Pile Design and Construction Practice, Fifth edition

Schmertmann et al.(5.26)have established an improved curve for obtaining the vertical-strain
influence factor based on elastic half-space theory where the factor Izis related to the
foundation width, as shown in Figure 5.30.
The vertical strain influence factor is obtained from one of the two curves shown in
Figure 5.30. For square pile groups (axisymmetric loading) the curve in Figure 5.30a should
be used. For long pile groups (the plane strain case) where the length is more than 10 times
the breadth, use the curve in Figure 5.30b. Values for rectangular foundations for L/Bof less
than 10 can be obtained by interpolation.
The deformation modulus for square and long pile groups in normally consolidated sands
is obtained by multiplying the static cone resistance, qc, by a factor of 2.5 and 3.5 respec-
tively. The deformation modulus applicable for a stress increase of pabove the effective
overburden pressure, , is given by the equation:

(5.38)

Where standard penetration tests only are available the static cone resistance (in MN/m^2 )
can be obtained by multiplying the SPT N-values (in blows/300 mm) by an empirical

Ev E

vo(p/2)

vo

vo

270 Pile groups under compressive loading

Rigid foundation vertical strain influence factor Iz

Relative depth below foundation level

0

0

B

2B

3B

4B

0

B

2B

3B

4B

B

p

B/ 2

L/B 1

L/B 10

B/ 2 for L/B 1

B for L/B > 10

Depth to peak Iz

(use Ed 2.5qcone)

(use Ed 3.5qcone)

Peak Iz 0.5 + 0.1

0.1 0.2 0.3 0.4 0.5 0.6

(a) (b)

p

s9vp

Dp p – s9vo

s9vo

s9vo

Figure 5.30Schmertmann’s influence factors for calculating immediate settlements of foundations on
sands (after Schmertmann et al.(5.26)).