Environmental Engineering Reference
In-Depth Information
Table 11.5.
Skempton “A
f
” values for clays of varying degrees of
overconsolidation (Skempton, 1954).
Type of clay
A
f
Highly senstive clays
0.75 to
1.5
Normally consolidated clays
0.50 to
1.0
Compacted sandy clays
0.25 to
0.75
Lightly overconsolidated clays
0 to
0.50
Compacted clay gravels
0.25 to
0.25
Heavily overconsolidated clays
0.50 to 0
where
u
change in pore pressure;
1
change in major principal stress;
3
change
in minor principal stress.
A and B are pore pressure parameters, which are related to the compressibility of the soil as
follows:
1
B
(11.8)
nC
C
v
1
sk
where n
porosity; C
v
compressibility of the voids; C
sk
compressibility of the soil
skeleton.
The pore pressure equation can be rewritten as:
u
3
3
BB
A1
(11.9)
1
1
1
1 but, as shown in Figure 6.13, B varies considerably with
degree of saturation and the degree of over-consolidation of the soil. “A” also varies with
the degree of over-consolidation of the soil, the stress path followed and with the strain in
the soil. Usually A at “failure” (i.e. at maximum principal stress ratio or maximum devi-
ator stress) is used and is known as A
f
. Some typical values are shown in Table 11.5.
The values of A will also be dependent on the lateral strain conditions within the dam
embankment. It is normal to assume that K
o
conditions apply, i.e. that there is zero lateral
strain. This is a reasonable (and conservative) assumption for the bulk of the failure sur-
face in an embankment. The use of A at failure is also slightly conservative in most cases.
A more general pore pressure equation was proposed by Henkel (1960) to take into
account the effect of the intermediate principal stress. It is:
For saturated soils, B
u (
a
)
(11.10)
oct
oct
where
1
3
(
oct
)
(11.11)
1
2
3
1
3
(
)
2
(
)
2
(
)
2
(11.12)
oct
1
2
2
3
3
1
