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Canadian Geotechnical Journal, 7, 3, pp. 297-312, 1970-08
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On the yielding and mechanical strength of leda clays
Mitchell, R. J.
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On the yielding and mechanical strength
of
Leda clays
R. J.
MITCHELL
Department of Civil Engineering, Queen's University, Kingston, Ontario
Received September
23,
1969
The pronounced yielding observed in laboratory tests on Leda clay has been associated with
the destruction of cementation bonds in the clay. Triaxial test data presented in this paper show
that a yield curve can be established for a Leda clay. The form of the yield curve deviates from
that expected for an isotropic material and the pre-yield strains are shown to be different for
vertically and horizontally orientated specimens.
The shear strength of the clay is dependent on the mean normal stress at failure.
A
portion
of the failure envelope is different for specimens orientated in different directions and this
'strength anisotropy' is associated with anisotropic yielding.
Le
fluage important observd lors des essais de laboratoire sur l'argile LCda a et6 reli6
B
la
destruction des liens de
cementation dans l'argile. Les resultats d'essais triaxiaux presentds ici
montrent qu'une courbe de fluage peut Ctre ttablie pour l'argile Leda. La forme de cette courbe
de
fluage diffkre de celle qu'on anticiperait pour un materiau isotrope et l'on obtient des
dtformations avant
fluage differentes pour des Cchantillons
B
orientation verticale et horizontale.
La rdsistance au cisaillement de l'argile est fonction de la contrainte
normale moyenne
B
la
rupture. Une
partie de l'enveloppe de rupture est variable en fonction de l'orientation des
dchantillons; cette anisotropie de rdsistance dtant relide
B
l'anisotropie de fluage.
One-dimensional consolidation of naturally
cemented clays such as the Leda clays of
Eastern Canada gives rise to a pressure
-
void
ratio relation similar to that shown in Fig.
la.
The very abrupt increase in compressibility and
the inflection in the curve, apparent in the vicin-
ity of the "preconsolidation pressure," is char-
acteristic of Leda clay and has been associated
(Jarrett 1967; Walker and Raymond 1968)
with the rupture of cementation bonds. From
the consolidation curve a preconsolidation pres-
sure,
PI,
can be estimated (Fig. la). The curve
from Fig. la is plotted on a linear stress scale
in Fig.
lb, and a yield point is defined for the
clay material in the oedometer test. At stresses
less than the yield stress, the specimen com-
pression is small, mostly recoverable, and nearly
a linear function of the applied stress. At
stresses exceeding the yield point stress, the
specimen compression is relatively large and
mainly irrecoverable.
The oedometer test represents only one of an
infinite number of stress paths that may be
imposed on a soil specimen to cause consolida-
tion. The amount of distortion accompanying
volume compression will be a function of the
stress-strain properties of the soil and the
boundary conditions of the test. Yield points
under other stress paths may be most easily
NRCC 11514
investigated using triaxial test apparatus.
A
sufficient number of yield points may then de-
fine a yield curve for the clay material.
A
knowledge of the various combinations of stress
that will cause yielding in a body of clay
(i.e.
the yield curve) is essential in order to predict
the occurrence of zones of plastic deformation
when the clay is used to support a structure.
Together with stress-strain data this information
U
may bc used, to various degrees of sophistica-
tion, in predicting the excess pore water pres-
sures and eventual settlements that will develop
from point-to-point within the clay.
Although not explicitly stated, this work im-
plies that clay behaves as an elastic-plastic
continuum material. Detailed consideration of
the plastic flow and the development of
stress-
strain relations are, however, beyond the scope
of the present paper. Inherent in this general
approach is the assumption that the initial
yielding is not time dependent. The time lag
associated with the hydrodynamic process of
excess pore pressure dissipation is not of par-
ticular concern because this process may be
described bv conventional
theoiies of consolida-
tion. Time-dependent deformation of the soil
skeleton is not, however, compatible with the
conventional elastic-plastic approach. Consid-
erable effort has been devoted to the studv of
time-dependent consolidation in Leda clay
(Crawford 1964; Jarrett 1967; Walker and
Canadian Geotechnical Journal,
7, 297 (1970)
CANADIAN
GEOTECHNICAL
JOURNAL. VOL.
7.
1970
I
-
Preconsolidation
Pressure
-
-
-
-
(After Crawford. 1964
Fig
3,
Specimen 96-1-18]
I
I
I
I
I
I
1
I
I
FIG.
1.
Typical consolidation curve for Leda clay.
Raymond 1968). The following conclusions of
immediate interest are drawn from these
studies.
(
1
)
Major time effects occur only bithin a
range of pressures in excess of the
precon-
solidation pressure.
(2) Very slow constant rate of loading tests
give rise to a void ratio
-
pressure curve essen-
tially the same as the curve obtained from slow
incremental loading tests.
(3) Volume compression in the region of
the preconsolidation pressure is largely a secon-
dary (or creep) phenomenon not associated
with the dissipation of excess pore water pres-
sures in laboratory specimens.
The first two conclusions suggest that a yield
curve can be established, providing excess pore
pressures are allowed to dissipate fully.
Secondary deformation rates, calculated on
the basis of a linear relation between volume
data indicate that the structural time effects
decrease in significance as the shearing stresses
increase
(i.e. the time delay for the breaking
of cementation bonds is greatest in isotropic
compression and least in pure shear).
Test
Program
Fully drained triaxial tests were carried out
on specimens (10 cm2 area) trimmed from
12.7 cm diameter Osterberg samples taken at
the Canadian Forces Base Rockcliffe site in
Ottawa. The tests were designed to provide
information (as far as possible in triaxial com-
pression tests) on the yield curve and the
failure states of this clay in the stress region
applicable to most civil engineering works. The
results are presented in terms of the following
effective stress and strain parameters.
[I]
Mean normal stress,
p
=
(al'
+
2u3')/3
change and the logarithm of time, have been
[2]
Deviatoric stress,
q
=
(al'
-
agf)
correlated between laboratory oedometer tests
and field consolidation (walker 1969). These [3] Volumetric strain increment,
correlations indicate that stress-strain data
ob-
8v
=
861
+
2863
tained from triaxial test specimens that have
been allowed to undergo extended secondary
[4]
Distortional strain increment,
deformation may be usefully extrapolated to
the analysis of long-term field deformations.
86 $'(a61
-
863)
The test program described in this paper was
Natural strains are used, and compressive
carried out in view of these conclusions. Some stresses and strains are considered positive.