Pile Design and Construction Practice, Fifth edition

Methods of drawing sets of p–ycurves have been established for soils which have a
linearly increasing modulus, that is, soft to firm normally consolidated clays and coarse
soils. Empirical factors were obtained by applying lateral loads to steel tubular piles driven
into soft to firm clays and sands. The piles were instrumented to obtain soil reactions and
deflections over their full embedded depth.
The method of establishing p–ycurves for soft to firm clays is described by Matlock(6.17).
The first step is to calculate the ultimate resistance of the clay to lateral loading. Matlock’s
method is similar in concept to those described in Section 6.3.1. but the bearing-capacity
factor Ncis obtained on a somewhat different basis.
Below a critical depth xrthe coefficient is taken conventionally as 9 as in Section 6.3.1.
Above this depth it is given by the equation:

(6.36)

where is the density of the overburden soil, xis the depth below ground level, cuis the
undrained cohesion value of the clay, Jis an empirical factor, and Bis the width of the pile.
The experimental work of Matlock yielded values of Jof from 0.5 for a soft clay to 0.25
for a stiffer clay. The critical depth is given by the equation:

(6.37)

The ultimate resistance above and below the critical depth is expressed in the p–ycurves as
a force puper unit length of pile, where puis given by the pile width multiplied by the
undrained shear strength cuand a bearing capacity factor Nc, usually taken as 9.
Up to the point ain Figure 6.32 the shape of the p–ycurve is derived from that of the
stress/strain curve obtained by testing a soil specimen in undrained triaxial compression, or
from the load/settlement curve in a plate loading test (Figure 5.15). The shape of the curve
is defined by the equation:

(6.38)

p

pu^ 0.5

3 y

yc

xr 6 B

B

cuJ

Nc 3 

x

cu

Jx

B

Piles to resist uplift and lateral loading 343

Curve defined by :

Ultimate resistance for static loading

Ultimate resistance for cyclic loading

p 5 pb ×

y 515 yc

x (^50)
y 58 yc
a
b
y 53 yc y
p
pb 5 0.72pu
pa 5 pu
x ≥ xr
x < xr
xr
x
p/pu 5 0.5^3 y/yc
Figure 6.32Determining shape of p–ycurve in soft to firm clay (after Matlock(6.17)).