Foundation depths


Foundation depths in shrinkable clay soils

(NHBC Chapter 4.2)


Foundation depths in shrinkable clay soils

If new foundations are to be constructed on a shrinkable clay soil, then the depth of the foundations needs to be considered.  If there are any trees close to the building, then deeper foundations will be needed. However, if the soil is a granular soil, such as sand or gravel, then the foundation depth is not affected by trees.

Shrinkable clay soils change in volume as their moisture content varies. Where there are trees present, the roots from the trees can take out enough water to cause building subsidence. In fact, this is the most common reason for foundation subsidence in the UK.

In broad terms if a tree is closer to a building that its mature height, then deeper foundations will be required.  The the foundations will need to be taken down below the zone of influence of the tree roots.

Unless existing trees are to be removed at the time the building or extension is being built, then the foundation depth should also be designed for the mature height of the trees.

Often this issue only raises its head when the work starts on site and 9 times out of 10 nobody has noticed the small 2m high Leylandii tree in the neighbour’s garden right next to the building. The best answer is to get you neighbour to remove the tree before you start work, but if they won’t, the foundations will need to be designed to allow for the tree to mature.

Our designs minimise the foundation depth wherever possible, by using stepped foundation depths as the distance from the trees increases.

We need the following information to design the foundation depths in a shrinkable clay soil

a site plan showing the trees which are closer to the building than their mature height

details of the types of tree

the distances of each type of tree from the building

the approximate location of the site

the type of soil (low, medium or high shrinkage potential), your local Building Inspector will give you guidance with this.

Design information

All designs are carried out in accordance with BS 8110, NHBC Chapter 4.2 and BS 8004:1986

The calculations provided will be suitable for Building Regulation approval.

You should satisfy yourself that the excavation for the foundations will not undermine any adjacent foundations or neighbouring ground

Excavations may fall under the Party Wall Act Please seek specialist advice