Settlement, subsidence and underpinning
Firstly, definitions. There have been many discussions about the difference between settlement and subsidence. The best definition I have heard is that subsidence is when the ground shrinks away from the foundation, while settlement is when the foundation pushes the ground down. So, settlement is often a function of weakness somewhere, while subsidence is due to changes in ground conditions. This, it would seem, is a very important differentiation when it comes to buildings insurance.
When conversations take place about subsidence and underpinning it always seems strange to me that, often, nobody had taken the time and trouble to explain to the householder what subsidence is and why we underpin. So here goes.
Imagine a single tree standing alone in a field. We know that it takes water from the ground and the zone from which the water is grabbed can be likened to a fruit bowl under the ground, centred on the tree. The size of the fruit bowl depends on the species of tree and how susceptible the ground is to shrinkage and swelling. If the foundations of a property are within this fruit bowl they can – and we need to emphasise the word “can” – be susceptible to movement.
It can be seen that there is a relationship between the type of tree, type of soil, distance from the tree and the depth of the foundations. Victorian properties, with their shallow foundations are more likely to be in this zone. New properties have foundations whose depths are determined by charts based on soil etc etc. The problem is trees don’t read the same books as us and sometimes houses that are prime candidates for subsidence don’t move and houses that have nice deep foundations do.
The bottom of the foundations is known as the formation level and the purpose of underpinning is to lower the formation level so that it is outside the fruit bowl. This is done by digging a hole under the foundations deep enough to be outside the influence zone and filling it with concrete.
On balance, it is better not to have holes under the foundations to walls so the exercise has to be carried out one bit at a time and generally by hand. This is time consuming, labour intensive and expensive. In extreme cases the required depths are so great that mini-piles are installed by specialist companies. Either way, there is considerable disruption to the household.
The curious thing about all of this is that a property that has been underpinned can be difficult to insure or sell. This makes no sense engineering-wise. There was a problem, but the underpinning has fixed it. There is no longer a problem. Why, then, are such properties considered to be “prone” to movement and a higher risk? They are now less likely to move, therefore should be considered less of a risk.
If you are standing in a tree lined street full of palatial Victorian properties and the one on the left has been underpinned and the one and the right has not, an engineer would pick the one on the left every time because the problem has been solved. The one on the right is a problem waiting to happen.