Environmental Engineering Reference
In-Depth Information
Davis in Thorne (1984) gave the relationship:
c
cos
U
A
sin
sin
(6.17)
o
Su
(
)
tc
112
f
where U o
vo /3
vo
Ho )/3; A f
(1
2K o )
(
2
Skempton “A” at failure;
c
earth pressure coefficient at rest.
This relationship is applicable to both normally consolidated and over-consolidated
clay, with A f and K o dependent on the degree of over consolidation.
effective cohesion;
effective friction angle; K o
6.1.7
Estimation of the undrained strength of cohesive soils from in-situ tests
The undrained strength of cohesive soils in a dam foundation or within the embankment
can be determined by in-situ test methods. These include:
- Cone Penetration Test (CPT) and Piezocone Test (CPTU);
-Vane Shear;
-
Self boring Pressuremeter.
6.1.7.1 Cone penetration and piezocone tests
The undrained strength of clays can be estimated from the cone resistance q c of the static
cone penetrometer test (CPT).
The undrained strength S u is related to q c by
q
(6.18)
c
vo
S
u
N
K
where q c
vo
in-situ total overburden stress and N K
cone resistance;
empirical
cone factor.
Lunne and Kleven (1981) presented graphs showing that for normally consolidated
clays, N K varies depending on the plasticity index of the soil. When uncorrected vane
shear strengths are used as the base for correlation the values shown in Figure 6.22 are
obtained. Note that in Figure 6.22 Po
vo )
Aas et al. (1986) showed that when the vane shear strengths were corrected using
Bjerrum's (1973) correction factor
in-situ total vertical stress (
S uv , the cone correction factor N K val-
ues were less dependent on the plasticity index of the soil, as shown in Figure 6.23 .
As summarised in Jamiolokowski et al. (1985) and in Aas et al. (1986) water pressure
acting on the cone modifies the cone resistance by an amount which depends on the cone
geometry and the pore pressure generated as the cone is inserted. The corrected cone resist-
ance q r is calculated from:
, where S u
(
)
qqu1a
T
(6.19)
c
where q T
corrected cone resistance; q c
measured cone resistance; u
pore pressure
at cone and a
area ratio
A N /A T (see Figure 6.24 ) .
q
T
vo
S
(6.20)
u
N
KT
where N KT is the empirical cone factor related to q T .
 
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