Causes of Subsidence and Foundation Failure


The most common causes of subsidence are as follows:


Trees

Trees can be damaging on buildings, due to their effect in taking up moisture within the ground. This is particularly problematic in clay soils, the effect of which can cause the downward movement of foundations resulting in Subsidence. Problems can also occur, where a mature tree has been removed, as the sudden increase in moisture levels can cause a process known as Ground Heave.

Tree roots can also be problematic, their growth and size can displace surrounding soil, leading to damage to foundations and ground movements.


Frost Heave

This is a problem that can occur where there is a high ground water table, or after a period of heavy rainfall in cold water. Water expands as it turns to ice, causing soil displacement, the effect of which is a term known as Frost Heave.


Sub soils containing organic matter

The building may lie on soils which contain a substantial amount of organic material; this may be topsoil that has not been removed to its full depth, or a soil type such as ‘Peat’. As a consequence, any applied loads will cause substantial compression, leading to settlement and potential fracture of the foundations.


Shrinkable Clay soils

Shrinkable clay soils are capable of absorbing and releasing large amounts of water. As a result, this exacerbates normal climatic changes and seasonal variations, which can during particularly dry or wet periods cause the downward movement of the foundation, ‘sagging’ or upward heaving, ‘hogging’.


Drainage leaks

A very common cause of ground movements and Subsidence are drainage leaks. Drains may have been incorrectly laid too close to the building, at shallow depths or there may be damage caused by tree roots. These broken drains contribute to Ground Movements and Building Subsidence by washing away sandy sub soils, or may increase water content of a shrinkable clay soil.


Builder mistakes

There are a number of common on site mistakes, which if not picked up by the Building Control office can lead to inadequate and unsafe foundations. The foundation may not be aligned with the walls correctly; there may be an incorrect concrete mix specified, or the contractor could have used the incorrect aggregate, such as brick, which can cause and lead to Sulphate Attack, as explored below.


Sulphate attack

Sulphate attack tends to occur in favourable conditions where there is a high water table and a sub soil, (usually clay) that includes calcium, magnesium and/or sodium sulphate. These sulphates react with the cement based materials, causing expansion and consequential deterioration of the concrete foundations.


Landfill

Building on Landfill is the largest single course of failure in new build housing. By their very nature landfill sites are unstable and prone to settlement, as such appropriate foundation design needs to be considered, such as pile or raft foundations to avoid the obvious consequential settlement.


Mining related problems

Mining was undertaken extensively for coal, lead, copper and stone, many of which are known and identifiable on plans and some which are not. Areas above mines may be prone to sudden and catastrophic collapse. This may be due to the breakdown of pillars left to support the ground above the mine or the collapse of timber covers on mine shafts.