WHAT’S THE DIFFERENCE BETWEEN SUBSIDENCE AND SETTLEMENT?
SETTLEMENT – is caused by the weight of a new building/structure or part of it. Buildings are heavy things and, as their weight is taken up by the ground, a little movement caused by this adjustment sometimes occurs as the ground consolidates under the new load- this is settlement. It usually occurs early in the life of a building and rarely recurs, although, there are exceptions, for example, in soft clay soils. Settlement rarely causes problems, although differential settlement (differing degrees of settlement between connected parts of the same structure) can cause damage.
SUBSIDENCE – results from external factors which cause the disruption, displacement, contraction or distortion of the ground under or around a building. Some of the more common causes include:
- TREES – trees extract moisture from the ground which then contracts, particularly in shrinkable clay soils, causing buildings above to move (subside).
- DRAINS – leaking drains can wash away or erode the adjacent ground which then partially collapses reducing the lateral (sideways) strength of the ground. The support provided by this ground will then be reduced causing any building above to move (subside).
- MINING – mining removes part of the ground below the surface (leaving shafts and tunnels) and which reduces its ability to support the ground above. If these shafts or tunnels collapse, the ground above drops as well, causing damage to any buildings.
- BIODEGRADATION – domestic refuse disposed of in the ground usually breaks down and consolidates at a steady rate. However, the apparent soundness of the ground can be misjudged and further decay/compaction occur resulting in movement (subsidence) of the ground and any buildings above.
- COLLAPSE – the collapse of a sewer, for example, can cause the ground/buildings above to drop (subside).
HOW CAN I TELL IF MY HOUSE IS SUBSIDING?
There could be movement in the ground beneath your home if you find:
- * New or expanding cracks in plasterwork
- * New or expanding cracks in outside brickwork or rendering
- * Sticking doors or windows
- * Rippling wallpaper with no other apparent cause e.g. damp
HOW LONG WILL IT TAKE TO SORT OUT?
First of all, it is vital to establish that there is, in fact, a significant problem and then to correctly identify its cause. Only then can the correct way of solving the problem be specified. Establishing whether or not there is a problem and, if so, its potential extent, can take some time. This is because subsidence usually takes place very slowly and the affected property will need to be carefully monitored over a period of time, often between 6 months and two years. There is rarely any cause for real concern, however, unless cracks appear suddenly and are more than 3mm wide.
WHAT ARE THE SOLUTIONS
This depends on the cause of the problem.
- NOTHING – Sometimes, the subsidence happens and then stops, with further significant movement being unlikely. In these circumstances, when little damage has been caused, it is quite often unnecessary to do anything, apart from fill cracks and renew decorations.
- UNDERPINNING – Underpinning or strengthening/supplementing the foundations may be necessary to prevent further movement. Although it is a costly and disruptive process, it is estimated that only 20-30% of properties suffering from subsidence need underpinning.
- TREE PRUNING/REMOVAL – Tree roots sucking water from the ground is one of the most common causes of subsidence. However, great care must be taken, as the removal of a tree can cause even more severe problems. This is because the ground then has to absorb the water which the tree was taking and which usually results in ‘heave’ or the expansion of the ground causing lifting. Simply pruning a tree, on the other hand, reduces the amount of water that the tree takes up and, therefore, the extent of the problem that the water extraction is causing.
- REPAIR/RENEWAL OF PIPEWORK – Where the soil beneath or adjacent to a property is being washed away because of leaking or broken drains or water pipes, the repair or renewal of defective pipework is often enough to stabilise the situation without the need for expensive underpinning.