Environmental Engineering Reference
In-Depth Information
Figure 6.2.
Relationship between peak, softened and residual strengths.
samples of the same soil will tend to achieve the same critical state or fully softened
condition.
-With continuing displacement of soils with a high clay fraction content, particle reori-
entation occurs, resulting in a further reduction in shear strength. The minimum value
of shear strength achieved at large displacements is the “residual” strength.
-
The residual and fully softened strengths are significantly different for high clay frac-
tion content soils, but not for soils with low clay fraction content (see
Figure 6.1
).
Sands behave similarly to the low clay fraction soils.
-
Strength envelopes for these cases are defined by:
- peak strength
c
;
- softened strength
c
s
s
;
- residual strength
c
R
R
.
Note that often c
s
c
R
0, and
s
. Figure 6.2 shows the relationship between
these parameters.
Skempton (1985) adopts the use of the term “field residual” strength (
R
) as the
strength of fully developed shear or slide surfaces in nature. As discussed in Section
6.1.4.4 this value differs from the laboratory residual strength depending on the testing
method.
In practice, it may also reflect the roughness and waviness of the field failure surface
compared to the planar surface in the laboratory sample.
An undisturbed sample of soil may behave in an overconsolidated manner at low nor-
mal stress and in a normally consolidated manner at high normal stresses (in excess of the
preconsolidation pressure). This affects the pore pressure response of the soil during shear
(see Section 6.1.2) and also the drained load-deformation behaviour as shown in
Figure 6.3
.
6.1.2
Development of drained residual strength
R
There is a large amount of evidence that softening, with increased water content, and particle
reorientation, occurs on slide planes and that these lead to a reduction in shear strength
from the peak strength. Skempton (1985), Lupini et al. (1981), Mesri and Cepeda-Diaz
(1986) and Hawkins and Privett (1985) give good summaries of these effects.
Lupini et al. (1981) carried out a series of ring shear tests on sand-bentonite mixtures, and
suggested that there were different mechanisms of residual plane development depending on
the clay fraction percentage present. The mechanisms are turbulent or rolling shear, where the
presence of rounded silt size particles prevents alignment of clay particles on the shear surface;
