Shrinks Well

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UK Geohazard NoteMay 2012

Ground shrinking and swelling

www.bgs.ac.uk [email protected]

What is a shrinkswell?Subsidence due to shrink-swell clays is ground

movement caused by clay soils that swell, and thus

increase in volume, when they get wet and shrink

when they get dry.

Why does shrinkswell occur?

Shrinkswell occurs as a result of changes in the

moisture content of clay-rich soils. This is relected

in a change in volume of the ground through

shrinking or swelling. Swelling pressures can causeheaving, or lifting, of structures whilst shrinkage can

cause differential settlement.

The amount by which the ground can shrink and/

or swell is determined by the water content in the

nearsurface and the type of clay. Fine-grained

clayrich soils can absorb large quantities of water

after rainfall, becoming sticky and heavy. Conversely,

they can also become very hard when dry, resulting

in shrinking and cracking of the ground. This

hardening and softening, with associated volume

change, is known as shrink-swell.

This can be a natural seasonal occurrence or one

enhanced by a range of factors, including:

normal seasonal movements associated with

changes in rainfall and vegetation growth

enhanced seasonal movement associated with

the planting, severe pruning or removal of trees

or hedges

changes to surface drainage and landscaping

(including paving)

shortterm unseasonal movements as a result of

leaks from water supply pipes or drains

longterm subsidence, as a persistent water

deicit develops

longterm heave as a persistent water deicit is

reversed by wetting.

What are the consequences of it occurring?

Damage to buildings may occur when the volume

change of the soil, due to shrinking or swelling, is

unevenly distributed beneath the foundations. For

Differential subsidence due to down-slope side of

house moving on shrinkable soil. Image Chris Page.

Overview

Shrinkage, also referred to as ground

shrinkage, is a form of subsidence

caused by the lowering or displacement

of the ground, which can be triggered

by man-made disturbances, a change in

drainage patterns, heavy rain or by water

abstraction. Swelling, or expansive, soils

increase in volume when they get wet and

can cause uplift, or heave.

Shrink-swell can cause damage to buildings

and infrastructure and is a major concern

for the insurance industry.

BGS maintains a National Geotechnical

Properties Database, which is continually

updated and the GeoSure National Ground

Stability Data, which provides geological

information about potential ground

movement or subsidence, including theGeoSure shrinkswell dataset.


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example, if there is a difference in water content in

the ground beneath a building, swelling pressures

can cause a wall to lift; often called heave. This can

happen at the corners or towards the centre of a

building.

What is the cost to the UK economy?

Shrinking and swelling of the ground (often

reported as subsidence) is one of the most damaginggeohazards in the UK today costing the economy

an estimated 3 billion over the past 10 years (ABI,

2006). The Association of British Insurers hasestimated that the average cost of shrinkswell

related subsidence to the insurance industry stands

at over 400 million per year (Driscoll and Crilly,

2000).

Scientific detail

Monitoring and measurement

The main factors chosen as relevant to the

determination of shrinkswell and the ability to

assess it on a national basis are:

Volume change potential (VCP) of bedrock and

supericial deposits

Thickness and type of supericial deposits

Variation in till (supericial deposits laid down by

the direct action of glacial ice)

The selection of those factors accord with the

assessment methodologies outlined by the Building

Research Establishment (BRE 1993).A meaningful assessment of the shrinkswell

potential of the UK requires a considerable amount of

UK Geohazard Note


UK Examples

In the UK, the effects of shrinking and

swelling were irst recognised by geotechnical

specialists following the dry summer of 1947;

since then the cost of damage due to shrinking

and swelling clay soils has risen dramatically.

After the drought of 197576 insurance

claims came to over 50 million. In 1991, after

the preceding drought, claims peaked at over

500 million.

Towns and cities built on clay-rich soils

most susceptible to shrinkswell behaviour

are found mainly in the south-east of the

country. Here, many of the clay formations

(e.g. London Clay, Oxford Clay, Gault Clay,

Kimmeridge Clay) are too young to have been

changed into stronger mudstones, leaving

them still able to absorb and lose moisture.

Clay rocks elsewhere in the country are older

and have been compacted and hardened by

deep burial and are less able to absorb water.

Some shrink-swell prone clays (e.g. around

The Wash and under the Lancashire Plain)

are deeply buried beneath other (supericial)

soils that are not susceptible to shrinkswell

behaviour. However, some supericial deposits

such as alluvium, peat and laminated clays

can also be susceptible to soil subsidence and

heave (e.g. in the Vale of York and the Cheshire

Basin).

Cracks in Gault Clay, Mundays Hill Quarry, Leighton

Buzzard. Image NERC.

May 2012


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high-quality, consistent and well distributed spatial

data. The BGS National Geotechnical Properties

Database contains a large amount of index test

data. At the time of writing, the database contained

data from more than 80 000 boreholes, comprising

nearly 320 000 geotechnical samples, with 100 000

containing relevant plasticity data, which is used to

calculate the VCP.

How is the hazard characterised?

GeoSure national datasets provide geological

information about potential ground movement or

subsidence that can help planning decisions.

The BGS has created a dataset which is based on the

properties of both the bedrock and the supericial

deposits. Bedrock comprises geological deposits

that are older then 2.6 million years, often found

at the surface as well as below supericial deposits.

Supericial deposits are unconsolidated 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 supericial deposits

generates a map of shrinkswell susceptible areas with

a rating for the potential for volume change in soils.

The classiication system used in the BGS GeoSure

shrinkswell hazards maps is based upon that outlined

by the BRE (1993), which provides a deinition for the

volume change potential for ine-grained rocks and

soils, based upon a modiied plasticity index.

Other considerations when characterising

the hazardIn addition to the shrink-swell properties of the

bedrock being classiied, consideration is also given

to the thickness and variation of supericial deposits

including variations in the type of glacial till. All of

these factors have been considered when producing

the GeoSure shrinkswell hazard maps.

Scenarios for future events

Indications are that future climate change will

have an increasingly adverse effect on shrink-swell soils and, therefore, on the 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 (Jones, 2004).

The change in the amount and distribution of

rainfall, as a result of climate change, will lead to

a signiicant increase in the damage done by the

shrinking and swelling behaviour of these clay soils.

The Association of British Insurers predicts that

subsidence (downward movement of the ground

surface) claims will reach 600 million a year by

2050 and the AA, which monitors over 40 different

home insurers, is predicting that the average home

insurance premium will rise signiicantly in the

years ahead (Jones, 2004).

Partnerships and links

British Research Establishment (www.bre.co.uk) National House-Building Council (www.nhbc.

co.uk)

UK Geohazard Note


Damage caused by shrinkage of London Clay

deposits beneath a building. Image Peter Kelsey

and Partners.

May 2012


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Subsidence Forum (www.subsidenceforum.org.uk)

Royal Institute of Chartered Surveyors

(www.rics.org/uk)

References

1. Association of British Insurers. 2006.

Subsidence Dealing with the problem

[online]. [Cited 3rd August, 2006]. Available

from http://www.abi.org.uk

2. BRE. 1993. Low-rise buildings on shrinkable

clay soils: BRE Digest, Vols. 240, 241 and 242.

CRC, London.

3. Driscoll, R and Crilly, M. 2000. Subsidence

damage to domestic buildings. Lessons learned

and questions asked. Building Research

Establishment, London.

4. Jones, L D. 2004. Cracking open the property

market. Planet Earth, Autumn 2004, pp3031.

NERC, UK.

5. Jones, L D, and Terrington R. 2011. Modelling

volume change potential in the London Clay.

Quarterly Journal of Engineering Geology and

Hydrogeology, 44, 115.

Further information

Contact the BGS Shallow Geohazards team by:

Email [email protected]

Telephone 0115 9363143

BGS GeoSure website: www.bgs.ac.uk/products/geosure/home.html

BGS shrinkswell website: www.bgs.ac.uk/science/landUseAndDevelopment/shallow_geohazards/

shrinking_and_swelling_clays.html

UK Geohazard Note


Interpolated volume change potential, values for the London Clay.Jones and Terrington, 2011.

May 2012

British Geological Survey NERC 2012

Shrinks Well

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