Part 1 discussed tree and soil characteristics. Part 2 will provide examples of the effects of trees on buildings with some guidance on defects and how to spot them. The subject of trees, soil and their influence on properties is complex with much engineering research having been done over the years and still continuing. Let’s look at soil first. The soils, which will not affect foundations within the vicinity of trees, are rocks (igneous) and sands (sedimentary). Rocks can be hard like granite or limestone or soft such as sandstone or chalk. Trees can split rocks but realistically in terms of the average house in UK, this is not a problem. Chalk is an exception because it is generally a soft rock but if a trench is dug in it and it rains, for example, the chalk will dissolve into a clay-like soil. So trees in any of the above soils will not normally affect foundations.

There are a range of soils however that will be affected by trees and hence foundations if the trees are near properties. These are clays. Sandy and chalk soils are permeable, which means the water can flow through unimpeded. Clays on the other hand are not and have a different molecular build-up. They are termed cohesive and are impermeable. A coarse soil is gravel or sand. A fine soil is silt or clay and will be less than 0.02mm in size. Puddle clay for example is still used in earth dam construction as an internal impermeable barrier. The photograph shows a trial pit in clay soil; note the different colours.

Clay is a complex material. Trees are simple. They are in constant demand for water and will seek it out anywhere through the soil. In a permeable soil like sand the roots grow easily and find water without trouble or water comes to them. In any soil they will seek out faulty drains and grow roots inside; plenty of water there. They open up the joints as the roots grow larger. The old stoneware drains are particularly prone to this defect; they were laid in short sections with therefore lots of joints. The newer plastic drains are much less so if laid in long lengths without joints around trees. So look out for blocked drains. Do a simple discharge test to check the flow. Blockage could be due to miscellaneous debris, earth fill due to damage or tree roots if a tree is nearby.

You will perhaps read out about Plasticity Index, which is a measure of a soil’s shrinkable characteristics. Shrinkability describes a clay soils ability to change its volume depending on its moisture level. It is influenced by the clay content (100% clay or a sandy clay), the fineness of the clay soil and its potential for absorbing water. Shrinkable soils are mainly in the East and South East of England but do occur elsewhere. They do not make good foundation bases.

A crack inspection of a property is one of the most difficult types of inspections to perform. However in this note we are only concerned with the effect a tree might have on the structure. Trees will generally affect houses if the foundations are too shallow, the tree is large and close by and generally if the soil is clay. Visible manifestations of tree effects on buildings will generally be corner diagonal cracking. The crack will be wider at the top than the bottom, although even this is not always true. Basically they are growing underneath the foundations and extracting the water.

So what happens is that the weaker zones and shorter stress paths, usually the external corners and bay windows for examples, show signs first. The effects of trees on basements and cellars are variable but sometimes if the walls are not thick enough, the roots will push the walls in or grow through them (less usual). Internally as shown in the 10mm wide crack photograph of a bay window attached, the crack will be quite pronounced.

On one project, a wood near a house had many oak trees. The combined effect of their water demand caused severe cracking externally and internally (see red dashed line in photograph). The other attached photograph shows the highly shrinkable clay soil in the garden with tension cracks.

The crack in the bay window of the SE London property shown in the photographs was due to soil and trees and was widening at 1.5mm per month. It was serious and the bay window required urgent underpinning. The subject of trees and their effects is very complex. Professional engineering advice should always be sought. This particular Note has only touched on this subject. But it is hoped that this has alerted the reader somewhat.

James Dale
Consulting Civil/Structural Engineer
www.dale-consulting.co.uk
10 July 2008