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74 Barry’s Advanced Construction of Buildings
Shrinkage and expansion due to seasonal variations will extend to 1 m or more in periods
of severe drought below the surface in Great Britain. Seasonal variation in moisture content
will extend much deeper in soils below trees and can be up to 4 m or more below large
trees. When shrubs and trees are removed to clear a site for building on cohesive soils, for
some years after the clearance there will be ground recovery as the soil gradually recovers
water previously taken out by trees and shrubs. This gradual recovery of water by cohesive
soils and consequent expansion may take several years.
Peat and organic soil
Peat and organic soils contain a high proportion of fibrous or spongy vegetable matter from
the decay of plants mixed with varying proportions of fine sand, silt or clay. These soils are
highly compressible and will not serve as a stable foundation for buildings.
Made-up ground and fill
Made-up ground is the term used to describe where the ground level has been raised by
spreading and sometimes compacting material from excavations or waste. Depending on
the materials used, the way that they have been levelled, layered and compacted will affect
the ability of the made-up ground to be used as a loadbearing material.
Fill is the term used to describe the tipping of material into pits or holes, which are left
after excavation, mining or quarrying, to raise the level to that of the surrounding ground.
Made-up ground and fill will not usually serve as a stable foundation for buildings due to
the extreme variability of the materials used to make up ground and the variability of the
compaction or natural settlement of these materials. Where it is anticipated that the area
to be filled will be used for further development, careful attention needs to be given to the
materials used to fill the site, the depth of the layers in which the fill material is placed and
the compaction method(s) to be used.
The characteristics of the individual constituents of ground, rocks, soils and organic soils
will provide an indication of the likely behaviour of a particular ground under the load of
foundations. In practice, soils often consist of combinations of gravel, sand and clay in
varying proportions, which combine the characteristics of the constituents.
Ground instability
There are areas of ground that are unstable due to natural processes, such as landslip of
sloping strata of rocks or soils, or due to human activities such as mining and surface
excavation. Under the load of foundations, the unstable ground may be subject to ground
movement, which should be anticipated in the design of foundation. Land instability may
be broadly grouped under the headings:
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Landslip
Surface flooding and soil erosion
Natural caves and fissures
Mining and quarrying
Landfill
The Environment Agency (http://www.environment-agency.gov.uk) provides a database of
useful information on landfill sites, floodplains, subsidence and contaminated land. The