Subsidence – Part 1
This is the term given when part of a property has lost the support of the ground on which it is built.
!f a house is built on ground with different quality soil the softer area will compress more than the firmer material, this will cause the foundation to crack and the walls above will drop.
When constructing a new home or extension the Building inspector needs to inspect the ground prior to pouring the foundations to ensure that the quality of the ground is consistent. If an area of the excavation is soft, the soft material will need to be excavated until better bearing soil has been reached.
In new build properties, a defect can become apparent within a few months, particularly ground movement. With established houses (over 10yrs) if subsidence occurs it will be because of recent changes in the soil below, in this case, we need to ask what has changed such that subsidence has occurred.
Normally houses will be constructed off a foundation, as mentioned above and the excavation should have reached a consistent layer of soil usually 900-1200mm deep.
In the UK we tend to hit clay at this depth unless of course, it’s a reclaimed site where other methods of foundation are employed. Clay or boulder clay is hygroscopic (it absorbs water) but uniformly, so in summer it dries out slightly and in winter absorbs moisture, but this uniform movement under the continuous concrete foundation has no adverse effect.
So, how does a house that is established become affected?
Well it usually comes down to changes in the moisture context of the ground which is localised.
Part 2 next week looks at some of the ways the moisture content of the ground can change…
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