residual strength of clay in landslides
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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
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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’
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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.
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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
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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
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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
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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
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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
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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
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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
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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-
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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
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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
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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
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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
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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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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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Sinclair, S. R. Brooker, E. W. (1967). The shear
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Skempton, A. W. (1972). Report on the investigations
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