Environmental Engineering Reference
In-Depth Information
6.1.3
Undrained strength conditions
Undrained strength conditions occur when the loading time and the properties of the soil
in the slope are such that there is insufficient time for pore pressures generated during the
loading period to dissipate. Duncan (1996) suggests this can be assessed from the dimen-
sionless time factor, T which is expressed as:
Ct
D
v
2
(6.1)
T
coefficient of consolidation (m
2
/year); t
where C
v
loading time (years); D
length of
the drainage path (metres).
Duncan indicates that, if the value of T exceeds 3.0, it is reasonable to treat the mate-
rial as drained, and, if T is less than 0.01, undrained. If the value of T is between 3.0 and
0.01, both undrained and drained strengths should be considered. The undrained strength
of a soil may be less than or greater than the drained strength depending on whether the
soil is contractive or dilatent when it shears.
It is important to note that the strength available in the case of undrained shear of the
contractive soil (Su) is less than that for drained loading and less than assumed for con-
ventional effective stress analysis, because of the positive pore pressures generated during
shearing. As discussed below, this can happen in coarse grained as well as fine grained
soils.
Figure 6.7 shows stress strain and stress paths of saturated loose cohesionless sand
loaded in triaxial compression under undrained conditions at a constant rate of strain,
undrained creep, and an initial shear stress and cycle loading. In all cases the large strain
or ultimate strength, also known as the steady state strength or residual undrained
strength, is reached at large strains. In an e-log p
diagram (
Figure 6.8a
)
, where e is the
void ratio and p
is the mean effective stress, such a test would have its initial point at I
1
,
and the ultimate critical or steady state at point U
1
on the steady state line. For initial con-
dition at point I
2
, below the steady state line, the undrained behaviour would be dilatant
with a steady state at point U
2
.
Whether a soil will be contractive or dilatent depends on its initial state condition, i.e.
whether it is above or below the steady state line, but also the stress path. This is discussed
further in Leroueil (2000) and Fell et al. (2000).
In some contractive soils, e.g. loose sands, loaded in saturated, undrained conditions,
the triaxial compression stress path is more complex, as shown in
Figure 6.9
.
The soil
reaches a peak undrained strength at stresses less than the peak strength (at A).
Figure 6.7.
Typical undrained response of loose sands in monotonic, creep and cyclic loading (Leroueil
et al., 1996).
