Civil Engineering Reference
In-Depth Information
8
7
Meas
u
red
6
Numerical: total
5
Numerical: shaft
4
3
Numerical: base
2
1
0
0
20
40
60
80
100
Displacement (mm)
Figure 4.2
Idealized load settlement response.
(Randolph, 2003), and the separate response of the pile shaft and pile base is indicated
in addition to the total response. The shaft capacity of 4.1 MN is mobilized at a
displacement of less than 6 mm (0.75% of the pile diameter), while the base response
is still rising for displacements in excess of 80 mm (10% of the pile diameter).
The nominal ultimate capacity of the pile may be taken as 7 MN, of which 60% is
provided by the shaft. However, at a working load of say 3.5 MN, Figure 4.2 shows
that 95% of the load is carried by the pile shaft, with only 5% reaching the base.
Choice of factor of safety for such a pile must be made in the light of the different
response of pile shaft and base. Once the pile shaft capacity has been fully mobilized
(at a load of 4.1 MN), the 'stiffness' of the pile-soil system reduces to that of the pile
base, and the displacements start to increase rapidly. This type of consideration is
of particular importance in the design of underreamed or partially end-bearing piles,
where the base capacity is large. Settlements under working load conditions will be
larger than for comparable straight-shafted friction piles.
The above example illustrates an important approach to the design of piles. In the
first instance, unit values of 'shaft friction' (the limiting value of shear stress between
pile and soil) and of end-bearing pressure must be estimated in order to calculate the
shaft and base capacities. The same overall capacity may be achieved with a variety
of combinations of pile diameter and pile length. However, before the final geometry
of the pile is determined, some consideration must be given to the load settlement
characteristics of the pile - and also, of course, to other factors such as the economics
of installing piles of one size or another.
In general, long slender piles may be shown to be more 'efficient' than short stubby
piles, both as regards their capacity per unit volume installed, and also in terms of the
stiffness of the pile (Randolph, 1983). Limitations on the slenderness of a pile come
from economic reasons (since, once a large boring or pile driving rig is mobilized, the
rate of pile installation becomes largely independent of the pile diameter), and also
from considerations of its stability during driving (Burgess, 1976) and of the axial
