Desiccation of Clay and Subsidence

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Posted by Robert Hopps | July 11th, 2018

What is Desiccation?

In a previous blog we looked at subsidence and heave of the soil however in this blog we will look in more detail at desiccation of shrinkable clay and other sub soils and what this could mean. It certainly seems an appropriate time to do so given the almost record dry period which the country has seen recently.

As we have seen, clay soil is often referred to as an expansive soil in that the minerals within the clay will expand when in contact with high levels of moisture and contract as that moisture leaves the soil such as during dry periods of the year. This can be further aggravated by the presence of large trees which will draw up moisture in the soil and so during dry periods with large trees nearby the soil can become excessively dry and desiccated and as a result will shrink dramatically sometimes causing the cracked and baked look we have all seen.

In the below image you can see a typical Victorian foundation which is normally a simple spreading of the bricks into the soil to spread the load. The foundations will rarely be very deep and often are 300 to 400 mm sometimes with some concrete below although this only tends to be 150 to 300 mm more.

Where these foundations are shallow they are within a layer of soil which can be prone to desiccation. Modern guidelines suggest that desiccation will occur up to a depth of not more than 1 m and so most modern foundations are at least 1 m in depth to avoid any movement.

Clearly Victorian foundations and other older property will be within a layer of soil which will be prone to desiccation and shrinkage and put simply where the soil shrinks the foundation will have nothing to sit upon and will simply follow the shrinking soil downwards. This is a simple subsidence mechanism and often translates into notable cracking in the walls above which is wider at the bottom and tapers as it rises.

As mentioned the desiccation can be made much worse where there are trees nearby extracting more moisture from the soil and sometimes after a long period of drought the rehydration of the soil will cause it to move upwards in a process known as heave which can also cause further damage to the already cracked wall or house above.

If you are concerned that this is occurring then feel free to give us a call to discuss the matter further.

The Hopps Partnership
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Wanstead, London E11 2RJ

The Hopps Partnership
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London EC1M 6EJ

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