clay from Sevenoaks Weald escarpment have been shown in Fig. 6. They are three of a total of seven such tests measuring field residual strength at pressures from 70 kPa to 400 kPa. The third clay in this context is the Upper Lias, for which Chandler (1982) gives valuable information on stability analysis and other details from eight different sites, covering pressures from 12 kPa to 120 kPa. Results for these and the four clays previously discussed are summarized in Table 2.

Curvature of envelope For most clays the relation between residual strength and normal effective pressure is nonlinear. The strength s at any given pressure u’ is conveniently expressed by the secant angle of shearing resistance 4 where tan 4 = s/u’

+4.5 *9 +17.5 -17

Values of 4 for (r’ = 50 kPa, 100 kPa and 150 kPa are given in Table 2. When comparing one clay with another it is best to fix on a ‘standard’ pressure, such as 100 kPa. Thus the value of & at u’ = 100 kPa

STRENGTH

RESIDUAL

11

OF CLAYS

A London

Clay

0 Llas OGaull I A@ = Fig.

12. Difference

@r,nrl

tan +/tan6 loo

A*

=

1.5”)

between ring shear and field residual strength

can be taken as a characteristic parameter of a clay. Curvature of the envelope can be expressed by the ratio of tan 4 at a pressure (T’ to the ‘standard’ tan 4 at 100 kPa. Mean values of this ratio for the clays listed in Table 2 are as follows: u’: kPa

d,, (mean

each point 6 an average otzor3 analyses

25

50

100

150

1.12

1.07

1.00

0.96

However, there are considerable variations in the degree of curvature between one clay and another. For design purposes it is often useful to take a ‘best-fit’ linear envelope over the range of pressures involved, in the form s=c+a’tanb COMPARISON OF FIELD RESIDUAL AND RING SHEAR TESTS Ring shear tests in the machine described by Bishop, Green, Garga, Andresen & Brown (1971) tend to give residual strengths, for high clay fraction materials, which are somewhat lower than the field values. Typically the difference is 1” or 2” in the angle of shearing resistance, as shown in Fig. 12 where comparisons are made with back analysis results. Chandler (1984) summarizes the data for Lias and London Clay, and a ring shear test on Gault from the M4 landslide at Burderop is quoted by Lupini (1980). At Bury Hill a ring shear result lay as much below the back analysis strengths as the slip surfaces tests lay above but, as previously mentioned, the clay at this site is variable. Various suggestions can be made in explanation, mostly based on the idea that shearing in the ring test is more concentrated or intense than in landslides, but the question is still unre-

solved, especially since Bromhead & Curtis (1983) indicate that with a different ring shear machine agreement with field residual strength is obtained in London Clay, despite the fact that this machine and Bishop’s give almost identical results on two samples of Gault Clay from Folkestone Warren (Bromhead, 1979). RELATION BETWEEN RESIDUAL STRENGTH AND CLAY FRACTION It is clearly a matter of great interest to obtain a relationship between residual strength and clay fraction for a natural material covering a wide range of particle size but having essentially the same clay mineralogy throughout. This is now close to being achieved by tests on Siwalik claystones and siltstones in Pakistan. Siwaliks Investigations at Mangla and a neighbouring site at Jari, and currently in progress at the proposed Kalabagh Dam on the Indus, provide data from within mutually similar suites of materials. At these locations rather thick beds of sandstone alternate with finer-grained beds of claystone and siltstone, ranging from the top of the Middle Siwaliks (late Pliocene) at Kalabagh into the Upper Siwaliks (early Pleistocene) at Mangla and Jari. The strata are heavily overconsolidated freshwater deposits and, owing to tectonic folding, most of the claystones contain bedding shears while thrust joints (many of them sheared) characterize the siltstones. Illite and kaolinite are the dominant clay minerals, with subordinate montmorillonite, and the PI/CF ratios vary between 0.5 and 0.8 with a slight tendency for lower values at Kalabagh than at Mangla and Jari. Typically there is a calcite content of about 5%. After many attempts to obtain satisfactory shear surface samples from these hard materials, seven sets of shear box tests were successfully

12

SKEWETON

carried out at the Mangla laboratory in 196% 67. Results for a high clay fraction bedding shear are shown in Figs 13 and 14. One test shows a small peak, as the shear surface could not be aligned perfectly with the plane of the box, but a steady minimum strength is attained after only 5 mm displacement. In the two other tests the shear surface (field residual) strength is

LL = 68

0

Sample 64144 PL = 28

CF = 58

400 o’ kPa

200

@&Sample

600

800

S’hear

64138

surface

recovered from the start, as was the case with most of the other samples. Tests on a thrust shear joint in siltstone are shown in Fig. 15. The displacement on this joint was quite small. Nevertheless the tests indicate that the residual strength has already been developed in nature, presumably to be accounted for by the low clay fraction (compare with Fig. l(b)) and also by the high pressure acting when the joint was sheared. Values of & (at o’ = 400 kPa) from these seven samples are plotted in Fig. 16. They reveal a relationship evidently corresponding to the ‘transitional’ and ‘sliding shear’ zones of the sand-bentonite tests of Fig. 2. However, it is possible to add further points and to extend the graph into the ‘sand’ or ‘rolling shear’ zone by including results of cut-plane multiple reversal shear box tests made at the Kalabagh laboratory. The cut plane acts rather like an unsheared joint, and five or six reversals usually produce a steady minimum strength (Fig. 17). The close correspondence between cut-plane and shear surface tests, demonstrated in Fig. 16, provides evidence that the cut-plane tests give a good measure of the field residual strength and justifies the use of such tests in delineating the picture, presented here for the first time, showing the relation between residual strength and clay fraction in a natural sedimentary deposit.

Fig. 13. Jari Dam: left abutment, shear zone A Sample LL = 150

-

68

6144

PL =

~,rst run

CF

28

---Second

=

58

run

n’--- =

300 I

831

---

f / /---0.0025

:

$=

292

I

mmlmr

Sample u’ =

s3

830

LL = -

40 Frst

76109

PL = run

21

CF

---Second

=

23

run

i,oo_/yK-T+

4 Dtsplacement:

6

8

10

mm

Fig. 14. Shear surface tests on Jari Dam, shear zone A, January 1%6

2

4 Displacement

6

8

10

mm

Fig. 15. Shear surface tests on Jari Valley no. 3, thrust shear joint, November 1965

FCESILXJAL STRENGTH

C&O3

<

PliCF

=

10%

.

Mangla

n

Jan

q

Kalabagh.

0.5 - 0.8

13

OF CLAYS

3

Values

Shear tests

surface

cut-plane

tests

of o,, at on’ =

400

kPa

40t--

SlItstone

.

Claystone

1 From field records

-

-0-1

E30D

\

‘,,,,,,,,

Bedding/,+,,, shears

\

B

\

20 -

OL

10 I

20 1

301 Clay

40 1

fraction

(after

16. Field residuals for Sialik

Fig.

50 I

60 I

80 1

90 L

%

pretreatment)

claystone

70 L

and siltstone,

April 1984

300 :

W =Sample 21 LL 1359 = 49 d, S, =

0

I

I

2

=

PL =Test 29 83CF

75 kPa

I

4 Dlsplacemenl.

Fig.

17. Reversal

= 42

10.6” o;

=

400

kPa

I

a

6

I

10

mm

shear box test on a cut-plane

sample

at Kalabagh,

October

1983

Variations with clay mineralogy The clay minerals can have little effect on residual strength when the clay fraction is less than 20%, as the strength is then controlled largely by the sand and silt particles. Conversely, with clay fractions exceeding 50%, residual strength depends almost entirely on sliding friction of the clay particles and therefore depends on their character. Thus the siltstone in Fig. 16 with 13% clay fraction has a strength equal to that of sand. At

the other end of the scale, clays such as the Lias and Atherfield having PI/CF ratios similar to those of the Siwalik claystones have much the same residual strength (Fig. 18), but the kaolinitic clay from Walton’s Wood (PI/CF = 0.4) has a somewhat greater residual, despite its high clay fraction, and lies in Fig. 18 not much below the point for kaolin itself (Lupini, 1980). In sharp contrast, if the PI/CF ratio exceeds about 1.5, as in some clay shales reported from the USA (Townsend & Gilbert, 1973) the residual angle

14

SKEMPTON

PIICF Values

40

+ Walton’s

of I$,,

at nn’ x 100

Wood (Upper

x JackfIeld

kPa

. Bury

t

HIII

I

0.4 0.6

Carbon-

Iferous)

0.6

o Siwallk

0.7

0 LIZIS

0.7

o Swmdon

(Gault)

0 Sevenoaks a London

0.8

(Atherfleld)

0.8

Clay

0.9

l\ l

Approximate for PVCF

0

I

Kaolin

-+--

0.4

Benlomte

--o---

bounds = 0.550.9

1’6

\

20

I

I

1

,

40

60

60

100

Clay Fig.

Aj_,-

fraction

%

18. Field residual and ring shear tests on sands, kaolin and bentonite

o

Kaolm

.

London (each

’= Clay point

>’

350

kPa

= 40-140

ave?age

CF =

82

CF

60

=

of 8 tests)

Usual range of slow laboratory tests

Tii g E

I

0.8

1

0~0001

0.001

L 0.01

2 v, 0.7

1

Fig. 19. Variation

0.01

0.1

1 100

1 mm/rmn i 100

10

I

I 10

0.1

1000

cm/day

, 10 000

cm/year

in residual strength of clays at slow rates of displacement

of shearing resistance falls below 7”, to values comparable with that of bentonite in which the clay mineral is montmorillonite. Finally there is the special case where the particles smaller than 0.002mm are non-platy clay minerals, such as halloysite, or rock flour consisting of very finely divided quartz etc. The angles of residual shearing resistance of such soils bear little if any relation to the content of clay-size particles and are usually greater than 25” (Kenney, 1967; Wesley, 1977). RATE EFFECIX Rates of displacement on pre-existing shear surfaces can vary by many orders of magnitude from exceedingly slow movements in some reactivated landslides to very fast displacements in-

duced by earthquakes. A knowledge of the effects produced by different rates of shearing is therefore a significant part of residual strength studies. Slow rates Tests on two clays over a range of speeds from about 100 times slower to 100 times faster than the usual (slow) laboratory test rate are plotted in Fig. 19 (data from Petley, 1966 and Lupini, 1980). On average, the change in strength is rather less than 2.5% per log cycle. It therefore follows that variations in strength within the usual range of slow laboratory tests (say 0.002-0.01 mm/min) are negligible. In the field, from observations on reactivated landslides and mud-flows, it is known (Skemp-

RESIDUAL STRENGTH OF CLAYS

Table 4. Variations ia residual slow rates of displacement

strength

of days at

~

Laboratory, typical

0.005

.

= 7 mm/day

ton & Hutchinson, 1969) that the highest daily rate of movement is of the order 50 cm/day and the lowest average rate is about 2cm/year, which probably corresponds to a daily rate of not less than 5 cm/year. If the strength at a typical laboratory rate of 0+00.5 mm/min is taken as standard, the variations over this entire range lie between -3% and +5%, as set out in Table 4. Thus it appears, to a first approximation, that all such movements can be regarded as ‘slow’ and as being related to a ‘static’ residual strength equal (from this point of view) to values measured in the usual slow laboratory tests. This is the justification for making a comparison, without any rate correction, between slow laboratory tests and back analysis. There is, however, an interesting corollary since Fig. 19 also implies that small changes in strength can cause large changes in rate of movement. This immediately accounts for the marked influence of seasonal variations in piezometric levels and for the success of remedial works which bring about a relatively small increase in factor of safety. Fast rates In connection with earthquake design of the Kalabagh Dam project, tests are being made at Imperial College to measure the effects of fast rates of displacement on residual strength. A Sample vv = O-St

0.01

205

62

LL

100

sample is remoulded with water to bring it to a plastic state and tested in the ring shear apparatus at pressures of 200 kPa and 500 kPa after preconsolidation at the maximum attainable pressure of 900 kPa. In all cases the water content during the shear tests is at, or a little below, the plastic limit. The slow residual state is first established by shearing at 0.01 mm/min to displacements usually of about 500mm (Fig. 4). The rate is then increased and maintained until approximately steady conditions obtain. After a pause to allow any pore pressures to dissipate, the slow rate is reimposed. The rate is then increased again, to some other high value and so on until tests have been made at three or four different fast rates under both pressures. Part of the first of this series of tests, in which the fastest rate was 400 mm/min, is shown in Fig. 20. In subsequent tests 700-800 mm/min has been achieved. All samples so far tested at fast rates show a rise in strength to a maximum, followed by a decrease to an approximately steady minimum value. To obtain characteristic parameters for any particular sample, 400 mmlmin is chosen as representing the fast tests and the strengths (residual, fast maximum and fast minimum) are plotted against normal pressure, in order to obtain by interpolation the values at a standard pressure of 400 kPa (Fig. 21). For clays the increase in strength becomes pronounced at rates exceeding 100 mm/min (Fig. 22) when some qualitative change in behaviour occurs. This is probably associated with disturbance of the originally ordered structure, producing what may be termed ‘turbulent’ shear, in contrast with sliding shear when the particles are orientated parallel to the plane of displacement. It is possible, also, that negative pore pressures are generated and, as displacement continues, these are dissipated within the g=

188L

27

15

kPa (p,

PL =

0 01

400

= 900

26

kPa)

CF

=

mm/mm

47

0.01

o-5 0.4 b 0.3 O-215 0.2 0.156 0.1 b

-___-_-.

12h

0.156

pause

-

0.155

12 h pause

\,

0 1 500

600

700 Displacement

Fig. 20. Kalabagh

800

900

mm

Dam ring shear test, August

1983

16

SKF.ME’rON 300Sample

704

LL = 45

Rmg

PL =

o Residual 400

23

shear CF =

40 X Max

Fast mm/mln

+ M,n

200 -

6 kPa

21. Kalabagb Dam ring shear tests, Febmary

Fig.

Sample LL = 45

1984

704

PL = 23

CF

= 40

kPa Max

Min

Slldmg shear

0000 1

10

Turbulent shear

400

100 Rate of displacement:

Fig.

22. Kalabagb

Dam ring shear tests, Febmary

1.4

Sample

1.2

w =

0.57

2094 24

(r = LL =

490

1000

mmlmln

kPa

39

1984

(p,

=

PL = 27

900

kPa)

CF

= 3

0.52

-____“z,

0.4 0.2 0

800

I 900

3 h pause , \ 1000

Displacement: Fig.

23. Kalabagb

4 h pause I 1200

1100 mm

Dam ring shear test, April 1984

\

, 1300

1400

RESIDUAL

STRENGTH

Sample LL =

39

PL =

17

OF CLAYS

91 OL 21

CF

=

21

kPa D =

200

g =

495\

01 1

I

1

10 Rate

100 of displacement:

400 1000 mm/mln

J 10 000

Fig. 24. Kalabagh Dam ring shear tests, October 1983

body of the sample thus leading to a decrease in strength. That some structural change has taken place in clays at ratios of 400 mm/min or more seems apparent from the fact that on reimposing the slow rate a peak is observed, the strength falling to the residual only after considerable further displacement (Fig. 20), an effect not seen after shearing 100 mm/min or slower. By contrast, in a low clay fraction siltstone 5 o-

4 OValues

of I$

at (T = 400

kPa

3,O-

P D 0

2‘o-

1 o-

slitstone

0‘

20

LOW CF 30

40

@, deg

Fig. 25. summary of ring shear tests for Kalabagh Dam, June 1984

(CF = 3) there is no qualitative change at rates even as high as 800 mm/min; the strength at once rises to a maximum and then falls sharply towards the residual, and on restoring the slow rate the residual is almost immediately regained (Fig. 23). These effects point to pore pressure changes only; certainly there can be no clay particle orientation or disordering in this sample. As an intermediate material, a clayey siltstone with about 25% clay fraction shows a remarkable drop in strength, at fast rates (400 mm/min or more), from the maximum to a minimum equal approximately to one-half of the residual (Fig. 24). It is surely significant that this material lies in the ‘transitional’ zone, but why it should show a normal increase in strength at fast rates followed by an abnormal decrease is not clear. However, two specimens from this sample, one with 21% and the other with 27% clay fraction, show almost identical patterns of behaviour. Clearly more research is needed better to define the limits of this phenomenon and, for all types of soil, to measure pore pressures at fast rates of displacement and to explore the effects in still more rapid tests. Meanwhile the results at present available are summarized in Fig. 25; their significance in earthquake engineering design is obviously considerable. ACKNOWLEDGEMENTS Permission to quote results from the Mangla and Kalabagh laboratories has kindly been given by the Pakistan Water and Power Authority (WAPDA). Other tests not taken from published papers were carried out as part of a

SKEMPT0N

18

general research programme at Imperial College and in connection with investigations for Kent County Council (Sevenoaks bypass), Sir Alexander Gibb & Partners (M4 landslides near Swindon) and WAPDA (Kalabagh Dam project). The fast ring shear tests are being made by Mr Luis Lemos. In preparing the lecture much benefit has been derived from discussions with Dr R. J. Chandler and Dr P. R. Vaughan. All the tracings are by Mrs Anne Langford. REFERENCES Bishop, A. W., Green, G. E., Garga, V. K., Andresen, A. & Brown, J. D. (1971). A new ring shear apparatus and its application to the measurement of residual strength. G&technique 21, No. 4, 273328. Bromhead, E. N. (1978). Large landslides in London Clay at Herne Bay, Kent. Q. J. Engng Geol. 11,

291-304. Bromhead, E. N. (1979). A simple ring shear apparatus. Ground Engng 12, 40-44. Bromhead, E. N. & Curtis, R. D. (1983). A comparison of alternative methods of measuring the residual strength of London Clay. Ground Engng 16,

39-41. Burland, J. B., Longworth, T. I. & Moore, J. F. A. (1977). A study of ground movement and progressive failure caused by a deep excavation in Oxford Clay. G&otechnique 27, No. 4, 557-591. Calabresi, G. & Manfredini, G. (1973). Shear strength characteristics of the jointed clay of S. Barbara. Gdotechnique 23, No. 2, 233-244. Chandler, R. J. (1982). Lias clay slope sections and their implications for the prediction of limiting or threshold slope angles. Earth Surf. Process Landforms 7, 427-438. Chandler, R. J. (1984). Recent European experience of landslides in over-consolidated clays and soft rocks. Proc. 4th Int. Symp. Landslides, Toronto, 1,61-81. Early, K. R. & Skempton, A. W. (1972). Investigations of the landslide at Walton’s Wood, Staffordshire. Q. J. Engng Geol. 5, 19-41. Henkel, D. J. & Skempton, A. W. (1955). A landslide at Jackfield, Shropshire, in heavily overconsolidated clay. Giotechnique 5, 131-137. Hutchinson, J. N. & Gostelow, T. P. (1976). The development of an abandoned cliff in London Clay at Hadleigh, Essex. Phil. Trans R. Sot., A 283, 557-604.

Hutchinson, J. N., Somerville, S. H. & Petley, D. J. (1973). A landslide in periglacially disturbed Etruria Marl at Bury Hill, Staffordshire. Q. J. Engng Geol. 6, 377-404. Kenney, T. C. (1967). The influence of mineral composition on the residual strength of natural soils. Proc. Geotechnical Conf.. Oslo 1. 123-129. Lupini, J. F. (1980). The residual strength of soils. PhD thesis, University of London. Lupini, J. F., Skinner, A. E. & Vaughan, P. R. (1981). The drained residual strength of cohesive soils. Ge’otechnique 31, No. 2, 181-213. Morgenstern, N. R. & Price, V. E. (1965). The analysis of the stability of general slip surfaces. Gdotechnique 15,No. 1, 79-93. Morgenstern, N. R. & Tchalenko, J. S. (1967). Microstructural characteristics on shear zones from slips in natural clays. Proc. Georechnical Conf., Oslo 1, 147-152. Petley, D. J. (1966). The shear strength of soils at large strains. PhD thesis, University of London. Sarma, S. K. (1973). Stability analysis of embankments and slopes. GCotechnique 23, No. 3, 423433. Sinclair, S. R. & Brooker, E. W. (1967). The shear strength of Edmonton Shale. Proc. Geotechnical Conf., Oslo 1,295-299. Skempton, A. W. (1964). Long-term stability of clay slopes. Gioorechnique 14, No. 2, 75-101. Skempton, A. W. (1971). Report on tests on and adjacent to the slip surface in the Gault clay at Burderop Wood, Wiltshire. Sir Alexander Gibb & Partners. Skempton, A. W. (1972). Report on the investigations and remedial works at Burderop Wood and Hodson landslides on the M4 motorway near Swindon. Sir Alexander Gibb & Partners. Skempton, A. W. & Hutchinson, J. N. (1969). Stability of natural slopes. Proc. 7th In?. Conf. Soil Mech. Fdn Engng, Mexico City, State of the art volume, pp. 291-340. Skempton, A. W. & Petley, D. J. (1967a). The strength along structural discontinuities in stiff clays. Proc. Geotechnical Conf., Oslo 2, 29-46. Skempton, A. W. & Petley, D. J. (1967b). Sevenoaks by-pass. Shear tests on clays. Report for Kent County Council. Townsend, F. C. & Gilbert, P. A. (1973). Tests to measure residual strength of some clay shales. Gkotechnique 23, No. 2, 267-271. Weslev. L. D. (1977). Shear strength properties of hafioysite and allophane clays in-Java, Indonesia. Ggotechnique 27, No. 2, 125-136.