Building Failure and Subsidence Investigation
What is subsidence and heave damage?
Subsidence and heave damage occurs when differential movement in the ground causes movement of the building foundations. The damage often shows up as cracks in walls or ceilings, and often found around door or window openings.
What causes this?
Subsidence most commonly occurs when clay soil under the foundations dries up and shrinks, frequently due to moisture extraction by the roots of nearby trees. This type of subsidence typically occurs in the Summer following hot, dry weather. The second most common cause is leakage from underground pipes washing away or softening the soil beneath the foundations. Buildings can also move and be damaged by other factors including the movement of old coal, flint and chalk mine workings and the collapse of shafts; natural features such as sinkholes; infilled quarries; soft soil and unstable slopes. Alternatively, heave is the expansion in clay soil which most commonly occurs as a result of rehydration following the removal of trees. This causes differential upward movement of foundations.
A series of investigations are carried out to determine the cause of the damage. These consist, where appropriate, of trial pits to research the buildings foundation, boreholes to obtain details of soil at depth, laboratory testing of soil samples and monitoring of the building.
In many cases, subsidence damage can be repaired relatively easily and effectively and without foundation work. This is usually after the cause has been addressed which will typically involve some tree work or repair of a leaking service pipe. In a small proportion of cases, normally where the cause cannot be removed, it is necessary to undertake foundation works.