Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 255

Marsland (5.10)obtained Eu/cuequal to 348 for an upper glacial till, and 540 for a laminated
glacial clay at Redcar, North Yorks.
The influence factor Ip 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, Ip F 1. When the
ratio is zero,Ip 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:

(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 Euis constant with depth. Calculations based on a con-
stant value can over-estimate the settlement. Usually the deformation modulus in soils and rocks

average (centrecornercentre long edge)/3

Figure 5.17Relationship between Eu/cuand axial strain (after Jardine et al.(5.9)).

Eu

/c

u

3000

Axial strain (%)

Typical strain range

10 –^310 –^210 –^1100101

2500

2000

1500

1000

500

0

Foundations

Table 5.1Poisson’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