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Ground Stability, Foundations and Substructures 69
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Different ground conditions under the building
Water courses that destabilise the ground
Weak compressible strata
Adjacent loads causing overloading under existing foundations
Changes in adjacent vegetation that result in different water content of clay soil, causing
it to shrink or swell
Freezing of ground below, or adjacent to, foundations. As the water in the ground
freezes, it expands, which may cause shallow foundations to lift as the ground below
it freezes.
Movements of the foundation independent of the applied loads of buildings are due to
seasonal changes or the effects of vegetation, which lead to shrinking or swelling of clay
soils, frost heave, changes in groundwater level and changes in the ground due to natural
or artificial causes. The expansion of water in soils with low permeability due to freezing
was described in Barry’s Introduction to Construction of Buildings. The expansion and consequent
heaving of the soil occur at the surface and for a depth of some 600 mm. The NHBC
(2000) Standards recommend a minimum depth of 450 mm for all excavations to avoid
frost action; however, most foundations are excavated to a minimum distance of 750 mm
to avoid volume changes due to seasonal movement. The foundations of large buildings are
generally some metres below the surface, at which level frost heave will have no effect in
the UK. Correctly designed and constructed, the foundations will provide a firm and
durable base, helping to prevent distortion of the structure and damage to underground
services.
Rocks
Rocks may be classified as sedimentary, metamorphic and igneous according to their geological
formation as shown in Table 3.1 or by reference to their presumed bearing value
(Table 3.2), which is the net loading intensity considered appropriate to the particular type
Table 3.1 Rocks
Group
Sedimentary
Formed as particles are laid on top of one another
under pressure of the above ground, air or water
Metamorphic
Transformation of an existing rock –
metamorphism. These rocks are formed subject
to high temperatures and pressures deep
beneath the earth’s surface.
Igneous
Formed from molten rock, magma.
Rock type
Sandstones (including conglomerates)
Some hard shales
Limestones
Dolomite
Chalk
Some hard shales
Slates
Marble
Quartzite
Schists
Gneisses
Granite
Dolerite
Basalt