UK Geohazard Note
June 2014
Compressible Ground
Why does it occur?
Many ground materials contain water-filled pores
(the spaces between the solid particles). Ground is
compressible if a building, or other applied load,
causes the water in the pore space to be squeezed
out, causing the ground to decrease in thickness
(compress). Peat, alluvium, laminated clays, some
silts and sands, and made ground are the types of
deposit commonly associated with various degrees
of compressibility. This deformation of the ground
is usually a one-way process that occurs during or
soon after construction. Compressible materials,
such as peat, undergo both primary and secondary
settlement. Primary settlement takes place in days
and occurs due to water expulsion or loading;
secondary settlement may last years and is due to
the restructuring of the material.
Damage to the internal structure of a house due to
settlement of compressible ground. Image © NERC.
Overview
If ground is extremely compressible the building
may sink below the surface of the surrounding
ground or relative to adjacent structures that apply
lesser or greater loads to the ground. If the ground
is not uniformly compressible, different parts of the
building may sink by different amounts (differential
settlement), or at different rates, causing tilting,
cracking or distortion. It isn’t just the risk that
the soft layers might be compressed that could
present an elevated danger, but also the threat from
groundwater levels changing. When moisture is
extracted highly compressible deposits shrink and
the compression process that follows is very difficult
to reverse. The most common consequences are
damage to existing properties that were not built
to a sufficient standard, and possible damage to
underground services.
• Compressible ground is caused by soft
superficial layers, which can undergo volume
reduction under the weight of overlying
structures or changes in water level.
• Loading causes water in pore spaces to be
squeezed out causing the ground to compress.
• Compressible materials include peat,
alluvium, laminated clays, some silts and
sands and made ground.
• Compressible ground can cause damage to
building foundations, utility connections and
can create cracks, tilting or subsidence in
buildings.
• The BGS produces maps of potential for
compressible ground in England, Scotland
and Wales.
What problems can it cause?
There are a number of problems that may affect
properties or services built on compressible ground:
What is compressible ground?
Certain types of ground contain layers of very soft
materials like peat and clays. These layers are likely to
compress if they are loaded by overlying structures,
or if the groundwater level changes around them.
This compression may result in depressions
appearing in the ground surface or under structures,
potentially damaging foundations and services.
www.bgs.ac.uk • structural damage to foundations and to the
fabric of the building
• strains or break in service connections to water,
gas and electricity
• cracks in walls, floors or ceilings of a building
(first noticeable signs of damage)
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UK Geohazard Note
Susceptible locations
June 2014
Compressible ground is most commonly associated
with peat, alluvium, laminated clays, some silts and
sands and made ground.
Scientific detail
Methodology
The BGS GeoSure methodology is used for producing
maps of potential compressible ground. The
approach identifies the presence of factors that
bring about a hazard at the site being assessed. The
causative factors are given a rating according to
their relative importance in causing the hazard, and
then combined in an algorithm to give a rating of
the relative susceptibility of the area being assessed
to the hazard occurring at some time. It does not
necessarily mean that the hazard has happened in
the past or will do so in the future, but if conditions
change and a factor intensifies, the hazard may be
triggered.
Damage to building caused by differential settlement
on compressible ground. Image © NERC.
• tilting of walls or buildings
• differential settlement of buildings with different
foundation solutions e.g. extensions
• cracks at the junctions of buildings or associated
structures
The factors were chosen on the basis of their
relevance to the determination of compressible
ground hazard and the ability to assess them within
reason on a national basis. Three factors were
chosen for inclusion in the assessment:
•lithology
• thickness of the compressible layer (where
known)
• the inferred variability of the material that
will cause settlement to be uneven and more
damaging (from knowledge of the material and
published literature)
How is the hazard characterised?
Map
showing the
potential for
compressible
ground in
England,
Scotland and
Wales. Image
© NERC.
www.bgs.ac.uk GeoSure national datasets provide geological
information about potential ground movement
or subsidence that can help planning decisions.
The BGS has created a dataset that is based on the
properties of both the bedrock and the superficial
deposits. Bedrock comprises geological deposits
that are older than 2.6 million years, often
found at the surface as well as below superficial
deposits. Superficial deposits are unconsolidated
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UK Geohazard Note
June 2014
damage caused to homes, buildings and roads. The
Government has recognised that future climate
change is one of the biggest problems that the UK
faces and, if current predictions are correct, we can
expect hotter, drier summers in the south-east of
England and milder, wetter winters in the rest of
the UK. The change in the amount and distribution
of rainfall, as a result of climate change, may lead
to a significant increase in the damage done by the
compressible behaviour of these deposits.
UK Example
Holme Post in Cambridgeshire is within
Britain’s largest area of peat soils. Peat is
a rich organic soil, ideal for farming but
unsuitable to construct upon. Engineering
works on the fens began in the early 1600s in
order to control the River Ouse and drain the
Bedford Levels. The ground subsidence that
followed was soon seen as being inevitable
and relentless.
BGS data
In 1851, in order to monitor the rate of
subsidence, the original Holme Post was sunk
into the clay substrate with the top flush with
the land surface. The post now rises 4 m above
the ground, exposed following the drainage and
wastage of the peat. The reclaimed farmland was
abandoned to birch woodland a century ago.
• BGS GeoReports (www.shop.bgs.ac.uk/
Georeports)
• BGS GeoSure: National Ground Stability
Data (www.bgs.ac.uk/products/geosure/
compressible.html)
Partnerships and links
geological deposits younger than 2.6 million years.
They are found at the surface, overlying bedrock
deposits. Using the properties of both the bedrock
and superficial deposits generates a map of
compressible ground susceptible areas with a rating
for the potential for ground subsidence to occur.
• British Research Establishment (www.bre.co.uk)
Indications are that future climate change could
have an increasingly adverse effect on highly
compressible deposits and, therefore, on the
• BGS GeoSure: National Ground Stability
Data (www.bgs.ac.uk/products/geosure/
compressible.html)
• National House-Building Council (www.nhbc.co.uk)
• Subsidence Forum (www.subsidenceforum.org.uk)
• Royal Institute of Chartered Surveyors
(www.rics.org)
• BGS GeoReports (www.shop.bgs.ac.uk/
Georeports)
Scenarios for future events
Further information
Contact the BGS Shallow Geohazards team by:
Email: [email protected]
Telephone: 0115 936 3143
BGS GeoSure website:
www.bgs.ac.uk/products/geosure/home.html
www.bgs.ac.uk British Geological Survey © NERC 2014
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