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68 Barry’s Advanced Construction of Buildings
As building loads are transferred to the ground, it will react to accommodate the load, and
some (minor) settlement should be expected.
The applied load of buildings on foundations may cause settlement either through the
compression of soil below foundations or because of shear failure due to overloading. Settlement
movements on non-cohesive soils, such as gravel and sand, take place as the building
is erected, and this settlement is described as ‘immediate settlement’. On cohesive soils,
such as clay, settlement is a gradual process as water, or water and air, is expelled from pores in
the soil. This settlement, which is described as ‘consolidation settlement’, may continue for
several years after completion of the building. Anticipated ground movements and potential
settlement will be accommodated in the design of the foundation system, which should
also include for relative movement between different parts of the foundation.
If the building loads are not properly distributed and foundations are not designed and
constructed correctly, differential settlement may occur. Differential settlement occurs
when different parts of the building settle into the ground at different rates. Figure 3.1
illustrates some of the causes of differential settlement.
Causes of differential settlement
❏ Differing building loads
○ Dead loads – building structure loads not properly accommodated by the
foundations
○ Unexpected live loads – services and equipment installed within the building where
the vibration or excessive load exceeds the foundation and soil design strength
If one part of the building is
loaded more than another (e.g. by
heavy equipment) and the
foundations do not allow for this,
settlement may occur in this area
as the ground is overstressed
Buildings or loads in
proximity
Loadbearing strata
Where foundations are close together
the stress exerted on the soil can
overlap increasing the force such that
the ground fails and settlement occurs
Weak strata
Where one part of the
build sits on firm ground
and another part rests on
weak or unstable strata,
the building is likely to
suffer differential
settlement
Excessive
settlement
may fracture
services
Figure 3.1 Causes of differential settlement.