Piling - Contiguous, Interlocking and Secant
Bored cast in-situ piles are often used as an efficient and
cost effective means of constructing temporary or permanent retaining
walls. Typically used in the construction of new build basements,
substructures in close proximity to existing structures requiring
restraint and where working space is limited. The techniques
avoids excessive excavation and assists in the control of ground
Typically there are three types of bored pile in current use.
Typically this technique is suitable to retain stiff and cohesive
subsoils and where ground water levels are below the eventual
depth of excavation.
Pile diameters range between 450mm and 900mm and are generally
installed at pile centres of between 500 and 1000mm respectively
thereby leaving gaps between the piles between 15 and 100mm.
This type of construction is in many ways similar to the contiguous
bored pile method, except the gap between the primary piles is
filled with a secondary ‘soft pile’ consisting an
unreinforced weak concrete mix constructed to a depth just below
the depth of final excavation. A sequence of secondary piles
are constructed followed by a sequence of primary piles constructed
to the full design depth, cutting into the secondary piles and
reinforced in the usual manner. This form of construction ensures
that water entry into the subsequent excavation is greatly reduced.
The procedure for constructing secant piles is similar to interlocking
piles, save that the secondary pile is not soft but constructed
of concrete similar to the primary pile and reinforced in a similar
fashion to the primary pile. The equipment used to construct
secant piles generally comprises a heavy duty CFA auger with
rotary cutting heads.
A secant pile wall when completed is a cost effective alternative
to diaphragm wall construction.
images below to enlarge.