Residual strength of clay in landslides

8/17/2019 Residual strength of clay in landslides

1/16

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

Residual strength of clays in landslides, folded strata

and the laboratory*

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 displace-

ments, the strength decreases to the ‘fully softened’ or

‘critical state’ value, owing to an increase in water

content (dilatancy). Second, after much larger dis-

placements, 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 en-

tirely 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 be-

haviour, residual strength being dependent on the

percentage of clay particles as well as on their nature.

The post-peak drop in strength of a normally-

consolidated 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 mul-

tiple 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.

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.

A. W. SKEMIlONt

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 feuil-

lets 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 com-

Porte a peu prts comme du sable ou du limon avec des

angles de resistance rtsiduelle au cisaillement typique-

ment 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 ap-

proximativement 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 par-

ticules. 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 glisse-

ments 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

vitesses superieures

a environ lOOmm/min des

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 aug-

mente, 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.

3


2/16

S-ON

Residual z N-C peak

Low (e g. < 20%) clay fraction

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 know-

ledge 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 over-

consolidated

clays.

Consequently, residual

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 reorienta-

tion 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

complete, the strength falls to and remains con-

stant at the residual value (Fig. l(a)).

In normally consolidated clays, which consoli-

date 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 be-

haviour 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 be-

haviour 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 frac-

tion of anoroximatelv 60 .


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RFSIDUAL STFtF NGTH OF CLAYS

5

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 & Vau-

ghan, 1981).

Plasticity index PI

critical state)

E

u zo-

-----e-o

EC

Clay fraction CF. %

_J

100

Normally consolidated at o’ = 350 kPa

PVCF = 1.55

Fig. 2. Ring shear tests on sand-bentonite mixtures

(after Lupini, Skinner & Vaughan, 1981)

A

I 1

0

Clay

pellet

,\”

.

organicncluslon

Z Partlcle orlentatlon

Fig. 3. Fabric of shear zone and slip surface at Waf-

ton’s Wood

Displacements at va r ious stag es 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 over-

consolidated clays, and after rather more move-

ment for normally consolidated clays: see Table

1. Water content changes (softening in over-

consolidated and consolidation in normally con-

solidated 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 shear-

ing resistance & However, strengths approach-

ing 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 dis-

placements. 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 30-

40 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 at various stages of

shear in clays having CF>30

Stage Displacement: mm

GC

N-C

Peak 0.5-3

3-6

Rate of volume change

approximately zero

4-10

At &,+1” 30-200

Residual 6,

100-500

Intact clays, with a’


4/16

6

SKEMlTON

Sample 188L

n = 525 kPa (p, = 900 kPa)

LL = 62 PL = 26 CF = 47

Rate of dlsplacemenl 0.01 mm/mln

b 0.3

2 Residual r/u = 0 152 -

o-2

@r = 8 6”

---__. -

____--_--_

---•

01

q, = 10.6” Q = 9.6”

200 300

Displacement. mm

Fig. 4. Kahbagh ring shear test, August 1983

S.

Barbara Clay

w = 51 LL = 76 PL = 43 CF = 37

I I

10 20

30 40

Displacement mm

20

15:

10..

5a

0

He. 5. Reversal shear box tests on intact day and on joint surfaces (from

CGebresi & Maafredini, 1973)

Petley, 1967a). A still sharper reduction in

strength was found in the shaly Lower Oxford

Clay tested parallel to bedding, though probably

not precisely on a bedding plane. Here the angle

of shearing resistance fell to within 2” of the

residual after displacements of only 4 mm and

almost to the residual itself at little more than

l(r2Omm (Burland, Longworth & Moore,

1977). All the tests mentioned in this paragraph

were made at pressures not exceeding 600 kPa.

They indicate the ‘brittleness’ of natural frac-

tures in clays.

FIELD RESIDUAL STRENGTH

When tests are satisfactorily carried out on

samples containing a fully developed slip or

shear surface the residual strength is recovered

at virtually zero displacement, since all water

content changes and particle orientation effects

have already been brought about by the shear-

ing movements in nature. The strength on such

shear surfaces is here defined as the ‘field re-

sidual’ value. In principle it should be the same

as the strength calculated from back analysis of

a landslide in which movement has been reacti-

vated along a pre-existing slip surface and, as we

shall see, this identity has in fact been estab-

lished within practical limits of accuracy.

Examples of ‘slip surface tests’ are shown in

Fig. 6 (Skempton & Petley, 1967b). The tests

were made in the shear box apparatus, care

being taken to locate the slip surface as exactly

as possible in the plane of the box and to

arrange the sample so that shearing follows the

natural direction of movement. It will be noted

that in second runs of the tests, after reversing

the travel of the box, the strengths return closely

to the first-run values. The ‘trough’ in the early

stages of the second runs is characteristic of

reversal shear box tests, although it may be

largely

r

wholly eliminated by unloading the

sample during the backward travel, an improve-

ment in technique introduced later than the date

of these particular tests.


5/16

RESIDUAL STRENGTH OF CLAYS

7

Before proceeding to examine case records

relating to the determination of field residual

strengths, two points must be mentioned. First,

in normal laboratory practice, tests to measure

residual strength are made at slow rates of dis-

placement not exceeding about 0.01 rnm/min to

avoid the possibility of generating unknown

pore pressures. However, it is demonstrated

later in this lecture that over the entire range of

rates of movement recorded in reactivated land-

slides residual strength is unlikely to vary by

more than *S from the value corresponding

to the usual laboratory testing rates. A direct

comparison can therefore be made between

laboratory and back analysis strengths.

The second point concerns stability analysis.

Ideally the reactivated landslide should have a

factor of safety of 1.0, i.e. it should be moving

slowly on a pre-existing slip surface, and the

shape of the slip surface and the relevant

piezometric levels should be known. It is then

possible to calculate the average normal effec-

tive stress and the average shear stress acting on

the slip surface from a two-dimensional analysis,

using the method of Morgenstern & Price

(1965) or Sarma (1973). Finally, a correction is

applied to allow for the strength developed on

the sides of the actual three-dimensional slide.

This amounts to a reduction in shear stress given

by the factor

1 KDIB

where

D

and

B

are the average depth and width

of the sliding mass, and K is an earth pressure

coefficient. In the cases considered here K is

taken as 0.5 and the correction is typically about

5 .

Pll WE1

60

LL 75 PL = 29 CF

= 58

- First run -- Second

run

m

B

0.002 mmlmln

‘-‘40-

TA

sr

=

31.0

Sr = 24.8

E

\

d kPa

”

--

172

I

103

6 20

sr

= 15-2

69

I

4

0

2 4

6 8

Dlsplacemenr mm

I

I

0

2

4 6

6

Displacement mm

Fig.

6.

Slip surface tests on Atherlield Clay from

Fig. 7. Slip surface test at Walton’s Wood landslide,

Sevenoaks Weald escarpment, 1 6

September 1962

Walton’s Wood landslide

The history of field residual strength begins in

September 1962 when the first successful slip

surface test was made on a sample from Wal-

ton’s Wood (Fig. 7) and found to give an angle

of shearing resistance in reasonably good agree-

ment with a conventional back analysis of this

old but still active landslide. Moreover, the

strength lay far below the peak and the fully

softened values for intact samples. Further tests

and more refined stability analysis gave results

(Fig. 8) proving, within the limits of accuracy

expected from field work, that slip surface tests

and back analysis yielded the same strength.

During this investigation, also, particle orien-

tation on the slip surface was observed in thin

sections under the polarizing microscope, and in

addition the residual strength was recovered

(approximately) by multiple reversal shear box

tests on intact clay.

A detailed description of this case record is

available (Early & Skempton, 1972), prelimi-

nary accounts having been given by Skempton

(1964) and by Skempton & Petley (1967a).

Clear evidence existed that the landslide had

undergone large displacements in the past, and

during 3 years preceding investigations it moved

about 1 m. The slip surfaces were in colluvial

clay derived from Upper Carboniferous mud-

stone, with kaolin&e as the predominant clay

mineral.

M4 landsli des near Swindon

Two quite large landslides were reactivated by

cuttings excavated for the M4 motorway, near

Swindon, in the winter 1969-70. A section

through the slide at Burderop Wood is shown in

Fig. 9. The other slide, half a mile away, near

Hodson village, had identical geological condi-

tions and closely resembled Burderop slide in

Sample 126/l 0

d = 59 kPa

w = 27


6/16

8

SKFtMFION

Colluwum from Carbontferous mudstone

LL = 57

PL = 27 CF = 70

SIIP surface tests

. Back analysis

Normal effective stress (T’. kPa

Fig. 8. Walton’s Wood landslide: field residual strength

Distance m

0 50

100

150

200 250

r

NNW

ssw

x slip surface

- 200

I Pwometer

600 - -

Top of Gault

Upper

- 180

1 Plerometrlc level

PrOfIle ,n March ,970

Greensand

Q GWL

Slope indlcalor

E

= 500.

Slip observed

an excavation

ZE

for remedlal works

pm

100 0

I

100

200

1

- 80

300 400

500 600

700 800

900

Distance ft

Fig 9 Burderop Wood landslide

other respects. The material involved was col-

luvium derived from Gault Clay with a few small

fragments of Greensand and pellets of un-

worked Gault.

During remedial works in 1970, block sam-

ples were taken for slip surface tests from three

locations at Burderop. At another position

nearby, organic matter of a woody nature was

found just below the slip surface. This gave a

radiocarbon age of 12 600 years, showing that

the landslide had originally taken place in a late

period of the last (Devensian) glaciation when

severe periglacial climatic conditions prevailed

in central and southern England.

The slip surface tests were carried out at

Portsmouth Polytechnic by the Author’s former

research assistant Dr D. J. Petley and are de-

tailed in an unpublished report (Skempton,

1971). They gave good results with an unusually

small scatter (Fig. 10).

At both sites the slip surfaces were well

defined by slip indicators, inclinometers and vis-

ual observation, and groundwater levels

(checked by piezometric readings) were known

while movements still continued. Back analyses

of the two slides (Skempton, 1972) differed by

about 0.7” in the angle of shearing resistance

and the slip surface tests gave an angle not more

than about 1” above the average back analysis

value.

Bury Hill

Regrading of the slope at the Bury Hill site

led to a reactivation in 1960 of a landslide which

had previously moved between about 1938 and

1955 in a thick mantle of soliflucted Etruria


7/16

RESIDUAL STRENGTH OF CLAYS 9

Gault Clay

LL = 64 PL = 29 CF = 47

Burderop

Hodson

I

back analysis

0 Shp surface tests

Normal effectwe stress d: kPa

Fig. 10. Field residual strengths or M4 laadslides near Swindon 1970-71

Table 2. Field residual strength of some English clays

Site

Walton’s Wood

Jackfield

>

Bury Hill

Various

M4, near Swindon

Sevenoaks bypass

various

Stratum

Upper Carboniferous

Etnria Marl

Upper Lias

Gault

Athetfield

London Clay

Water

in sheal

ZO”e

29

21

30

29

36

35

34

Index properties

(average values)

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 piezo-

metric 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 re-

cord is taken as 12.5” at 100 kPa with a curva-

ture 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

PL

27

28

29

29

29

CF

70

36

52

52

47

58

55

PIICF

0.4

0.6

0.6

0.7

0.8

0.8

0.9

150 kPa

= tan ’ (s/u)at the following

cr’ values: deg

12.8

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 im-

proved in 1978 when Bromhead published anal-

yses of several rather deep-seated slips at Herne

Bay, with normal effective pressures of lOO-

150 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


8/16

10 SKEMITON

loo-

London Clay

LL = 80 PL = 29

CF = 55

o GuIldford

Tests

D Dedham

on

Back

. Sudbury HalI

v Walthamstow

SllP

analysis

Hadleigh

surface

0 Warden Point

. Herne Bay

M Wraysbury

100

150 200

Normal ef fectw stress u’ kPa

Fig. 11. Field residual strength for London Clay

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 be-

tween 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 land-

slides 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 reac-

tivated landslides and slip surface test results (14 case

recolds)

Parameter

Angle of

A&l&:

shearing

resistance:

deg

Mean 4 from analysis

12.8

Mean 4 from tests 13.4

Mean A+

+0.6

+4.5

Standard deviation in A+

Zt1.2

*9

Maximum A+

+2.5

+17.5

Minimum A&

-2.2

-17

Other clays

Granted the above conclusion, it is possible to

collect values of field residual strength from

several other investigations. Three will be men-

tioned 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 pub-

lished 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 pres-

sures 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 de-

tails from eight different sites, covering pres-

sures 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 non-

linear. The strength s at any given pressure u’ is

conveniently expressed by the secant angle of

shearing resistance 4 where

tan 4 = s/u’

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


9/16

RESIDUAL STRENGTH OF CLAYS

11

A London Clay

0 Llas

I

each point

6 an average

OGaull

otzor3

analyses

A@ = @

,nrl d,, (mean A* = 1.5”)

Fig. 12. Difference 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 25 50 100 150

tan +/tan6 loo

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 pres-

sures 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 differ-

ence is 1” or 2” in the angle of shearing resis-

tance, as shown in Fig. 12 where comparisons

are made with back analysis results. Chandler

(1984) summarizes the data for Lias and Lon-

don Clay, and a ring shear test on Gault from

the M4 landslide at Burderop is quoted by Lu-

pini (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 explana-

tion, 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 identi-

cal 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 clay-

stones and siltstones in Pakistan.

iwaliks

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 ma-

terials. 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 over-

consolidated 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 min-

erals, 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


10/16

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

Sample 64144

LL = 68 PL = 28 CF = 58

0

200

400

600 800

o’ kPa

@&Sample 64138

S’hear

surf ce

Fig. 13. Jari Dam: left abutment, shear zone A

:

Sample 6144

LL = 68 PL = 28

CF = 58

150 - ~,rst run ---Second run

0.0025 mmlmr

u’ = 830

i,oo_/yK-T+

4 6 8 10

Dtsplacement: mm

Fig. 14. Shear surface tests on Jari Dam, shear zone

Fig.

15. Shear surface tests on Jari Valley no. 3, thrust

A, January 1 6

shear joint, November 1965

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 de-

veloped 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 re-

veal 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 ‘rol-

ling 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, show-

ing the relation between residual strength and

clay fraction in a natural sedimentary deposit.

300 I

n’ = 831

s3--

f / /----

---

= 292

I

Sample 76109

LL = 40

PL = 21 CF = 23

- Frst run ---Second run

2 4 6 8 10

Displacement mm


11/16

FCESILXJAL STRENGTH OF CLAYS

13

C&O3 < 10%

PliCF = 0.5 - 0.8

. Mangla Shear surface

tests

Jan

3

q

Kalabagh. cut-plane tests

Values of o,, at on’ = 400 kPa

40-

t-- SlItstone -

.

Claystone -

E30-

-0-1

D

\

B

\

‘,,,,,,,, Bedding/,+,,,

shears

20 -

\

1

rom

field

records

OL 10 20 30 40 50 60 70 80 90

Clay fraction (after pretreatment)

Fg. 16. Field residuals for Sialik claystone and siltstone, April 1984

300

: W =Sample1 LL 135949 PL = est 9 83CF = 42

d, = 10.6”

S, = 75 kPa

o; = 400 kPa

I

I

I I

I

0

2 4 6

a

10

Dlsplacemenl. mm

Fig. 17. Reversal 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 fric-

tion 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 kaolini-

tic 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


12/16

14

SKEMPTON

PIICF

40

t

Values of I ,,

+ Walton’s Wood

I

0.4

at nn’ x 100 kPa

x JackfIeld

(Upper Carbon- 0.6

. Bury HIII

Iferous)

0.6

o Siwallk

0 LIZIS

o Swmdon (Gault)

0 Sevenoaks

(Atherfleld)

a London Clay

0.7

0.7

0.8

0.8

0.9

Approximate bounds

for PVCF = 0.550.9

Aj_,- -+--

Kaolin 0.4

--o---

Benlomte 1’6

\

I I I 1 ,

0

20

40

60

60 100

Clay fraction %

Fig.

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

o Kaolm

’ = 350 kPa CF = 82

. London Clay >’ = 40-140 CF = 60

(each point ave?age of 8 tests)

Usual range of slow

laboratory tests

Tii

g 0.8

I 1

E 0~0001

0.001

0.01 0.1

1 mm/rmn

2

L

i

v, 0.01

0.1

1 10

100 cm/day

0.7

I

I

,

1

10 100

1000 10

000 cm/year

Fig. 19. Variation 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 reac-

tivated 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-


13/16

RESIDUAL STRENGTH OF CLAYS 15

Table 4. Variations ia residual strength of days at

slow rates of displacement

~

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 piezo-

metric levels and for the success of remedial

works which bring about a relatively small in-

crease 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 188

vv = 27

LL

sample is remoulded with water to bring it to a

plastic state and tested in the ring shear ap-

paratus at pressures of 200 kPa and 500 kPa

after preconsolidation at the maximum attaina-

ble 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 usu-

ally 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,

s

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 (re-

sidual, 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 be-

haviour 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 displace-

ment continues, these are dissipated within the

g =

205 kPa (p, = 900 kPa)

62 PL = 26

CF = 47

O St

o-5

0.4

b

b

0.3

0.2

0.1

0

1

0.01 100 0 01 400 mm/mm 0.01

O-215

-___-_-.

-.156 0.155

12

h 0.156

pause

\,

12 h pause

500 600 700 800 900

Displacement mm

Fig. 20. Kalabagh Dam ring shear test, August 1983


14/16

16

SKF.ME’rON

300-

200 -

Sample 704

Rmg shear

LL = 45 PL = 23

CF = 40

o Residual Fast

X Max

400 mm/mln

+ M,n

6 kPa

Fig. 21. Kalabagb Dam ring shear tests, Febmary 1984

Sample 704

LL = 45 PL = 23 CF = 40

kPa

Max

Min

Slldmg

shear

Turbulent

shear

0000

10

100

400 1000

Rate of displacement: mmlmln

Fig. 22. Kalabagb Dam ring shear tests, Febmary 1984

1.4

1.2

Sample 2094

(r 490 kPa (p, = 900 kPa)

w = 24

LL = 39 PL = 27 CF = 3

-____“z,

0.52

.57

0.4 -

0.2 -

3 h pause

0

I , \

4 h pause

I \ ,

800

900 1000

1100

1200

1300

1400

Displacement: mm

Fig.

23. Kalabagb Dam ring shear test, April 1984


15/16

RESIDUAL

STRENGTH

OF

CLAYS

17

Sample 91 OL

LL = 39 PL = 21 CF = 21

kPa

D = 200

g = 495\

01

1

I 1

J

10 100

400 1000

10 000

Rate of displacement: mm/mln

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

4

3

P

D

0

2

1

o-

O

O-

‘o-

o-

0‘

Values of I

at (T = 400 kPa

slitstone

LOW

CF

20

@, deg

30 40

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 remarka-

ble 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 de-

sign 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 pub-

lished papers were carried out as part of a


16/16

18

SKEMPT0N

general research programme at Imperial College

and in connection with investigations for Kent

County Council (Sevenoaks bypass), Sir Alexan-

der Gibb Partners (M4 landslides near Swin-

don) and WAPDA (Kalabagh Dam project).

The fast ring shear tests are being made by Mr

Luis Lemos. In preparing the lecture much ben-

efit has been derived from discussions with Dr

R. J. Chandler and Dr P. R. Vaughan. All the

tracings are by Mrs Anne Langford.

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A.

Brown, J. D. (1971). A new ring shear

apparatus and its application to the measurement

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Bromhead, E. N. (1978). Large landslides in London

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Bromhead, E. N. (1979). A simple ring shear ap-

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Engng

12, 40-44.

Bromhead, E. N. Curtis, R. D. (1983). A compari-

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Burland, J. B., Longworth, T. I. Moore, J. F. A.

(1977). A study of ground movement and progres-

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Documents

Residual strength of clay in landslides