Cracks in the walls of a house can often, but by no means always, be caused by subsidence or settlement of the property. One must, however, be careful to differentiate between the two.
Settlement is the downward movement brought about by the pressure from a foundation causing compression of the ground immediately below it. Most soils (rocks and perhaps dense gravel excepted) will, like any plastic materials, compress under pressure. Normally this is fairly evenly distributed and the house settles as a unit as it is built. This is commonly termed primary settlement. Once this occurs the property will remain in equilibrium with the loadbearing soil and no further movement need be anticipated unless some external factor becomes operable. However, if the ground is not uniform in character or the load on part of the foundations is disproportionately high, one section of the foundations may settle more than another causing differential settlement leading to cracking of the superstructure in the vicinity. The adding of an extension can often result in such settlement, resulting in cracks where it joins the original structure.
Settlement of this nature therefore is a relatively minor defect and is not normally a continuing problem. Minor repairs and repointing is often all that is necessary. Damage as a result of settlement is not normally covered by household insurance.
Subsidence on the other hand is downwards movement of the ground brought about by activity in the ground and includes:
· Mining subsidence, particularly coal mining in the North East, although due to the great reduction in this field, this is now a rare event.
· Compression of loose, manmade fill owing to its self-weight or the ingress of water, the most common examples in the North East being old filled quarries, the presence of which may not have been known when the house was built.
· Removal of water from the ground by pumping or, more commonly, the extraction of water from clay subsoils by tree roots during prolonged dry periods.
· Excavations which remove lateral support from adjacent ground.
Conversely, upward movement of the ground, or heave, can have very similar effects on a property and sometimes it is difficult to tell whether one side is going down or the adjacent side is going up. Causes of heave are:
· Rehydration or increase in water content of clay soils caused by the removal of trees when the clay swells as it becomes wetter.
· Formation of ice lens in silts, fine sands and chalk during prolonged freezing conditions where the water table is high and the foundations relatively shallow. Unheated buildings, buildings under construction and cold stores are most at risk.
The remedy for subsidence, depending on the circumstances, will sometimes involve underpinning either with mass concrete or piles, where a suitable and stable bearing can be located at a lower depth. Where movement is a result of mining subsidence there is little that can be done apart from patching up the damage, although once the surface subsidence has occurred little continuing movement need be expected.
Subsidence and heave due to ground movement is normally covered by a household insurance policy subject to an excess.
RM Higgins, July 1994
‘Subsidence of low rise buildings’, 1984 Institution of Structural Engineers