Pile Design and Construction Practice, Fifth edition

Miscellaneous piling problems 443
procedure on the adjoining piles. Where piles are installed in rows or closely spaced groups
by preloading or ‘pre-testing’ methods, the operation of jacking an individual pile relieves
some of the load on the adjacent piles which have already been installed and wedged-up. It
then becomes necessary to replace the jacks and re-load these piles, after which the inserted
struts are re-wedged. Alternatively, all the pre-testing jacks can remain in position until
the last pile in the group or row is jacked down. Then all the loads on the jacks are balanced,
the struts installed, and the jacks removed. The final operation is to encase the struts and
pile heads in concrete well rammed-up to the underside of the existing foundation.
Steel tube or box section ‘jacked piers’ such as those manufactured by Atlas Systems and
A B Chance in the USA and Rautoruukki Metform in Finland support the foundation being
underpinned on a steel bracket fixed to the top of the pile section. Diameters range from 60
to 320 mm, provided in short lengths appropriate to the jack stroke, with friction or welded
joints for the thin wall sections and threaded joints for 10 mm wall. The bearing capacity of
slender piles will be governed by the buckling resistance in weak soils. Corrosion protection
should be provided.
Where steel sections have to be added by welding during pile jacking to reach the required
stratum or resistance, the alignment and welding quality can be difficult to control when
working in constricted spaces excavated under existing foundations.
Whichever system of jacked piles is used, safeguards are needed to avoid a sudden drop
in the ram due to loss of oil pressure. Also care must be taken to restrain the existing
foundation, or the rows of jacks and struts, from moving horizontally due to lateral or eccentric
thrusts. Raking shores to the superstructure, strutting of the existing foundation to the walls
of the underpinning pit, or bracings between jacks and pile heads can be used to restrain
lateral movement.
The existing columns or walls of the structure to be underpinned can be used to provide
the reaction to jacking if they are sufficiently massive. Niches are cut into the faces of the
structure and concrete corbels or brackets are cast into these pockets to form the bearing
members for the jacks, as shown in Figure 9.7. The pairs of jacks must, of course, be
operated simultaneously to avoid applying eccentric loading to the existing structure.
Where H-section piles are used to provide underpinning combined with lateral support to
a deep excavation, as shown in Figure 9.1b, they can be installed by placing them in holes
previously drilled by mechanical auger or, in stable ground, by tripod rigs. If the drill holes
are given continuous support by a bentonite slurry or by casing there should be a negligible
loss of ground around the borehole and the installation of the piles is effected with little
noise or damaging vibration. Where it is necessary to use tripod rigs in unstable coarse soils,
there is a risk of loss of ground around the boreholes as described in Section 3.3.7. If there
is such a risk it will be necessary to shore the building temporarily with supports bearing on
the ground outside the zone of subsidence. Below the level planned for the base of the
excavation the space between the pile and the borehole is filled with a weak sand–cement
mortar. This provides the required passive resistance to lateral loads on the piles and allows
the latter to be removed if permitted by the planned sequence of underpinning and
construction of the permanent work.
Bored piles used in the combined role of underpinning and lateral support to the sides of
the excavation can be arranged in a single row (Figure 9.8a) or a double row of abutting
piles, or in a single row of interlocking piles (Figure 9.8b). When they are abutting the system
is known as ‘contiguous piling’ and when interlocking as ‘secant piling’. Contiguous
piles are cheaper to install, but because it is impossible to drill the holes in a truly vertical