We have been working on building restoration and repair projects for over a decade, as well as working with insurance companies on property damage claims – in that time, we’ve seen extensive subsidence and related problems with the interior decorations and drainage. One of the key tasks before repairing the subsidence is identifying why it happened and taking preventative measures to ensure that we don’t have to come out to make the same repairs again after a few years.
Subsidence happens for a number of reasons – some of these are to do with the environment, while others are due to improper building procedures. It can almost always be remedied if it is caught early enough.
If your home is built on clay soil, you may experience subsidence. Soil with high clay content tends to shrink and expand, depending on the amount of moisture available. When water evaporates or is extracted by trees and plants, the soil ‘shrinks’ which affects the stability of your foundations. The shrinkage can be temporary (just seasonal or until the next heavy rainfall) or permanent. If the water content is replaced, but there is more of it than when the house was original constructed, it can cause ‘heave’.
Wash out is another cause that is rooted in soil disturbance – the soil under the foundations of your house can be removed by groundwater. This can happen naturally, especially on granular soils, but can also be caused by broken or leaking drains, soak aways that have been built too close to the property, and poor ground water drainage around the property.
Shallow Foundations & Settlement
This is typically seen on house extensions, but can also be a culprit on some period properties that were built quickly. Shallow foundations on parts of the property (such as a kitchen extension) are more affected by seasonal moisture changes than the rest of the house, which means that it moves while the rest of the house stays still. This means that cracks will appear at the point of weakness, which is usually a door or window, or at the join between the two structures.
Settlement can also cause subsidence over time – soil takes some time to properly settle under the weight of a new structure, so there can be some movement over a number of years while the building settles into place. If there are different soil types, you can also see different rates of settlement which leads to ‘differential movement’.
Concentrated loading occurs when the structural load of the building exceeds the load bearing capacity of the soil – in short, the soil is too weak to properly support the building. Buildings have different load bearing concentrations – for example, Georgian townhouses have large, symmetrical and stacked windows, which increases the load bearing concentration under the walls between the windows. Once a building has settled, alterations and home improvements can alter the established load patterns and cause movements in the underlying soil – if these changes exceed the original foundation design, they will cause subsidence.
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This post was written by John Stylianou