Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 253

1.5qn

qn

B

C

D

Ax

Axial strain

Compressive stress

Stress–strain curve

from plate loading test

Strain corresponding to 1.5 qn

Eu 1.5qn/x 9

Strain corresponding

to net foundation pressure qn

Eu qn/x

x 9

Figure 5.15Determining deformation modulus Eufrom stress/strain curve.

(4) From relationships with the shear modulus (G) obtained in the field by pressuremeter
tests:

, where and are the undrained and drained values of Poisson’s

ratio respectively.

With regard to method (1) a typical stress/strain curve obtained by a plate-bearing test in
undrained conditions is shown in Figure 5.15. Purely elastic behaviour occurs only at low
stress levels (line AB in Figure 5.15). Adoption of a modulus of elasticity (Young’s modu-
lus) corresponding to AB could result in under-estimating the settlement. The usual proce-
dure is to draw a secant AC to the curve corresponding to a compressive stress equal to the
net foundation pressure at the base of the equivalent block foundation. More conservatively
the secant AD can be drawn at a compressive stress of 1.5 times or some other suitable mul-
tiple of the foundation pressure. The deformation modulus Euis then obtained as shown in
Figure 5.15.
As an alternative to direct determination of Eufrom field tests, it can be obtained from a rela-
tionship with the undrained shear strength cu, the plasticity index and over-consolidation ratio
of the clay established by Jamiolkowski et al.(5.6)(Figure 5.16). The latter value is derived
from oedometer tests or from a knowledge of the geological history of the deposit.(5.7)These
tests are used to calculate the long-term consolidation settlement of the foundation as
described below. Knowing the oedometer settlement (oed) provides another way of deter-
mining the immediate, consolidation and final settlements using the following relationships
established by Burland et al.(5.8)

E 2 G(1) u 

E 2 G(1u)