Pile Design and Construction Practice, Fifth edition

440 Miscellaneous piling problems
loaded columns. Heavier structures can be underpinned by pairs of piles located outside the
building but carrying a cantilevered bracket as shown in Figure 9.3b. This system can cause
difficulties in pile design. The compression pile is required to carry heavy loading and
there may be problems in achieving the required resistance to uplift in shaft friction on the
tension pile.
The Fondedile piling system employing the ‘Pali Radice’ (root pile) is suitable as a means
of underpinning structures undergoing settlement or for strengthening existing foundations
to enable them to carry heavier loads. Small-diameter holes lined temporarily with casing
tubes are drilled through the existing foundations or through both a load-bearing wall and
its foundation (Figure 9.4). A rich sand–cement mortar is then pumped down a tremie pipe
to fill the borehole and any cavities in the existing structure. Reinforcement is provided in
the form of a single bar for small-diameter (100 mm) piles or a cage or tube for the largerdiameter
(250–300 mm) piles. The casing is extracted with the assistance of compressed air
to push the grout into the cavities and against the walls of the drilled holes.
Holes drilled
through wall to
be underpinned
Cement–sand
mortar pumped
into place
Single bar
reinforcement
Figure 9.4 Underpinning with Fondedile piles (Pali Radice).