subsidenceCore Causes of House Subsidence

Soil Shrinkage

The predominant cause of subsidence in the UK is soil shrinkage (accounting for 75% of subsidence claims).

Soils are classified as either cohesive (shrink/swell) or non-cohesive but both types invariably contain water.

Cohesive Soils

Cohesive soils such as clay and silt vary in their ability to change their volume when wetted or dried (known as volumetric change), therefore when wet they will expand (swell) and when dry they will shrink.

Clay is particularly prone to ‘shrink-swell’ and is found extensively across England and Wales. Read more.

Cohesive: Clay soils are highly responsive to volumetric change due to their molecular structure. Clays have a repeating arrangement of atoms (known as a crystalline structure) with weak bonds that hold the clay crystals together (these allow water into the spaces between the crystal and push them apart causing swelling. Obviously the opposite is true and results in shrinkage. Clay is unique in containing water within the molecular structure as all other soil types where the water simply occupies the gaps between soil particles. To complicate matters further there is more than one type of clay each with different properties. Montmorillonite is the most highly expansive clay whilst kaolin is less expansive. The shrinkage / swelling potential of clay is measured through the ‘plasticity index’ (the range of moisture content over which clay retains is plastic properties). An index of over 20 denotes a soil susceptible to ‘medium’ shrinkage and swelling.

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Non-cohesive soil

Non cohesive soils, such as sand and gravel do not exhibit volumetric change, but they are susceptible for the fine particles in their composition to be washed away if subjected to water flow. Read more.

With non-cohesive soils fine particles in their composition can be washed away. Also sand that sits under the water table acts in a similar way to liquid and so will move into any gap surrounding it. In certain parts of the country (namely the Chilterns (Herts), Kent and Sussex) the soil is chalk and limestone and this can be dissolved over time by running water to create caves. These can sometimes collapse causing the ground to fall and create what is known as a swallow hole.

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Natural decomposition

Organic soils, such as peat, can naturally decompose due to oxidisation and this can lead to a reduction in its volume. Oxidisation occurs when the organic material comes into contact with the air and this is usually when the water table falls.

Man Made

Man-made subsidence is mainly caused by old in-filled sites and vibration see triggers
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In-filled sites are where pits and quarries have been filled in after excavation or mining with a variety of materials (sometimes including refuse) and covered over. The fill can consolidate or degrade over time (mainly due to decomposition of the materials) causing a reduction in the volume of the soil leading to subsidence.

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Triggers of House Subsidence

Trees and Vegetation

The main cause of subsidence movement in the UK is the influence of tree roots in clay soil. Over 60% of all subsidence claims are triggered by trees.

trees cause subsidenceTree roots absorb water for photosynthesis and moisture evaporates from the leaves through transpiration. The active period is predominantly in spring and early summer when tree growth is at its maximum. With larger trees in shrinkable clay soil the tree can extract sufficient moisture to cause soil shrinkage and hence may lead to subsidence. A mature deciduous tree can remove in excess of 50,000 litres of water a year.

Tree species vary considerably in their ability to cause clay soil shrinkage. Some species are not good at rooting to depth on clay soils while species like Oak, Willow, Poplar and Eucalyptus are able to maintain viable roots to a depth of several metres. In most soils, however, there tends to be a mixture of soil types and so many species will be able to grow roots to sufficient depth to influence buildings where the foundations are not deep enough. The age, health and past management of trees will also be an important factor. Trees of full maturity with little potential for future growth which have not caused damage to a building in the past may be less of a consideration as a risk than trees which are growing vigorously and increasing in size.

The zone of influence is the area from which a tree absorbs moisture. The potential impact on a property depends on whether a property sits within this zone of influence. The extent of the zone depends upon the type of tree and the location of other trees. A general rule of thumb to determine the zone of influence is to imagine cutting the tree at its base and lying it down.

Leaking Drains and Pipes

Leaking drains and pipes are the second highest cause of house subsidence accounting for about 15-20% of subsidence incidents. There are two ways in which a leaking drain or water main can cause house subsidence:

  • Softening the ground and therefore its load bearing capacity, resulting in the downward movement of the foundations
  • Washing away fine particles in a non-cohesive soil and so reducing the volume of the soil

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It is rare for the drain to have its joints forced open by a tree root, however more common is that the roots grow into any open joints or cracked drains causing further damage to the drain, which increases leakage. In addition drains are usually laid relatively shallow in the ground and the trenches are filled with excavated materials. As the drain trench acts like a sump and draws in ground water the trench can erode over time leading to consolidation in the fill and movement of the adjacent ground towards the trench.

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Man Made

Man-made triggers include removal of support around the foundations, vibration and changes in the water table. Read more.

Excavations around the foundations (e.g. extensions, new drainage or work on adjacent land) can cause a sideways collapse of the ground beneath the foundations.

Vibration can consolidate and settle ground leading to downward movement of the foundations. Vibration can be caused by heavy and constant traffic near properties or pile driving on an adjacent building site.

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