Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 299

piles to the maximum working stress on the concrete of the pile shaft. For 25-grade concrete,
max. working stress 0.25  25 6.25 MN/m^2 (factor of safety on quc 2.2) and since
max. working load on pile required number of piles
900/11 82.
A suitable arrangement of piles consists of fourteen rows of six piles placed at 5 m centres
in both directions.

Length of pile group L 13  5 65 m

Width of pile group B 5  5 25 m

The transfer of load in skin friction to the sand is relatively small and the piles can
be regarded as end bearing on the rock. The base of the equivalent raft will be shown in
Figure 5.3c.

Overall loading at base of raft

From Section 4.7.3 for RQD of 85%, mass factor 0.7 and from Section 5.5 the modulus ratio
of a well-cemented sandstone is 300, deformation modulus of sandstone Ed
0.7 300  14 2940 MN/m^2 , say 3000 MN/m^2.
From Figure 5.20 with , and L/B= 65/25 2.6, , and with D/B
(82)/25 0.4 and L/B 2.6,. From equation 5.22:

settlement of foundation

Example 5.5

A site, where the ground conditions consist of 5.5 m of soft organic silty clay overlying 35 m
of stiff to very stiff over-consolidated clay followed by rock, is reclaimed by placing and
compacting 4 m of sand fill covering the entire site area. Six months after completing the
reclamation a 12-storey building imposing an overall dead load and sustained imposed load
of 160 kN/m^2 on a ground floor area of 48 m by 21 m is to be constructed on the site. The
average undrained shearing strength of the stiff clay stratum is 90 kN/m^2 at the surface
of the stratum, increasing to 430 kN/m^2 at rockhead. Measurements of the deformation
modulus and modulus of volume compressibility show a linear variation, with average
values at the top and bottom of the stiff clay stratum as follows:

At top:

At bottom:

Design suitable piled foundations and estimate the settlement of the completed building.
Because of the heavy loading it is economical to provide large-diameter bored and cast in-situ
piled foundations. A suitable arrangement consists of fourteen rows of six piles (Figure 5.45).
Trial-and-adjustment calculations show that a pile diameter of 1200 mm is suitable.
The pile spacing must be a minimum of 3 diameters, giving a spacing of at least 3.6 m.

Eu 120 MNm^2 , mv 0.04 m^2 MN

Eu 40 MNm^2 , mv 0.08 m^2 MN

1.10.950.55^25 ^1000

3000

5 mm.

 0 0.95

HB  1 1.1

900

65  25

0.55 MN/m^2

1

4 ^ 1.5

(^2) 6.25 11 MN,