Civil Engineering Reference
In-Depth Information
example, for a bridge approach. The problem is normally associated with soft, lightly
overconsolidated deposits of clay. In some sensitive clays, remoulding of the soil during
driving may lead to downdrag of the piles, even where no fill is placed (Fellenius, 1972).
The settlement of the clay is then associated not with any long-term rise in the vertical
effective stress, but rather with a reduction in voids ratio and water content as the
remoulded soil consolidates.
The starting point for any estimation of the effects of negative friction is an assess-
ment of the likely free-field vertical movement of the soil. Where this movement exceeds
the estimated settlement of the pile, shear stresses acting in a reverse sense will be
imposed on the pile shaft. The magnitude of the shear stresses may be determined from
the amount of relative movement between soil and pile, with a limiting value reached
where downward slip of the soil occurs. The magnitude of relative movement neces-
sary to cause slip has been discussed in section 4.2.4 (see equation (4.52)). For relative
movements greater than this limit, the mobilized shear stress may be estimated by
σ
v
tan
φ
τ
s
=
K
(4.96)
where, since the soil is consolidating,
K
should be taken as the appropriate value of
K
0
, the at-rest earth pressure coefficient corresponding to the current overconsolida-
tion ratio. In most cases of practical interest, the consolidating soil will be normally
consolidated, and
K
0
may be approximated by (1
φ
).
For relative movements less than that to cause slip, the mobilized shear stress
(positive or negative) may be taken as a linear proportion of the limiting shaft friction.
Thus, for small relative movements, the local shear stress on the pile shaft is
−
sin
d
w
0
−
w
soil
2
G
ζ
τ
0
=
(4.97)
where
w
0
is the local settlement of the pile, and
w
soil
is the background consolidation
settlement of the soil (see section 4.2.1).
Assessment of negative friction for a given distribution of soil settlement necessitates
some iteration, since the profile of pile movement will depend on the distribution of
shear stress,
τ
0
down the length of the pile. The axial load at any depth,
z
, down the
pile is given by
z
0
π
P
=
P
t
−
d
τ
0
d
z
(4.98)
where
P
t
is the applied (structural) load. The axial strain in the pile may then be
integrated to give the settlement of the pile at any depth,
z
,as
L
P
(
EA
)
p
w
0
=
w
b
+
d
z
(4.99)
z
where
w
b
is the settlement of the pile base.
From the calculated pile settlement profile, and the estimated profile of soil
settlement, the distribution of shaft friction may then be calculated. This approach
