Geology Reference
In-Depth Information
Figure 6.4
Concept
of buoyant
foundation design.
The weight of the
building balances
the excavated soil
so that the net
increase or decrease
in pressure is
minimised.
load from building,
W
b
, balanced by
weight of excavated
soil, W
s
W
b
W
s
Holding down
anchors or piles
to counteract
buoyant forces
barrettes, as discussed below. Once the walls are in place, excavation is
conducted inside the walls, with either bracing and/or anchorages used
to stabilise the works.
Piles are used to transfer building loads, via pile caps, to deeper levels in
the ground pro
le. There are two main types: driven and bored. Driven
piles are hammered into the ground and are also termed displacement
piles. Hammering is sometimes done by dropping a large weight on the
top of the pile from a crane or using a diesel or hydraulic machine
(Figure 6.5)
. Bored piles are generally constructed using bucket augers,
soil grabs and rock roller bits, with heavy-duty rock cutting tools used to
grind their way into the underlying rock and to form rock sockets as
necessary. Even using the most powerful equipment, formation of sockets
can take a very long time, advancing perhaps only 100mm per hour in
strong rock, and therefore can be relatively expensive, so designers should
be wary of being ultra-conservative in their speci
cation of socket length.
6.4.3.1.1 DRIVEN PILES
Driven piles are generally made of timber, steel or concrete.
Figure 6.6
shows concrete piles being manufactured on site in a factory-type
operation to allow 20,000 piles to be driven in just 18 months for
Drax Power Station completion (Hencher & Mallard, 1989). The
purpose-made pile beds were heated to allow rapid curing of concrete,
