Craig's Soil Mechanics 7th Edition
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Craig's Soil Mechanics 7th Edition

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that there has been a reduction in void ratio during

Figure 4.10 Unconsolidated–undrained triaxial test results for saturated clay.

Shear strength of saturated clays 107

each test due to the presence of air in the voids, i.e. the specimen has not been fully
saturated at the outset. It should not be inferred that �u > 0. It could also be that an
initially saturated specimen has partially dried prior to testing or has been repaired.
Another reason could be the entrapment of air between the specimen and the mem-
brane.
In the case of fissured clays the failure envelope at low values of all-round pressure is

curved, as shown in Figure 4.10. This is due to the fact that the fissures open to some
extent on sampling, resulting in a lower strength and only when the all-round pressure
becomes high enough to close the fissures again does the strength become constant.
Therefore, the unconfined compression test is not appropriate in the case of fissured
clays. The size of a fissured clay specimen should be large enough to represent the mass
structure, otherwise the measured strength will be greater than the in-situ strength.
Large specimens are also required for clays exhibiting other features of macro-fabric.
Curvature of the undrained failure envelope at low values of all-round pressure may
also be exhibited in heavily overconsolidated clays due to relatively high negative pore
water pressure at failure causing cavitation, i.e. pore air comes out of solution.
The results of unconsolidated–undrained tests are usually presented as a plot of cu

against the corresponding depth from which the specimen originated. Considerable
scatter can be expected on such a plot as the result of sampling disturbance and
macro-fabric features if present. For normally consolidated clays the undrained strength
is generally taken to increase linearly with increase in effective vertical stress �0v (i.e. with
depth if the water table is at the surface); this is comparable to the variation of cu with �

0
3

(Figure 4.11) in consolidated–undrained triaxial tests. If the water table is below the
surface of the clay the undrained strength between the surface and the water table will be
significantly higher than that immediately below the water table due to drying of the clay.
The following correlation between the ratio cu/�

0
v and plasticity index (Ip) for

normally consolidated clays was proposed by Skempton:

cu

�0v
¼ 0:11þ 0:0037Ip ð4:13Þ

The consolidated–undrained triaxial test enables the undrained strength of the clay to
be determined after the void ratio has been changed from the initial value by consolida-
tion. The undrained strength is thus a function of this void ratio or of the corresponding

Figure 4.11 Consolidated–undrained triaxial test: variation of undrained strength with con-
solidation pressure.

108 Shear strength

all-round pressure (�03) under which consolidation took place. The all-round pressure
during the undrained part of the test (i.e. when the principal stress difference is applied)
has no influence on the strength of the clay, although it is normally the same pressure as
that under which consolidation took place. The results of a series of tests can be
represented by plotting the value of cu (�u being zero) against the corresponding
consolidation pressure �03, as shown in Figure 4.11. For clays in the normally consoli-
dated state the relationship between cu and �

0
3 is linear, passing through the origin. For

clays in the overconsolidated state the relationship is non-linear, as shown in Figure 4.11.
The unconsolidated–undrained test and the undrained part of the consolidated–

undrained test can be carried out rapidly (provided no pore water pressure measurements
are to be made), failure normally being produced within a period of 10–15min. However,
a slight decrease in strength can be expected if the time to failure is significantly increased
and there is evidence that this decrease is more pronounced the greater the plasticity
index of the clay. Each test should be continued until the maximum value of principal
stress difference has been passed or until an axial strain of 20% has been attained.
It should be realized that clays in situ have been consolidated under conditions of

zero lateral strain, the effective vertical and horizontal stresses being unequal, i.e. the
clay has been consolidated anisotropically. A stress release then occurs on sampling. In
the consolidated–undrained triaxial test the specimen is consolidated again under
equal all-round pressure, normally equal to the value of the effective vertical stress
in situ, i.e. the specimen is consolidated isotropically. Isotropic consolidation in the
triaxial test under a pressure equal to the in-situ effective vertical stress results in a
void ratio lower than the in-situ value and therefore an undrained strength higher than
the in-situ value.
The undrained strength of intact soft and firm clays can bemeasured in situ bymeans of

the vane test. However, Bjerrum [5] has presented evidence that undrained strength as
measured by the vane test is generally greater than the average strength mobilized along
a failure surface in a field situation. The discrepancy was found to be greater the higher
the plasticity index of the clay and is attributed primarily to the rate effect mentioned
earlier in this section. In the vane test shear failure occurs within a fewminutes, whereas in
a field situation the stresses are usually applied over a period of few weeks or months. A
secondary factor may be anisotropy. Bjerrum presented a correction factor (�), correlated
empirically with plasticity index, as shown in Figure 4.12, the vane strength being multi-
plied by the factor to give the probable field strength.
Clays may be classified on the basis of undrained shear strength as in Table 4.1.

Sensitivity of clays

Some clays are very sensitive to remoulding, suffering considerable loss of strength
due to their natural structure being damaged or destroyed. The sensitivity of a clay is
defined as the ratio of the undrained strength in the undisturbed state to the undrained
strength, at the same water content, in the remoulded state. Remoulding for test
purposes is normally brought about by the process of kneading. The sensitivity of
most clays is between 1 and 4. Clays with sensitivities between 4 and 8 are referred to as
sensitive and those with sensitivities between 8 and 16 as extrasensitive. Quick clays are
those having sensitivities greater than 16; the sensitivities of some quick clays may be
of the order of 100.

Shear strength of saturated clays 109

Strength in terms of effective stress

The strength of a clay in terms of effective stress can be determined by either the
consolidated–undrained triaxial test with pore water pressure measurement, or
the drained triaxial test. The undrained part of the consolidated–undrained test must
be run at a rate of strain slow enough to allow equalization of pore water pressure
throughout the specimen, this rate being a function of the permeability of the clay.
If the pore water pressure at failure is known, the effective principal stresses �01 and �

0
3

can be calculated and the corresponding Mohr circle drawn.
Failure envelopes for normally consolidated and overconsolidated clays are of the

forms shown in Figure 4.13. For a normally consolidated or lightly overconsolidated
clay the envelope should pass through the origin and the parameter c0 ¼ 0. The
envelope for a heavily overconsolidated clay is likely to exhibit curvature over the
stress range up to preconsolidation pressure and can be represented by either secant or
tangent parameters. It should be recalled that a secant parameter �0 only applies to a
particular stress level and its value will decrease with increasing effective normal stress
until it becomes equal to the critical-state parameter �0cv