Pile Design and Construction Practice, Fifth edition

346 Piles to resist uplift and lateral loading
driven into a soft sensitive clay the remoulded shearing strength could be in conjunction with
the Brinch Hansen method (Section 6.3.1), to obtain the ultimate resistance over a period of a
few days or weeks after driving. If the piles are not to be subjected to loading for a few months
after driving, the full ‘undisturbed’ shearing strength can be used. There is unlikely to be much
difference between the ultimate lateral resistance of short rigid piles driven into stiff over-consolidated
clays and bored piles in the same type of soil. The softening effects for bored piles
mentioned in Section 4.2.3 occur over a very short radial distance from the pile and the principal
resistance to lateral loads is provided by the undisturbed soil beyond the softened zone.
In the case of piles installed in coarse soils the effect of loosening due to the installation
of bored piles can be allowed for by assuming a low value of � when determining K q from
Figure 6.22. When considering the deflection of bored piles in coarse soils the value of the
soil modulus n h in Figure 6.20 should be appropriate to the degree of loosening which is
judged to be caused by the method of installing the piles.
p–y curves were developed primarily for their application to the design of long driven piles,
mainly for offshore structures. Because such piles are required to have sufficient strength
to cope with driving stresses, they have a corresponding resistance to bending stresses from
lateral loading. On the other hand, bored and cast-in-place piles are required to have only
nominal reinforcement, unless they are designed to act as columns above ground level, or to
carry uplift or lateral loading. Nip and Ng (6.21) investigated the behaviour of laterally loaded
bored piles. They noted that while allowance can be made, arbitrarily, by assuming that
the stiffness of a cracked reinforced pile section is 50% of that of an uncracked pile, this
assumption can result in over-predicting the deflections and under-predicting the bending
moments. By comparing the deflections measured in lateral load tests with predictions made
by calculations using p–y curves they concluded that the latter can be used to predict
deflections, bending moments, and soil reactions of laterally loaded bored piles with varying
EI values corresponding to uncracked, partially cracked, and fully cracked sections.
6.3.7 The use of the pressuremeter test to establish p–y curves
The pressuremeter test (see Section 11.1.4) made in a borehole (or in a hole drilled by the pressuremeter
device) is particularly suitable for use in establishing p–y curves for laterally loaded
piles. The test produces a curve of the type shown in Figure 6.34a. The initial portion represents
a linear relationship between pressure and volume change, that is the radial expansion of
the walls of the borehole. At the creep pressure pf the pressure/volume relationship becomes
non-linear indicating plastic yielding of the soil; at the limit pressure pl the volume increases
rapidly without increase of pressure as represented by the horizontal portion of the p–y curve.
Menard used a Poisson’s ratio of 0.33 to derive an expression for determining the
pressuremeter modulus of the soil from the initial portion of the curve in Figure 6.34a. This
equation as given by Baguelin et al. (6.22) is
�p Em � 2.66Vm �v where
�p � slope of the curve between V0 and Vf �v Vm � midpoint volume
(6.42)