Civil Engineering Reference
In-Depth Information
Fig. 4.26
Effects of overconsolidation on the pore pressure coefficient A.
4.10.1 Values of A
For a given soil, A varies with both the stress value and the rate of strain, due mainly to the variation of
Δ
u
d
with the deviator stress. The value of
Δ
u
d
under a particular stress system depends upon such factors
as the degree of saturation and whether the soil is normally consolidated or overconsolidated. The value
of A must be quoted for some specific point, e.g. at maximum deviator stress or at maximum effective
stress ratio
(
1
/
at maximum deviator stress it can vary from 1.5 (for a highly sensitive clay) to
−
0.5 (for
a heavily overconsolidated clay).
′
′
)
σ σ
4.10.2 Variation of A
An important effect of overconsolidation is its effect on the pore pressure parameter A. With a normally
consolidated clay the value of A at maximum deviator stress, A
f
, is virtually the same in a consolidated
undrained test no matter what cell pressure is used, but with an overconsolidated clay the value of A
f
falls
off rapidly with increasing overconsolidation ratio (Fig.
4.26)
.
Overconsolidation ratio is the ratio of preconsolidation pressure divided by the cell pressure used in
the test. When the overconsolidation ratio is 1.0 the soil is normally consolidated.
Example 4.8:
Consolidated undrained triaxial test (ii)
A series of consolidated undrained triaxial tests were carried out on undisturbed samples
of an overconsolidated clay.
Results were:
Deviator stress
at failure (kPa)
Pore water pressure
at failure (kPa)
Cell pressure (kPa)
100
410
−
65
200
520
−
10
400
720
80
600
980
180
(i) Plot the strength envelope for the soil with respect to effective stresses.
(ii) If the preconsolidation to which the clay had been subjected was 800 kPa, plot the
variation of the pore pressure parameter A
f
with the overconsolidation ratio.
