Introduction to civil engineering projects - Practical Engineering Geology - page 30

500 tonnes load

a

b

Load

Steel reinforcement bars

Compression

500 tonnes = 5000kN

a b

Bearing pressure
kN / m 2

m

Bending 1.0

2.0
3.0

1.0 5000

1250
556

Tension

2.0
3.0

Figure 2.4 Demonstration of how the bearing pressure on the ground can be reduced by increasing
the dimensions of the footing whilst carrying the same building load. The weaker the ground, the
larger the foundation will need to be or some other solution might be necessary, such as piling to
stronger material at depth. As the size of the foundation increases, so will the cantilever effect with
bending, as illustrated in the second diagram. Steel reinforcement will be necessary to resist tensile
stresses throughout the structure and also to resist buckling in the column.

and it might be necessary to carry out local dentition to excavate
pockets of soil or weak rock and replace with structural concrete.
Excavations should be examined by a competent engineering geologist
or geotechnical engineer, to check that the conditions are as good as
assumed for the design. Such checks and approvals should be well
documented. If the required pad sizes between individual columns are
large, it may make sense to combine the foundations in a single raft
over the full building footprint. It must be remembered, however, that
the wider the foundation, the greater the volume of ground stressed, as
illustrated in Figure 2.3, and the ground investigation must establish
the nature of ground over that full depth. There are many cases
where weak compressible material at depth has caused problems for
foundations (e.g. Poulos, 2005).
Instead of using a raft it is often cost-effective to take the foundations
deeper using piles, which might be timber, steel or concrete. The entire
building load might be transferred to some stronger stratum at depth
and this is called end-bearing on rockhead, a term that is discussed in
Chapter 3 ( Box 3-1).
Other piles rely on support from skin friction on the side of the piles,
for example, by driving a pile into sand until it can be driven no
further. Many piles are designed to be part end-bearing and partly
relying on skin friction (Figure 2.5) . The design of foundations and the
many different options are discussed in more detail in Chapter 6.