Pile Design and Construction Practice, Fifth edition

Compressive stress
1.5 q n
q n
B
A x
C
D
x9
Axial strain
Stress–strain curve
from plate loading test
Strain corresponding to 1.5 q n
E u �1.5q n /x9
Strain corresponding
to net foundation pressure q n
E u � q n /x
Pile groups under compressive loading 253
(4) From relationships with the shear modulus (G ) obtained in the field by pressuremeter
tests:
E � 2G(1 � � u)
E � 2G(1 � ��) , where �u 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 modulus)
corresponding to AB could result in under-estimating the settlement. The usual procedure
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 multiple
of the foundation pressure. The deformation modulus E u is then obtained as shown in
Figure 5.15.
As an alternative to direct determination of E u from field tests, it can be obtained from a relationship
with the undrained shear strength c u, 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 determining
the immediate, consolidation and final settlements using the following relationships
established by Burland et al. (5.8)
Figure 5.15 Determining deformation modulus E u from stress/strain curve.