Pile Design and Construction Practice, Fifth edition

196 Resistance of piles to compressive loads
where
W s and W b � loads on the pile shaft and base respectively
L � shaft length
A s and A b � cross-sectional area of the shaft and base respectively
E p � elastic modulus of the pile material
B � pile width
v � Poisson’s ratio of the soil
I p � influence factor related to the ratio of L/R
E b � deformation modulus of the soil beneath the pile base
For a Poisson’s ratio of 0 to 0.25 and L/B � 5, I p is taken as 0.5 when the last term approximates
to 0.5 W b/(BE b). Values of E b are obtained from plate loading tests at pile base level
or from empirical relationships with the results of laboratory or in-situ soil tests given in
Sections 5.2 and 5.3. The value of E b for bored piles in coarse soils should correspond to the
loose state unless the original in-situ density can be maintained by drilling under bentonite
or restored by base grouting.
The first term in equation 4.38 implies that load transfer from pile to soil increases linearly
over the depth of the shaft. It is clear from Figure 4.22 that the increase is not linear for a
deeply penetrating pile. However, with the present-day availability of computers it is possible
to simulate the load transfer for wide variations in soil stratification and in cross-sectional
dimensions of a pile. One of the principal programmes represents an elastic continuum model.
A pile carrying an axial compression load is modelled as a system of rigid elements connected
by springs and the soil resistance by external non-linear springs (Figure 4.29). The load at the
pile head is resisted by frictional forces on each element. The resulting displacement of each
of these is obtained from Mindlin’s equation for the displacement due to a point load in a
semi-infinite mass. The load/deformation behaviour is represented in the form of a t–z curve
(Figure 4.29). A similar q–z curve is produced for the settlement of the pile base.
The concept of modelling a pile as a system of rigid elements and springs for the
purpose of determining the stresses in a pile body caused by driving is described in
Section 7.3.
It was noted at the beginning of this section that the adoption of nominal safety factors in
conjunction with conventional methods of calculating pile-bearing capacity can obviate the
necessity of calculating working load settlements of small-diameter piles. However, there is
not the same mass of experience relating settlements to design loads obtained by EC7 methods
based on partial safety factors. Hence, it is necessary to check that the design pile capacity
does not endanger the serviceability limit-state of the supported structure. Equation 4.38 can
be used for this check. A material factor of unity should be adopted for the design value of E d.
EC7 (Clause 7.6.4.1) states that where piles are bearing on medium-dense to dense soils
the safety requirements for ultimate limit state design are normally sufficient to prevent a
serviceability limit state in the supported structure.
4.7 Piles bearing on rock
4.7.1 Driven piles
For maximum economy in the cross-sectional area of a pile it is desirable to drive the pile
to virtual refusal on a strong rock stratum, thereby developing its maximum carrying capacity.