Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 9 FOUNDATION ENGINEERING
(e) Presence or otherwise of a ground-bearing cap, and
(f) Relative stiffness of the structure, the piles and the ground.
Traditionally, the assessment of group effects is based on some 'rules-of-thumb' or semiempirical
rules derived from field observations. Recent advances in analytical studies have
enabled more rational design principles to be developed. With improved computing capabilities,
general pile groups with a combination of vertical and raking piles subjected to complex loading
can be analysed in a fairly rigorous manner and parametric studies can be carried out relatively
efficiently and economically.
9.4.2 Minimum Spacing of Piles
The minimum spacing between piles in a group should be chosen in relation to the method of
pile construction and the mode of load transfer. It is recommended that the following
guidelines on minimum pile spacing may be adopted for routine design:
(a) For bored piles which derive their capacities mainly from shaft resistance and for all types
of driven piles, minimum centre-to-centre spacing should be greater than the perimeter of the pile
(which should be taken as that of the larger pile where piles of different sizes are used); this
spacing should not be less than 1 m as stipulated in the Code of Practice for Foundations (BD,
2004a).
(b) For bored piles which derive their capacities mainly from end-bearing, minimum clear spacing
between the surfaces of adjacent piles should be based on practical considerations of positional and
verticality tolerances of piles. It is prudent to provide a nominal minimum clear spacing of about
0.5 m between shaft surfaces or edge of bell-outs. For mini-piles socketed into rock, the minimum
spacing should be taken as the greater of 0.75 m or twice the pile diameter (BD, 2004a).
The recommended tolerances of installed piles are shown in Table 9.6 (HKG, 1992). Closer
spacing than that given above may be adopted only when it has been justified by detailed
analyses of the effect on the settlement and bearing capacity of the pile group. Particular
note should be taken of adjacent piles founded at different levels, in which case the effects of the
load transfer and soil deformations arising from the piles at a higher level on those at a lower
level need to be examined. The designer should also specify a pile installation sequence within a
group that will assure maximum spacing between shafts being installed and those recently
concreted.
Table 9.6 Tolerance of Installed Piles
Description
Tolerance
Land Piles Marine Piles
Deviation from specified position in plan,
measured at cut-off level
75 mm 150 mm
Deviation from vertical 1 in 75 1 in 25
Deviation of raking piles from specified batter
Deviation from specified cut-off level
1 in 25
25 mm
The diameter of cast in-place piles shall be at least 97% of the specified diameter
9.4.3 Ultimate Capacity of Pile Groups
Traditionally, the ultimate load capacity of a pile group is related to the sum of ultimate capacity of
individual piles through a group efficiency (or reduction) factor, η, defined as follows:
9-46 March 2009