Ground heave

[edit] Introduction

Ground heave is the upward movement of the ground usually associated with the expansion of clay soils which swell when wet. As the soil generally cannot expand downwards or sideways, the result is that the exposed upper surface of the soil rises up. The impact of heave is opposite to the effect of subsidence which is where soil is unstable and sinks downward, or settlement which is caused by the weight of a building.

Displacement is generally less than 150 mm, however, even this level of movement can lead to serious structural damage to building foundations and fabric. Long-term damage could be incurred that may not be detected for some time, but can significantly affect a property’s value.

[edit] Causes of ground heave

The most common cause of ground heave is associated with trees which have died or been removed. As the root network no longer draws water from the subsoil, water accumulates in the ground, resulting in swelling that can move building structures upwards.

Additional causes of ground heave may include:

‘Stress relief’ (or overburden recovery) attributable to soil being removed from an excavation and so relieving pressure on layers below. This may occur for example in the construction of basements.
A change in the level of the water table.
Broken drains or nearby building works that interfere with existing ground drainage.
Swelling of the sub-soils due to seasonal weather changes.
Water expanding as it freezes causing soil to swell. Some particular soils, such as silty and sandy clays, can be more susceptible than others.

[edit] Signs of ground heave

Common signs of ground heave may include:

Cracking to brickwork and windows. This cracking is more likely to be vertical, whereas subsidence cracking is more likely to be diagonal.
Doors sticking as their frames become out of square.
Lifting of paths and patios surrounding buildings.

[edit] Prevention and remedying of ground heave

Surveys will indicate whether heave has occurred that may impact upon a building, and may advise, for example, on the potential risks of proposed works such removing trees near buildings. Investigations may include visual inspection, historic research and drilling boreholes to determine the moisture profile of the soil.

Where heave is likely to be a problem, cellular structures may be installed beneath foundations and floor slabs to reduce the upward force of heave from transmitting to the structure above. For more information, see Cellular raft foundation.

Remedying damage due to ground heave can be a lengthy and expensive process. Where heave is the result of an underlying problem, remedial works such as repairing leaking drains or removing vegetation may resolve the problem.

Where the soil itself is prone to heave, underpinning may be necessary to stabilise structures. Excavated soil from beneath existing foundations is replaced with material, usually concrete, in a series of phases to form a new foundation beneath the existing one, reaching down to subsoils that are not prone to heave. It may be necessary to leave a cavity beneath the structure to accommodate future movement of the upper layers of soil.