Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 271

factorNfor which Schmertmann suggests the following values:

Silts, sandy silts, and slightly cohesive silty sands: N 0.2

Clean fine to medium sands, slightly silty sands: N 0.35

Coarse sands and sands with a little gravel: N 0.5

Sandy gravel and gravels: N0.6

Where static cone resistance data are available the relationships in Figures 5.28 or 5.29
can be used to obtain values of for substitution in equation 5.35.
The procedure for estimating settlements by the Schmertmann method is first to divide
the static cone resistance diagram into layers of approximately equal or representative values
of qcin a manner shown in Figure 5.31. The base of the equivalent raft representing the pile
group is then drawn to scale on this diagram and the influence curve is superimposed
beneath the base of the raft. The settlements in each layer resulting from the loading pat
the base of the equivalent raft are then calculated using the values of Evand I 2 appropriate
to each of the representative layers. The sum of these settlements is corrected for depth and
creep from equations 5.36 to 5.38. The various steps in the calculation are made in tabular
form as illustrated in Example 5.3.
Where piles are terminated in a coarse soil stratum underlain by compressible clay,
the settlements within the zone of clay stressed by the pile group are calculated by the
methods described in Section 5.2.2. The form of load distribution to be used in this
analysis to obtain the dimensions of the equivalent raft on the surface of the clay layer
is shown in Figure 5.21.

Ev

Base of equivalent

raft foundation 0

B

2B

3B

4B

0 0.1 0.2 0.3 0.4 0.5

Long pile group L/B>10

Ground level

Layer 1

Layer 2

Layer 3

B

Layer 4

Settlements

calculated

over depth


 4 B

Static cone

resistance qc

Iz

Layer 5

Figure 5.31Establishing the vertical strain from static cone penetration tests.