Environmental Engineering Reference
In-Depth Information
Bishop and Henkel (1957, 1971) used consolidation theory to develop the follow-
ing approximate equations to estimate the time for failure:
(a) Drained tests (CD)
t
. H/C
2
(6.6)
f
a
for top and bottom drainage
2
t
. H/C
(6.7)
f
a
for top, bottom and side drainage
(b) Undrained tests with pore pressure measurement (CUDPP)
t
. H/C
2
(6.8)
f
a
without drainage
2
t
.07H /C
(6.9)
f
a
with top, bottom and side drainage. where t
f
time for failure; H
half height of
coefficient of consolidation.
To determine the strain rate, the strains to failure must be estimated. This may be
up to 20% for normally consolidated soils, and only 1 to 2% for heavily over con-
solidated soils.
C
a
should be determined from the consolidation phase of the test, but it should be
noted it will vary with degree of over consolidation (Germaine and Ladd, 1988;
Leroueil et al., 1988), so should be measured at the end of the consolidation.
Blight (1963), Lade (1986), Germaine and Ladd (1988), Leroueil et al. (1988) all
discuss the problem of efficiency of the side drains (usually strips of filter paper
placed between the soil sample and the rubber membrane in the triaxial test).
Leroueil et al. (1988) point out that, in the over-consolidated range, the drains are
usually able to cope with the required water flow, but in the normally consolidated
range, they may not have adequate discharge capacity. Germaine and Ladd (1988)
indicate this is a problem for soils with a permeability greater than 10
10
m/sec (i.e.
virtually all soils) Lade (1986), recommends the use of
Figure 6.15
based on the the-
ory of Bishop and Henkel and experimental results.
Akroyd (1957, 1975), Head (1985) and Bowles (1978) all give relationships for
estimating time for failure based on monitoring the consolidation stage of the test.
Lade (1986) and Germaine and Ladd (1988) indicate that, by using lubricated end
plates, pore pressures are more uniform and CUDPP tests can be carried out more
quickly. However no guidelines are given to determine an acceptable strain rate.
Lubricated end plates do not reduce the required time for failure in CD tests since
pore pressure dissipation, not equalisation, is required.
(iv)
Staged testing
. In staged testing, the one soil sample is usually saturated, consolidated
and sheared at the lowest selected confining stress, then consolidated to the second
confining stress and sheared, then further consolidated to the third confining stress
and sheared. This reduces the amount of sample preparation time, and hence the cost
of testing. The procedure often gives acceptable results, but may tend to give lower
strengths for the second and, particularly third stage, when testing strain weakening
or compacted dilative soils due to sample dilation and softening, and loss of strength
sample
diameter; C
a
