Pile Design and Construction Practice, Fifth edition

direction or to maintain a constant shaft diameter, there are always gaps present between
adjacent piles. Below the water table, sand and silt can bleed through these gaps causing
a considerable loss of ground, and the installation of the piles in a double staggered row
cannot prevent this happening. Contiguous piles are best suited to underpinning and to
support excavations in firm to stiff clays or damp silts and sands above the water table.
Where the excavation is to be performed in water-bearing coarse soils, any loss of ground
can be avoided by adopting secant piling (Figure 9.8b). Alternate piles are first installed by
conventional drilling and casting relatively weak concrete in-situ. The soil in the space
between the pile shafts is then drilled out and a ‘secant’is cut into the wall of the ‘soft’pile
on each side, using appropriate drilling tools, including CFA techniques. Concrete is next
placed to fill the drill hole, thus forming the interlocked and virtually watertight wall.

444 Miscellaneous piling problems

Figure 9.7Underpinning load-bearing wall by jacking piles from corbels.

Niches cut in

masonry wall

Concrete corbel

Hydraulic jack

Underpinning pile

jacked down in

short sections

Figure 9.8Bored piles used for combined underpinning and lateral support (a) Contiguous piles

(b) Secant piles.

Piles cast in

first stage

Piles cast in

second stage

(a)

(b)