Pile Design and Construction Practice, Fifth edition

E u/c u
3000
2500
2000
1500
1000
500
Foundations
Axial strain (%)
Typical strain range
10 –3 10 –2 10 –1 100 101 0
Pile groups under compressive loading 255
Figure 5.17 Relationship between E u/c u and axial strain (after Jardine et al. (5.9) ).
Table 5.1 Poisson’s ratio for various
soils and rocks
Clays (undrained) 0.5
Clays (stiff, drained) 0.1–0.3
Silt 0.3
Sands 0.1–0.3
Rocks 0.2
Marsland (5.10) obtained E u/c u equal to 348 for an upper glacial till, and 540 for a laminated
glacial clay at Redcar, North Yorks.
The influence factor I p in equation 5.14 is obtained from Steinbrenner’s curves
(Figure 5.18) using the method developed by Terzaghi (5.11) . Values of F 1 and F 2 in Figure 5.18
are related to the Poisson’s ratio (�) of the foundation soil. For a ratio of 0.5, I p � F 1. When the
ratio is zero, I p � F 1 �F 2. Some values of Poisson’s ratio are shown in Table 5.1.
When using the curves in Figure 5.18 to calculate the immediate settlement of a flexible
pile group, the square or rectangular area in Figure 5.5 is divided into four equal rectangles.
Equation 5.14 then gives the settlement at the corner of each rectangle. The settlement at the
centre is then equal to 4 times the corner settlement. In the case of a rigid pile group such
as a group with a rigid cap or supporting a rigid superstructure, the settlement at the centre
of the longest edge (twice the corner settlement) is obtained and the average settlement of
the group obtained from the equation:
� average � (� centre � � corner � � centre long edge)/3
(5.20a)
These calculations can be performed by computer using a program such as VDISP in the
OASYs GEO suite.
The curves in Figure 5.18 assume that E u is constant with depth. Calculations based on a constant
value can over-estimate the settlement. Usually the deformation modulus in soils and rocks