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BGS Technical Report WM97/03 15 December 1997
valley alluvium. Such deposits are highly compressible compared to the surrounding deposits
and could give rise to excessive and differential settlement of overlying structures.
7.4.3 Man-made deposits
Man-made deposits represent a hazard in three main ways:
1. The commonly uncompacted nature of man-made deposits can give rise to unstable
foundation conditions. The composition can be very variable from site to site and
within short distances on a single site. In places it may be very weak or cavernous and
cause excessive and uneven settlement. Organic material within made ground may rot,
causing cavitation and settlement below buildings. When spoil is dumped on a slope,
the buried soil/organic layer may be weak and therefore might form a potential failure
surface.
2. Toxic fluids, either as a primary component of the man-made deposit or generated
secondarily by chemical or biological reactions within it, can migrate both within the
deposit itself and into adjacent permeable strata.
3. Toxic or explosive gases, particularly methane, can be generated within waste tips and
landfill sites. Such gases can migrate - sometimes through permeable strata adjacent to
the site - and accumulate within buildings or excavations either on the site itself or some
distance from it.
These possibilities should be addressed by appropriate geotechnical investigations in areas where
man-made deposits are likely to be present. The man-made deposits shown on sheets SK 23 SE
represent those that were identifiable at the time of survey. They were delineated principally by
the examination of documentary sources and the recognition - in the field and on aerial
photographs - of areas where artificial modification of the natural topography has taken place.
Only the more obvious man-made deposits can be mapped by this method and the boundaries
shown are likely to be inferred and approximate.
8. BOREHOLES
There is only one deep borehole in the area. This is the Pastures Hospital Borehole (SK 23 SEA9
[2968 33 171 drilled 1899-1901. As usual in the case of these older records, the stratigraphy is
open to interpretation. One interpretation is given below. Abbreviations are as follows: OD
Ordnance Datum; SL Surface level; TD Terminal depth. Soil has been omitted fiom the logs.
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