Civil Engineering Reference
In-Depth Information
4.1.5.1 Pore pressure dissipation in clay
The question of the effect of time since installation is of much more consequence,
particularly for displacement piles installed in soils of low permeability. Piles driven
into cohesive soil generate high excess pore pressures close to the pile. The pore pres-
sures arise partly as a result of a decrease in effective stress, as the soil is sheared and
remoulded, and partly due to the increase in total stress, as the pile forces soil out of
its path. Typically, the excess pore pressures may be as high as the effective overbur-
den stress, and may extend out over a zone up to 10 times the diameter of the pile.
Randolph
et al
. (1979) suggest that the excess pore pressure,
u
, close to a driven pile
may be estimated from the expression
p
u
=
4
cu
−
(4.28)
p
is the change in mean effective stress due to shearing and remoulding the
soil. In normally or lightly overconsolidated clay,
where
p
will be negative and may be as
high as 2 to 3
c
u
for sensitive clays. In more heavily overconsolidated clay,
p
will
become positive as the clay attempts to dilate on shearing. Figure 4.20 shows typical
field measurements of excess pore pressures due to pile installation. The values reflect
the trend implied by equation (4.28).
After the pile is installed, the excess pore pressures will dissipate, primarily by radial
flow of pore water away from the pile, and the soil will consolidate
During this pro-
cess, the water content of the soil will decrease, and its shear strength increase. Seed
and Reese (1955) report a decrease in water content of 7% close to a pile driven
into San Francisco Bay mud. This was accompanied by a threefold increase in the
remoulded strength of the soil (measured from samples taken from close to the pile).
A similar figure for the decrease in water content adjacent to a displacement pile has
been reported by Francescon (1983), from tests on model piles installed in soil of
varying overconsolidation ratio.
The axial capacity of a driven pile will vary with time after installation, because of
this consolidation process. Seed and Reese report a sixfold increase in pile capacity,
over a period of 30 days. Similar increases in capacity have been discussed by Vesic
(1977) and by Thorburn and Rigden (1980). Analytical studies of the stress changes
caused by pile installation, and of the consolidation process, have shown that the shear
strength of the soil may increase by between 30 and 100% close to the pile, over a time
.
Figure 4.20
Field measurements of excess pore pressure.
