A. The most common causes of subsidence are as follows:
1. Settlement of old mine workings, often at great depth, resulting in damage to structures on the surface. These may often be a long way from the cause of the damage. Access shafts are notoriously difficult to identify as mine operators rarely kept good records of these points.
2. Damage to subsoils caused by water flowing through them. This may be from naturally occurring ground water or leaking drains and water mains. When a drain begins to leak it can soften or wash away the body of a soil and create weakness. This can result in the weight of any nearby structures causing the soil to crush, allowing the building to subside.
3. Long term consolidation of fill.
4. Decomposition and degradation of soils that contain a large organic content. Peat soils are made up of vegetable matter laid down by historic forests. When these materials are below the water table in the ground they remain relatively stable for hundreds of years. If, for any reason, the water table should become lower then the soils will be prone to drying-out or biodegrading - either of which will result in a reduction in its volume. This will cause any buildings supported by the soils to settle and crack.
5. Instability of uneven ground. When soils rest at an angle to the horizontal they can sometimes become unstable and begin to slip. This is typical of coastal cliffs and most people are aware of the type of dramatic collapses that appear on the news from time to time. In a lesser way this can happen on naturally occurring slopes and in areas where a man-made excavation has created large differences in ground levels. This may be a large excavation for a tall building or open earthworks for a major road scheme. This type of movement is both difficult and expensive to deal with.
6. Shrinkage of clay soils. Clay is a natural soil created in historic rivers by the deposition of tiny rock particles. In Great Britain and Northern Europe clays occur with an amount of water dispersed within their matrix.. Being highly impermeable to the passage of water, clays will resist the tendency to dry out and will normally maintain a consistent moisture content over many decades. If a clay dries out it can shrink. Upon rewetting it expands. This results in seasonal shrinkage and swelling which can cause cracking of nearby buildings. Tree roots spread out in search of water. Clay soils are a ready source of water that a tree can remove by suction. When trees grow in shrinkable clay soils they locally dry out the soils and can often cause subsidence if they affect the soils beneath the footings. Alternatively, if a well established tree should die then the ground in which its roots are dispersed will no longer be subject to the long term drying-out effect and as a result will begin to take up moisture from surrounding soils and from the atmosphere. This in turn will result in swelling of the soil and this is known as heave. In many respects, heave is capable of producing greater damage to a building than its counterpart, subsidence.