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Ground Stability, Foundations and Substructures 77
subterranean water. Where there are caves or small cavities in these areas near the surface,
land instability and subsidence may occur.
Mining and quarrying
Mining and quarrying of mineral resources has been carried out for centuries over much
of England and parts of Wales and Scotland. The majority of the mines and quarries have
by now been abandoned and covered over. From time to time, mining shafts collapse and
the ground above may subside. Similarly, ground that has been filled over redundant quarries
may also subside. There is potential for land instability and subsidence over those areas
of the UK where mineral extraction has taken place. The Environment Agency has commissioned
surveys and produced reports of those areas known or likely to be subject to
land instability due to mining and quarrying activities. There are regional reports and
atlases indicating the location of areas that may be subject to land instability subsidence.
Coal mining areas have been comprehensively surveyed, mapped and reported. Other areas
where comparatively extensive quarrying for stone, limestone, chalk and flint has taken
place have been surveyed and mapped and reported. Less extensive quarrying, for chalk
for example in Norwich, has been included. The reports indicate those areas where subsidence
is most likely to occur and the necessary action that should be taken preparatory to
building works (http://www.environment-agency.gov.uk).
Landfill
Landfill is a general term to include the ground surface which has been raised artificially
by the deposit of soil from excavations, backfilling, tipping, refuse disposal and any form
of fill which may be poorly compacted, of uncertain composition and density and thus
have indeterminate bearing capacity, and may be classified as unstable land. In recent years,
regional and local authorities have had some control and reasonably comprehensive details
of landfill, which may give indication of the age, nature and depth of recent fill. The land
over much of the area of the older cities and towns in the UK, particularly on low-lying
land, has been raised by excavation, demolition and fill. This overfill may extend some
metres below the surface in and around older settlements and where soil excavated to form
docks has been tipped to raise ground levels above flood water levels. Because of the variable
and largely unknown nature of this fill, the surface is in effect unstable land and should
be considered as such for foundations. There are no records of the extent and nature of this
type of fill that has taken place over some considerable time. The only satisfactory method
of assessing the suitability of such ground for foundations is by means of trial pits or boreholes
to explore and identify the nature and depth of the fill.
3.2 Functional requirements
The primary functional requirements of foundations are strength and stability. To comply
with Building Regulations, the combined dead, imposed and wind loads of the building
should be safely transmitted to the ground without causing movement of the ground that
may impair the stability of any part of another building. Loading is concerned with the
bearing strength of the ground relative to the loads imposed on it by the building. The