Pile Design and Construction Practice, Fifth edition

(4) Obtain the safe end-bearing load on the pile from where Fis a safety factor
greater than 3
(5) Obtain qfrom q= Wb/ B^2 and hence determine q/qf
(6) From a curve of the type shown in Figure 4.28, read off i/Bfor the value of q/qfand
hence obtain i(the settlement of the pile base).

Merely increasing the size of the base by providing an under-ream will not reduce the
base settlement, and if the settlement is excessive it should be reduced by one or more of the
following measures:

(1) Reduce the working load on the pile
(2) Reduce the load on the base by increasing the shaft resistance, i.e. by increasing the
shaft diameter
(3) Increase the length of the shaft to mobilize greater shaft friction, and to take the base
down to deeper and less-compressible soil.

For piles in London Clay, Kin equation 4.36 has usually been found to lie between 0.01
and 0.02. If no plate bearing tests are made, the adoption of the higher value provides a
conservative estimate of settlement. Having estimated the settlement of the individual pile
using the above procedure it is still necessary to consider the settlement of the pile group as
a whole (see Chapter 5).
The greater the length of the pile the greater is the pile head settlement. From their analyses
of a large number of load/settlement curves, Weltman and Healy(4.7)established a simple
relationship for the settlement of straight shaft bored and cast-in-place piles in glacial till.
The relationship given below assumed a pile diameter not greater than 600 mm, a working
stress on the pile shaft of about 3 MN/m^2 , a length to diameter ratio of 10 or more, and stiff
to hard glacial till with undrained shear strengths in excess of 100 kN/m^2. The pile head
settlement is given by

in millimetres (4.37)

where lmis the length of embedment in glacial till in metres.
Precast concrete piles and some types of cast-in-place piles are designed to carry working
loads with shaft stresses much higher than 3 MN/m^2. In such cases the settlement should be
calculated from equation 4.37 assuming a stress of 3 MN/m^2. The settlement should then be
increased pro ratato the designed working stress.
The above methods of Burland et al., and Weltman and Healy, were developed specifically
for piling in London Clay and glacial till respectively and were based on the results of field
loading tests made at a standard rate of loading as specified by the Institution of Civil
Engineers (Section 11.4) using the maintained loading procedure. More generally the pile
settlements can be calculated if the load carried by shaft friction and the load transferred to
the base at the working load can be reliably estimated. The pile head settlement is then given
by the sum of the elastic shortening of the shaft and the compression of the soil beneath the
base as follows:

 (4.38)

(Ws 2 Wb)L

2 AsEp

^

4

.

Wb

Ab

.

B(1v^2 )Ip

Eb



lm

4

1

4

Wb QbF,

Resistance of piles to compressive loads 195