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What is subsidence?

Subsidence on buildings is where the foundation/footing supporting it is not substantial, and therefore cannot support the weight of the building and begins to sink, as this happens, cracks appear on walls as the ground underneath moves and walls can become very unstable. The reason this happens is to do with the shrinking and expansion of ground as it goes through stages of wet and dry throughout the years. When the ground is dried out, (especially in clay type areas), shrinking occurs. Therefore when the ground becomes moist again it expands, creating a 'push and pull' effect.

How can subsidence be prevented / halted?

Subsidence can be stopped by excavating the ground underneath to a specified depth and width, (calculated by a qualified structural engineer). There are also steel reinforcing bars that go either all the way through the new foundation, or between designated bays of concrete usually about a metre wide.

Are there different types of underpinning?

Yes, there are two different common methods used to address most subsidence situations, Mass concrete underpinning and Beam Base underpinning. Buildpin have a proven track record in specialising in the delivery of both of these and other solutions.

Mass Concrete Underpinning

This includes a calculated sequence of bays, usually around a meter wide each, with a depth and width all calculated by a structural engineer. These bays are dug out individually and concreted in the same day, (the bays are staggered so that the building remains structurally sound at all times). Between these bays, steel reinforcing bars are placed to tie them together to act as one solid beam when completed.

Beam and Base Underpinning

The other method that we also take on is the beam and base type. In this situation when preferred by the structural engineer, the whole area that is affected by the subsidence is excavated, again to the calculated specifications. During this process the underneath of the building is supported using special equipment. After this a level concrete base is formed a certain depth from the underneath of the existing footing along the whole area. There is then a steel, ring beam structure that is placed on top of the base and poured as one solid beam of concrete.

After either of the two methods described above have been completed, they have been left down roughly about 75mm from the existing footing, this is because as concrete cures, it also slightly shrinks. The concrete is left 48 hours to totally cure and thereafter the gap is filled with a dry-pack compound, in which there is an additive that doesn't allow the compound to shrink, giving you a totally stable footing.

What could happen if I do not get my scenario dealt with?

Your building can become very unstable, and potentially very dangerous to you and others around you.

Can trees contribute towards the effects of subsidence?

Yes, trees when taking in water, can suck the ground dry if there has not been a lot of rainfall for a while, again creating the 'push and pull' effect described above. Trees that demand high amounts of water are the worst as the process can happen a lot quicker.

Can Leaking Drains Cause Problems?

Yes, damaged drains can also contribute towards subsidence. A lot of subsidence problems area a result of drains leaking into the ground causing soil erosion beneath a building's foundations.

Will my home insurance cover for subsidence?

Most home insurance policies include subsidence insurance, either as standard or an added extra – check your policy's small print for details.

Most insurance companies have further details regarding subsidence such as this useful document from Direct Line.