Civil Engineering Reference
In-Depth Information
where
is the area ratio (ratio of annular area to total cross-sectional area). For
steel piles, the ratio of diameter,
d
, to wall thickness,
t
, is typically around 40, cor-
responding to an area ratio of 0.1. This would therefore lead to a reduction in
ρ
σ
ri
of
2
3
c
u
, with a corresponding reduction in the overall extent of the excess pore pressure
field. However, while this approach gives reasonable adjustments for lightly overcon-
solidated clays, leading ultimately to a reduction in shaft friction of 15 to 20%, it
appears to give unreasonably low radial stresses and shaft friction for more heavily
overconsolidated clays.
.
4.1.2.5 Bored piles
Turning to bored piles, assuming that no chemical bond develops between the pile and
soil, an effective stress approach is again more attractive than the empirical approach
based purely on the undrained shear strength of the soil. Provided the pile is formed
promptly after excavation of the shaft, little change in the in-situ effective stress state
in the soil should occur, and equation (4.19) may be applied with
K
K
0
. In heavily
overconsolidated clay, where the value of
K
0
is large, some allowance for stress relax-
ation should be made, reducing the value of
K
by 20%. Alternatively, the mean stress
between the in-situ horizontal stress and that due to the concrete poured into the pile
shaft may be taken, replacing
K
by (1
=
2. It must be emphasized, however, that
delays in pouring the concrete may reduce the in-situ horizontal effective stress consid-
erably. Back analysis of tests of bored piles in heavily overconsolidated London Clay
show deduced values of
K
below unity, where delays have occurred between augering
and forming the piles (Fleming and Thorburn, 1983).
The question of what friction angle should be used in equation (4.19) is still sub-
ject to debate. The surface of most bored piles will be sufficiently rough to ensure
that failure takes place in the soil immediately around the pile, rather than actu-
ally on the interface. However, there have been suggestions (notably Burland and
Twine, 1988) that a residual angle of friction is appropriate, at least for bored piles
in heavily overconsolidated clay. The friction angle mobilized on the (vertical) failure
surface at peak shear stress will depend on the complete stress state. Randolph and
Wroth (1981) have argued that, where the horizontal effective stress exceeds the ver-
tical effective stress, the friction angle will be significantly lower than that obtained
from a triaxial compression test, and will be similar to that mobilized on horizontal
surfaces at failure in a simple shear test, conducted under appropriate normal stress
conditions.
Adoption of the residual angle of friction in equation (4.19) (based on arguments of
slip-planes formed during the augering process) does not appear consistent with the
assumption of failure
within
the clay (due to the roughness of the pile shaft). However,
residual friction angles maybe appropriate where the augered shaft is relatively smooth,
and failure occurs at the concrete-soil interface. Overall, a reasonable design approach
seems to be to take the horizontal effective stress as 0
+
K
0
)
/
σ
v
, and the friction
angle as that mobilized at failure in a simple shear test. Where the horizontal effective
stress is greater than the vertical effective stress (i.e. in heavily overconsolidated clays),
the friction angle will lie in the range 15 to 20
◦
(Randolph and Wroth, 1981).
For the design of bored piles in soil deposits where the profile of
K
0
with depth is not
known, either the approach for driven piles may be used with an appropriate factor to
.
5(1
+
K
0
)
