Pile Design and Construction Practice, Fifth edition

Bored piles are suitable for underpinning light structures and they are installed outside
the periphery of the existing foundations as shown in Figure 9.1a. In addition to light tripod
percussion rigs many short-masted rotary auger rigs are available for installing the piles
inside buildings in conditions of low headroom. However, it is desirable, for reasons of
economy, to install piles outside a building as far as possible. This can be done at the corners
of buildings as shown in Figure 9.2. Precast reinforced concrete sections or steel H-piles can
be concreted or grouted into the pile boreholes in cases where it is desired to transfer the
loading to underpinning piles as quickly as possible after installing them. When using bored
piles inside basements spoil removal can be a drawback.
Light structures can be underpinned from a single row of bored piles located outside the
building. After concreting the piles, cantilever brackets are cast onto their heads as shown in
Figure 9.3a. The bending resistance of a small-diameter pile is relatively low, and therefore
the form of construction is limited to strip foundations of light buildings or to lightly

Miscellaneous piling problems 439

Figure 9.2Layout of piles for light structures.

Underpinning

piles

Capping beams

(a) (b)

Cantilever

bracket

Head of pile

cut down

Tension

pile

Compression

pile

Cantilevered

capping beam

Figure 9.3Cantilevered brackets for supporting light structures (a) From single piles (b) From pairs

of piles.