Pile Design and Construction Practice, Fifth edition

9.2 Piling for underpinning
Miscellaneous piling problems 437
9.2.1 Requirements for underpinning
Underpinning of existing foundations may be required for the following purposes:
(1) As a remedial measure to arrest the settlement of a structure
(2) As a precautionary measure carried out in advance to prevent the excessive settlement
of a structure when deep excavations are to be undertaken close to its foundations, and
(3) As a strengthening measure to enable existing foundations to carry increased loading,
or to replace the deteriorating fabric of a foundation.
An example of the use of piling as a remedial measure is shown in Figure 9.1a. The column
has settled exclusively due to the consolidation of the soft clay beneath its base. Piles
are installed on each side of the base and the load transferred to the pile heads by needle
beams inserted below the base.
A typical use of piles as a precautionary underpinning measure is shown in Figure 9.lb.
A deep basement is to be constructed close to an existing building on shallow strip foundations.
Underpinning of the foundation adjacent to the basement is required since yielding of
the ground surface as a result of the relief of lateral pressure due to the excavations would
cause excessive settlement. Rows of piles are installed close to the wall foundation inside
and outside the building and the loads are transferred to them by a system of longitudinal
and transverse beams. The external row of piles also serves as a support for the horizontal
sheeting members used to retain the face of the excavation.
Piling as a strengthening measure is shown in Figure 9.1c. Pits are excavated beneath the
existing foundation and piles are jacked down to a bearing on a hard incompressible stratum.
Underpinning of the foundations may be required where the existing piles have deteriorated
due to attack by aggressive substances in the soil or groundwater. New piles can be installed
in holes drilled through the cap or raft (Figure 9.1d). The new pile heads are bonded to the
reinforcement of the existing substructure.
Piling has a somewhat limited application to underpinning work. This is because it is usually
necessary to excavate pits below the existing substructure to place supporting beams or
pads. In a high proportion of the cases where remedial or strengthening works are required
a suitable bearing stratum exists at no great depth and it is cheaper to take the pits down to
this stratum and to backfill the void with mass concrete rather than to install piles in conditions
with a low headroom and a restricted working area. Also a considerable force may be
required to jack down an underpinning pile even though the soil is removed from the piling
tube at each stage of jacking. There may be insufficient mass in the existing structure to provide
the required reaction to this jacking force. It is not usually feasible to employ jacked
piles beneath two-storey or three-storey buildings of load-bearing wall construction, since
the usual mass concrete strip foundation and brick footing walls have insufficient bending
strength to withstand the loading that results from jacking down a pile, even though a
spreader beam is used between the ram of the jack and the foundation.
9.2.2 Piling methods in underpinning work
Before underpinning by piling is considered, it is essential to determine the cause of structure–
foundation instability and confirm the ground conditions at depth. Means of checking pile
capacity and integrity once installed should be available.