Written by Peter French.
First of all, let me introduce myself; I am a chartered engineer and the managing director of Canham Consulting, a local firm of civil and structural engineers as well as surveyors. We are now being told we are in a drought, with only around 63pc of the average winter rainfall so far.
Concerns have been raised with regard to future water supply, agriculture and wildlife, but are there issues for property? Well there might be if your house is in an area with clays soils. Unlike sands and gravels, clay experiences volume change with variation in moisture; swelling as water content rises and shrinks as it dries out. The potential is that during periods of drought the depth and area to which the seasonal drying out occurs within the clay increases. If the area affected is below the house foundations then clay shrinkage subsidence can occur.
This can result in significant cracking and distortions depending on the nature of the clay and extent of the moisture deficiency. Should you be worried? The severest drought in recent time was 1976. Since then the depth of foundations to new properties founding in clay have been designed dependant on the clay type and the proximity of vegetation at the time of design, to allow for seasonal moisture variation. Vegetation such as trees and shrubs will have increased in size since the last dry spell or since the foundations where designed.
As trees mature the water demand increases as does the area and depth from which they extract moisture. What typically happens with a new property is that it is landscaped upon completion and 15-20 years later trees and shrubs begin to dry out soils beside and beneath the foundations. The increased water demand causes the clay to shrink. How do you confirm the cracks are caused by clay shrinkage? Often the cracks will appear in late summer or early autumn and increase in size reasonably quickly. Crack widths vary greatly, but generally they will taper. The crack widths will then be seen to reduce during the following winter when soils begin to rehydrate. Clay shrinkage subsidence is normally covered by buildings insurance. When damage occurs insurers should be notified.
You may also wish for an independent view on the cause and remedy. Resolution may come in the form of a reduction in the vegetation, for example, removal of trees. This should be followed by a period of monitoring to confirm stability has returned prior to repairs. In my view monitoring should extend over a 12 month period to allow the soils to stabilize. On some occasions underpinning maybe required.
Peter French can be contacted on 01603 430650 or e-mail firstname.lastname@example.org