The usual method of application is to install the piles in relatively large numbers at differing
angles of rake so as to provide resistance to lateral loads and overturning moments as well as
to support loads in a vertical direction. The rotary drilling machines used by Fondedile require
a working space of only 21.5 m and a headroom of 1.8 m, and the drilling is performed
with relatively little vibration. Thus the piles can be installed in confined surroundings and
very close to the foundations to be underpinned (Figure 9.5). Other systems employ minipiles
of the type described in Section 2.6 such as small-diameter steel tubes which are drilled down
or set in drilled holes. Grout is pumped down the tubes which remain in place as permanent
reinforcement. A detailed account of the Fondedile system and underpinning with other types
of small-diameter piles with numerous case histories is given by Lizzi(9.7).
Bored piles installed by continuous flight auger as described in Section 2.4.2 are also
suitable for strengthening and underpinning work since the rotary drilling methods and
continuous support given to the soil result in little vibration and a negligible loss of ground
in properly controlled operations.
Bottom driven piles using thin wall steel tubes up to 300 mm diameter may be used in
difficult conditions such as brownfield sites, peaty soils and soft clay, and founded on a
competent stratum. Such piles will have limited tension capacity and should not be used in
heaving ground or where vibration may cause problems. A pneumatic piling hammer may
be used for driving tubes up to 100 mm diameter.
Heavily loaded foundations can be underpinned by jacking piles down to the bearing
stratum using the dead load of the existing foundations and superstructure as the reaction to
the jacking operation. The Presscoreprecast jacked-in pile with a central hole is described
Miscellaneous piling problems 441Figure 9.5Fondedile piling rig working close to foundations to be underpinned.