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Ground Stability, Foundations and Substructures 113
alternative process must be used. One option is to insert pairs of piles on each side of the
wall. Steel or reinforced concrete beams (often called needles) are then inserted through
the wall above the foundation but below ground level. The needles will be used to support
the wall and transfer the loads to the piles on either side of the original foundation. When
piles are formed on both sides of the wall, they are jacked in against temporary units loaded
with kentledge. As there is no building foundation to jack against, a temporary loaded
structure (kentledge) must be used so that the jack can drive piles from the structure into
the ground. Once the piles are jacked into position, the jack and kentledge are removed.
The piles and needles can then be tied together using a reinforced concrete pile cap. Figure
3.48 shows some underpinning arrangements.
Section
Wall underpinned using
cantilevered needles
Wall underpinned
Cantilevered reinforced concrete
or rolled steel needle (beam)
Packing shims or expanding
grout fills the gap between
the needle and wall being
supported
Reinforced concrete or
rolled steel needle
(beam)
Load removed from
existing foundation
Load removed from
existing foundation
Short piles segments
fixed together with
splicing collars
Jacked steel or concrete
piles
Plans
Plan of
cantilever
needles
Figure 3.48 Various underpinning arrangements.