Pile Design and Construction Practice, Fifth edition

344 Piles to resist uplift and lateral loading
where y c is the deflection corresponding to the strain ‡ c at a stress equal to the maximum
stress resulting from the laboratory stress/strain curve. The strain ‡ c can also be obtained
from the established relationship between c u and the deformation modulus E u (see
Section 5.2.2). Matlock (6.17) quotes values of ‡ c of 0.005 for ‘brittle and sensitive clays’ and
0.020 for ‘disturbed or remoulded clays or unconsolidated sediments’. These values of ‡ c
have been based on the established range of E u/c u of 50 to 200 for most clays, and they can
be applied to stiff over-consolidated clays, for example the value of E u/c u for stiff London
Clay is 400. Matlock (6.17) recommends an average value of 0.010 for normally consolidated
clays for use in the equation:
y c � 2.5ε cB
(6.39)
The effect of cyclic loading at depths equal to or greater than x r can be allowed for by cutting
off the p–y curve by a horizontal line representing the ultimate resistance p b of the clay under
cyclically applied loads. From the experimental work of Matlock (6.17) , the point of intersection
of this line with the p–y curve (shown in Figure 6.32 as point b) is given by
p b�p u � 0.72
(6.40)
The p–y curves for cyclic loading with values of y/y c from 3 to 15 and for depths of less than
x r, at x � 0 are shown in Figure 6.32.
There are little published data on values of p b for various types of clay. The application of
a static horizontal load after a period of cyclic loading, say in a deep-sea structure where a
berthing ship strikes a dolphin after a period of wave loading, produces a more complex
shape in the p–y curve and a method of establishing the curve for this loading condition has
been described by Matlock (6.17) .
The shape of a p–y curve for a pile in sand as established by Reese et al. is shown in
Figure 6.33. It is in the form of a three-part curve up to the stage of the ultimate failure p u.
p
p u 5Ap c
p m 5Bp c
k
yk5
c
n x
h
m
n
n–1
l
m
Curve defined by
p5Cy 1 n
ym5 B 60
Figure 6.33 Determining shape of p–y curve in sand (after Reese et al. (6.10) ).
u
x5x 1
3B
yu 5
8O
Y