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Trees, foundations and moving the earth

The NHBC is currently highlighting the need to consider trees and ground conditions when planning and designing the foundations for buildings.

The NHBC identified that the majority of claims related to foundations being affected by trees and that foundations had not been adequately constructed to be able to cope with the effect of the trees; either lacking protective measures or not constructed in accordance with the design.

Trees and shrubs affect the moisture content of cohesive (such as clay) soil.  Plant uptake of water increases during hot/dry weather causing the clay to shrink. In colder/wetter conditions, plant uptake decreases, resulting in swelling of the clay and this movement can affect foundations.

The problem can stem from three areas:

  • Existing trees and shrubs on site, causing the movement of the soil.
  • Tree and shrubs that have been removed can result in swelling of the clay to its natural state over an extended period of time, creating a “heave” effect.
  • Planting new trees and shrubs can create new problems.

Complexity is added through the differences in soil types and tree species, all of which affect the requirements for foundation design. Fortunately, the NHBC has well established guidance that specialists can use to determine the required foundation design.

There are two key steps in avoiding the problems posed by trees on foundations:

  1. Undertake a ground investigation to discover if trees and shrubs may present a risk to foundations.
  2. Undertake a tree survey so that the type of tree and distance to the development can be accurately mapped. This may also highlight measures required to protect the tree from damage.

agb Environmental has extensive experience of undertaking both ground investigations for foundation design, including using the NHBC guidance, as well as tree surveys. Our specialists work together to ensure our clients are provided with accurate and reliable information.

For more information on ground investigations for foundation design, or to discuss a tree survey, contact Alex Brearley at agb Environmental on 01638 663226 or email alex@agbenvironmental.co.uk.

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