Geology Reference
In-Depth Information
As discussed in detail in
Chapter 6,
there are two key considerations
for foundation design. Firstly, there should be a check against bearing
capacity failure or ultimate limit state of the underlying soil or rock.
This involves analysis of the various loads and calculating the strength
of the supporting ground. Generally, in traditional design methods a
Factor of Safety (FoS) of between 2.5 and 3 is adopted against ultimate
bearing capacity failure, i.e. the allowable bearing stress should be at least
2 or 3 times lower than the load that the ground could theoretically
support without failing catastrophically.
The second check is for settlement (otherwise known as a service-
ability state). Settlement is inevitable as a building is constructed and the
ground loaded, but there are certain tolerances that the designer needs to
be aware of. Many structures can cope with perhaps 25mm of vertical
(total) settlement and some, such as an earth embankment dam (con-
structed from soil and rock
fill), may settle by metres without distress;
the key question is usually the tolerance of a structure to differential
settlement, whereby some parts of the structure settle more than others,
causing shear stress between different parts. This may happen where the
ground is not uniform
perhaps one corner of the building footprint is
less weathered and therefore stronger and less compressible than the
rest. If the design of the foundations and/or load distribution of the
building does not properly account for this variability, the building will
settle more towards one end. Generally, the limiting rotation for a
framed structure is taken as about 1 in 500 to 1 in 300, to avoid cracking
in walls and partitions (Skempton &Macdonald, 1956); for a high-rise
building the tolerance may be lower. Some structures may be even more
sensitive and have special requirements for restricting settlement.
Generally, the structural designers will need to tell the geotechnical
engineer and engineering geologist what is the tolerance for the project
so that foundations can be designed accordingly.
Generally, it is cost-effective to design shallow foundations for
structures. For framed structures comprising columns and beams of
concrete or steel, the load is carried on the columns, which are then
founded on pads of reinforced concrete. The size of pad will control the
bearing pressure on the underlying soil or rock. If the ground cannot
carry the applied stress from the building without unacceptable settle-
ment, then the pad size may be increased, as illustrated in
Figure 2.4.
For a concrete frame structure, steel reinforcement would be placed in
the columns and towards the base of the foundation where the con-
crete may be subject to tensile stress by bending; concrete (and rock) is
relatively weak in tension
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typically about one tenth of its strength in
compression. Examples of design calculations are given in Tomlinson
(2001). In
Figure 2.4,
because of the variable ground conditions below
the pad, great care would be needed not to overstress any weaker zones
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