Subsidence, Landslip and Heave

Subsidence, Landslip or Heave

These are major perils covered by a
household insurancepolicy are defined as follows:-

What is Subsidence?

Subsidence is the vertical downward movement of a building foundation caused
by the loss of support of the site beneath the foundations.
This is usually associated with a change in the volume of the subsoil
(e.g. the shrinkage of clay) which occurs as a result of an external factor
(e.g. the influence of trees) as defined
in the Subsidence Forum Handbook.

Possible causes of Subsidence

Trees and shrubs extract moisture from the subsoil to facilitate thetransport of nutrients and minerals and also assist in cooling the tree.

In clay subsoils, the extraction of moisture may result in desiccation of the clay subsoil, which causes clay to shrink. This causes the house foundations to settle, which will cause cracking in the walls of the property.

Leaking or defective drains can soften the supporting subsoil or wash out the fine minerals within the soil, causing settlement of the house foundation.

Mining/Solution Features - Voids beneath the soil beneath the property foundations can form due to mining or naturally occurring features caused by ground water wash out causing settlement.

What is Heave?

Heave is the expansion of the ground beneath part or all of the building. This is normally taken to mean an upward movement of the site caused by expansion or swelling of the subsoil (or the opposite to subsidence). The most common cause of heave is removal of trees and other vegetation, causing the clay subsoil to re-wet and swell.

If a tree pre-dates the house, then removing the tree may cause the soil to uplift to a greater volume than when the property was built. A tree that postdates the age of the property may possibly be removed and allow the ground to revert to equilibrium without causing damage.

Heave or long-term recovery can only occur if a persistent soil moisture deficit exists. This is where the soil remains in a partially desiccated condition following the process of re wetting during winter months. This is more likely to occur if the removed tree was mature and had a high water demand, such as an oak tree.

Possible causes of Heave

Removal of Vegetation - When a tree is removed in a clay subsoil, the soil will rehydrate and swell in volume. The expansion may lift the
foundations and cause cracking to the property.

Drains - It is possible for a leaking drain to cause previouslydesiccated clay subsoil to swell due to over-wetting and cause heave damage.

What is Landslip?

Landslip is the sudden movement of soil on a slope or gradual creep of a slope over a period of time.

Possible causes of Landslip

Landslip can be caused by slope destabilisation (caused by new adjacent works - undercutting), alteration of water courses, dumping of fill up slope (which causes a surcharge) and failure of retaining walls allowing movement ofretained earth.

How to recognise Subsidence damage

    Vertical and diagonal cracking concentrated in
    specific areas and tapering in width between the top and bottom of the property

Cracks extending through the DPC down into the

    External cracking reflected internally in the
    same area of the wall

Rucking of wallpaper at the corner between wall
and ceiling junctions

    Re-pointing of mortar joints diagonally or
    vertically up walls indicating past problems or possible subsidence

    Distortion of openings, which are weak points in
    the structure, causing doors and windows to stick

    Rotation of buildings towards trees on
    shrinkable clay subsoils

    Cracks appearing after a prolonged period of dry

Seasonal opening and closing of cracks.

What constitutes a significant crack?

Generally it can be considered that hairline cracks which appear on an annual basis are not structurally significant, but cracks which increase in width gradually over a period of time should be investigated. When they become between 16-25mm, they are classified as severe under the BRE Digest 251 (classification of visible damage to walls).

RICS suggests
that regular checking of your property for evidence of cracking or distortion of window/door openings is advisable.

Should you find
cracking of any significance, you should immediately notify your insurance company or your landlord and make a claim.

Previous page: ART SAVES LIVES