Civil Engineering Reference
In-Depth Information
4 A load transfer stiffness, relating the load transferred at a given depth to the local
displacement may be expressed as
=
π
τ
0
w
s
d
2
π
ζ
k
=
G
(4.35)
Equation (4.34) is of particular importance in deducing the magnitude of deflection
necessary to mobilize full shaft friction (see section 4.2.4). The theoretical basis for
the load transfer approach, and analytical expressions for the ratio
k
/
G
have been
explored by Mylonakis (2001).
The overall load taken by the pile shaft is
P
s
=
π
τ
0
is the average shear
stress mobilized at the pile shaft. Thus the load settlement ratio (or stiffness of the
pile-soil system) is
d
/τ
0
, where
LG
ζ
P
s
2
π
w
s
=
(4.36)
where
G
is the average shear modulus of the soil over the embedment depth,
L
,of
the pile.
4.2.1.2 Pile base
At the pile base, it is sufficient to ignore the pile shaft and surrounding soil, and treat
the base as a rigid punch acting at the surface of a soil medium (that, in reality, starts at
a depth,
z
L
). The base stiffness is obtained from the standard solution (Timoshenko
and Goodier, 1970) as
=
P
b
2
d
b
G
b
(1
w
b
=
(4.37)
−
ν
)
where the subscript
b
refers to the pile base.
4.2.1.3 Combining shaft and base
For a stiff pile, the base settlement and shaft settlement will be similar to the settlement
of the pile head,
w
t
. The total load,
P
t
, may thus be written as
w
t
P
b
P
s
w
s
P
t
=
P
b
+
P
t
=
w
b
+
(4.38)
In developing a general solution for the axial response of a pile, it is convenient to
introduce a dimensionless load settlement ratio for the pile. The pile stiffness is
P
t
/
w
t
and this may be made dimensionless by dividing by the diameter of the pile and an
appropriate soil modulus. It has been customary to use the value of the soil modulus at
