SUBSIDENCE

In engineering & surveying, subsidence is the motion of a surface (usually, the Earth's surface) as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation. There are several types of causes for subsidence:

Seasonal effects: Many soils contain significant proportions of clay which because of the very small particle size are affected by changes in soil moisture content. Seasonal drying of the soil results in a reduction in soil volume and a lowering of the soil surface. If building foundations are above the level to which the seasonal drying reaches they will move and this can result in damage to the building in the form of tapering cracks. Trees and other vegetation can have a significant local effect on seasonal drying of soils. Over a number of years a cumulative drying occurs as the tree grows, this can lead to the opposite of subsidence, known as heave or swelling of the soil, when the tree declines or is felled. As the cumulative moisture deficit is reversed, over a period which can last as many as 25 years, the surface level around the tree will rise and expand laterally. This is often more damaging to buildings unless the foundations have been strengthened or designed to cope with the effect.

Damage to subsoil's: Mainly caused by increased levels of water. 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.

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.

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

How to make an initial assessment of your own house.

If you suspect that your house has suffered or is suffering from ground movement then pay particular attention to the following points:

• Are any of the doors or windows in the house sticking or jamming? If you ease them by planning off part of their edge, do they jam again?
• Is there an area of wallpaper in the house that is rippling or torn where it was previously flat?
• Are there signs of cracking to the internal plastering that are either fresh movements or enlargements of existing cracks? Examine the external brickwork.
• Are there any signs of fresh cracking, or perhaps old cracks that may have opened recently? If there are any signs of clean brick in any part of the crack then the movement may be recent.
• Have you any reason to believe that the drains are not working properly or a water main is leaking? Does the external gulley take far longer to drain than previously? Have you undertaken a drain survey and found cracks or dislocations in them?

Repairs to Structural Damage.

Where an element of a building has been damaged by movement in the ground the first requirement is that the stability of the structure must be reinstated. Where brickwork has cracked it is essential that the strength is restored. Where damage is severe this can sometimes only be achieved by demolishing the brickwork and rebuilding it again. In less serious cases the brickwork can be repaired by injecting resins into the cracks and effectively gluing the brickwork back together again. When carried out correctly it can produce joints that are stronger than the bricks themselves.

Any part of a building may suffer damage due to ground movement. To all external elevations the damage is similar i.e. cracks in the brickwork. However, the function of the affected parts has a significant effect on the type of repair that may be required. Important load bearing elements require a structural repair whilst lesser elements might receive only cosmetic and weatherproof repairs. There are a number of proprietary repairs to brickwork and concrete that are presently available. These usually involve the introduction of steel reinforcement to the masonry and locking it in place using either resins or cement-based materials. Many of these processes are more effective than the earlier types of repair, and at HB Jones we are able to give a sound, insurance-backed guarantee to our work.