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156 Barry’s Advanced Construction of Buildings
(a) Contiguous piled wall – gaps left between each
pile
(b) Hard/soft (interlocking) piles – soft primary piles
bored and cast first, hard secondary piles secant
(cut into) soft piles, also called interlocking piles.
Typical strength of female piles 1–3 N/mm 2
Soft piles cement and
bentonite or cement,
bentonite and sand
Spacing
50–100 mm
1st phase of piles
Diameter 300–2400 mm
2nd phase (male
piles)
(c) Hard/firm piles. Female piles which are
intersected by the male piles have characteristic
strength of 10–20 N/mm 2 . Retarder is also used in
female piles
Female piles Male piles
Minimum overlap 25 mm
Diameter 600–1200 mm
(d) Hard/hard piles. Both male and female piles are
reinforced and cast with full strength concrete.
Reinforcement in the female piles is positioned so
that it is not damaged by the auger when forming
the male pile. Male piles can also be cased in steel
liners for extra strength
(e) Hard/hard piles with I section steel columns.
Rolled steel I section columns can be introduced to
add extra strength
Figure 3.75 Contiguous and secant piles.
basements are usually stabilised by ground anchors and cross-bracing to resist lateral forces.
Secant piles can be constructed as hard/soft, hard/firm or hard/hard walls.
Hard/soft secant piles
For hard/soft piles, the female piles, which are cast first, are constructed of a soft pile mix;
this usually takes the form of a cement and bentonite mix or cement, bentonite and sand
mix. The mix has a weak characteristic strength of 1–3 N/mm 2 . The female piles are used
as a water retaining structure rather than a loadbearing column. Soft piles can retain up to
8 m head of groundwater. The unreinforced soft pile is not usually used as a permanent
wall material. As the bentonite and cement mix dries, it will shrink and crack, losing its
water-resisting properties. Some soft piles have been designed to retain their water-resisting
properties for the life of the structure; often this necessitates the mix remaining hydrated
throughout the life of the building.