Fast-growing trees near buildings can cause uneven settlement when active tree roots dry out the soil and cause differential soil shrinkage. Shrinking clay affects the bearing capacity and leads to movement in the building, especially in the case of shallow foundations.
Tree roots can extend a considerable distance and can draw moisture from as deep as 6 m below the surface. It is therefore necessary to make an accurate survey of their position and obtain details of the type of tree, at the same time establishing that the tree is the cause of the damage (see Fig. 1).
Poplars and elms with fast-growing root systems are expected to cause severe seasonal changes.
One way to prevent root problems in tall trees is to use a ‘safe distance’ between the tree and the building. Some types of trees are likely to cause more problems than others. Table-1 shows the different types of trees known to have caused damage, listed in descending order of threat. It also shows their expected maximum height on clay soils. Planting a tree near a new or existing building usually carries some risk of damage. It is therefore proposed to follow the recommendations described in Table 1.
Buildings can also be damaged when established trees are removed. The resulting pressures from the removal of trees and shrubs act both vertically and horizontally. In most cases it is the horizontal movement that causes the most damage, especially in the top clay layer. In such cases, there is a danger that the clay will expand over a period of years as it reabsorbs moisture, causing the foundation’sswell‘.
Where window sills crack and rise in the center is an indication of soil build-up.
Differential movements will occur causing cracks in walls and partitions. In such cases, removing a tree can do more harm than good.