Environmental Engineering Reference
In-Depth Information
Apparent cohesion
results from capillary forces in partially saturated fine-grained granu-
lar soils, such as fine sands and silts, and provides a temporary strength which is lost upon
saturation or drying. The apparent cohesion has been expressed in terms of the depth
D
to the water table (Lambe and Whitman, 1969) as
c
a
D
γ
w
tan
φ
(3.27)
Pore-Pressure Parameters
Definition
Pore-pressure parameters express the portion of a stress increment carried by the pore
fluid in terms of the ratio of the pore-pressure increment (
∆
u
) to the total stress increment
(
). As indicated in Lambe and Whitman (1969) and Bishop and Henkel (1962), the
parameters are
∆σ
C
∆
u
/
∆σ
1
(3.28)
for loading in the odeometer (one-dimensional compression),
B
∆
u
/
∆σ
(3.29)
for isotopic loading (three-dimensional compression),
A
(
∆
u
∆σ
3
)/(
∆σ
1
∆σ
3
)
(3.30)
for triaxial loading and
A
∆
u
/
∆σ
1
(3.31)
for the normal undrained test where
0.
Pore-pressure parameter A
is the most significant in practice. Values depend on soil type,
state of stress, strain magnitude, and time. Typical values are given in Table 3.17 for con-
ditions at failure but important projects always require measurement by testing.
High values occur in soft or loose soils. Negative values indicate negative pore pres-
sures, which occur in dense sands and heavily preconsolidated clays as the result of vol-
ume increase during shear (dilatancy). Pore pressures are most responsive to applied
σ
3
TABLE 3.17
Typical Values of Pore-Pressure Parameter
A
at Failure
a
Soil Type
Parameter
A
Sensitive clay
1.5-2.5
Normally consolidated clay
0.7-1.3
Overconsolidated clay
0.3-0.7
Heavily overconsolidated clay
0.5-0.0
Very loose fine sand
2.0-3.0
Medium fine sand
0.0
Dense fine sand
0.3
Loess
0.2
a
From Lambe, T.W.,
Proc. ASCE, J. Soil Mech. Found. Eng. Div.,
88, 19-47, 1962.
