4.2.4 The effects of trees on shrinkable soils
Foundations shall be designed to make allowance for the effect of trees, hedgerows and shrubs on shrinkable soils. Items to be taken into account include:
- soil classification, shrinkage and heave
- water demand, tree heights and zone of influence of trees
Soil classification, shrinkage and heave#
Shrinkable soils, that are widely distributed throughout the UK, often change volume as moisture content fluctuates seasonally and as a result of factors, including the action of tree roots. The resulting shrinkage or swelling can cause subsidence or heave damage to foundations, the structures they support and services.
Table 1: Modified Plasticity Index related to volume change potential
|Modified Plasticity Index
|Volume change potential
|40% and greater
|20% to less than 40%
|10% to less than 20%
Alternatively, the Plasticity Index may be used without modification. For pure clays and other soils with 100% of particles less than 425μm, the result will be the same. However, for mixed soils such as glacial tills, use of the Modified Plasticity Index may result in a more economic design.
The volume change potential should be established from site investigation and reliable local knowledge of the geology. Sufficient samples should be taken to provide confidence that the results are representative. High volume change potential should be assumed if the volume change potential is unknown.
Water demand, tree heights and lateral zone of tree influence#
Water demand varies according to tree species and size. Water demand categories of common tree species are given in the table below.
Where the species of a tree has not been identified, high water demand should be assumed.
Where the species of a tree has been identified but is not listed, the assumptions about water demand as listed in Table 2 may be made for broad-leafed trees:
Table 2: Water demand of broad-leaf trees by species
|All elms, eucalyptus, hawthorn, oaks, poplars and willows
|High water demand
|Moderate water demand
Table 3 shows the water demand categories and the average mature heights to which healthy trees of the species may be expected to grow in favourable ground and environmental conditions. This information:
- should be used for trees that are to remain or are scheduled to be planted
- may be used even when actual heights are greater.
Table 3: Water demand of tree species in relation to their height
Tree identification can be assisted by reference to a tree recognition book. Information may be obtained from suitable alternative authoritative sources for trees not listed in this chapter.
When the species is known but the subspecies is not, the greatest height listed for the species should be assumed.
Where hedgerows contain trees, their effect should be assessed separately and the height of the species likely to have the greatest effect should be used.
Table 3a provides guidance to be used in assessing the effects of trees and their water demand when they are subjected to heavy crown reduction or removal. For trees which have been or are to be removed, allowance should be made for the fact that the water demand of a tree varies with its size and rate of growth.
Table 3b, on the other hand gives the lateral extent of the zone of influence of trees. So any foundation located within these zones will be influenced by the tree and should be designed to meet the requirements of this chapter.
Table 3a: Guidance for factors affecting the mature height and water demand of trees
Table 3b: Zone of influence (lateral extent) of trees
|Zone of influence
|1.25 x mature height
|0.75 x mature height
|0.5 x mature height
High rainfall reduces moisture deficits caused by trees and hedgerows, while cool, damp weather reduces the rate of water loss from trees thus reducing the risk of soil movement.
The driest and hottest areas in the UK generally exist in southeast England; therefore, the greatest risk occurs in that area and diminishes with distance north and west. A 50mm decrease can be made to the foundation depth determined in accordance with this chapter for every 50 miles distance north and west of London. Where it is unclear which zone applies, the lower reduction value should be used.
Last updated: 2nd January 2024