Foundation design needs to take account of the water absorbing capacity of nearby trees as well as the potential groundswell caused by trees that have been removed. In this exclusive extract from Shortcuts: Book 1 by Austin Williams, we highlight the dangers to the structure that may be caused by the proximity of certain species. 

Radial roots

Most trees in the UK have a significant radial root system, sometimes extending out to a distance of 1 to 1.5 times the height of the tree. Severing just one of a tree's major roots during careless excavation for construction or services can cause the loss of up to 20% of the root system; undermining its water-absorption and also leaving it unstable in high winds. Therefore, when laying service pipes, if it is possible to tunnel under the root system then it is advisable to do so - running the pipes under the middle of the tree (on plan) if necessary.

In general, 80-90% of all tree roots are to be found in the top 600 mm of soil; and almost 99% of the tree's total root length occurs within the topmost 1m of soil (with some variations depending on soil porosity). The undoubted nuisance that fine root systems create for the development of specific sites has to be weighed up against the importance that they play in soil stabilisation on sloping ground (acting in a similar way to geotextile matting).

Only a few mature species, like oak, pine and fir have significant central tap roots (those main central roots from which the others spread) and, in most instances, even these extend downwards by only about 2 metres. So it is the radial tree roots that extend outwards that are of primary concern here; these can influence soil conditions well beyond the circumference of the tree's leaf canopy. But with around £400 million-worth of tree related insurance claims per annum in the UK, what precautions are needed to build near them?

Precautions when building near trees

It is always recommended that construction takes place as far away from trees and established vegetation as possible because the distance at which tree roots can detrimentally affect a building is quite significant. Damage directly caused by proximate tree roots (e.g. roots exerting pressure on underground services and cracking drains) is quite a rare occurrence. It is the secondary effects caused by the changing ground conditions near a tree - caused by varying degrees of moisture removal - that should be the most important consideration for designers.

Check ground samples and liaise with the local authority and/or NHBC guidance to see whether they have any specific requirements to deal with local conditions. But as a rule, for construction near trees, clay soils are more problematic than porous/sandy soils due to their increased water retention and their potential to swell in heavy rain. Conversely, healthy trees take large amounts of water out of the soil, often forcing the clay soils to shrink. Each action exerts a significant pressure on the foundation, causing cracking and subsidence. It should be borne in mind that around 60 percent of all UK's housing is currently built on shrinkable clay.

It is important to remember though that removing trees will also affect ground conditions. A recently removed tree means that the moisture that would otherwise have been absorbed diurnally from the ground will remain, allowing the soil to swell and heave (the condition whereby water which would otherwise have been removed swells the clay soil causing pressures on trenches and slab foundations).

Similarly, deciduous trees have a seasonal impact on the ground moisture content, whereby winter rainfall rehydrates the dry summer soil, with less of it being taken out of the ground by the dormant root action. Given that mature elms, oaks, horse chestnuts, planes and ashes can draw up to 50,000 litres of water a year from the surrounding soil, the consequent soil water retention, or frost, can lead to significant heave. Worst case examples have resulted in concrete slabs 'humping' as the soil expansion exerts an upward pressure on the floor slab. Similarly, trench foundations can crack with consequent movement affecting the structure above.

Shortcuts provide an eclectic mix of at-a-glance guides to the minefield of regulations, new materials, and technologies that confront building designers today. Comprising hand-drawn sketches, technical drawings, and punchy articles, each Shortcut takes you right to the heart of a key topic in architecture and construction, presenting a wealth of invaluable information in an accessible and lively way.

About this article

Shortcuts: Structure and Fabric externallinkby Austin Williams focuses on issues to do with the structure and fabric of buildings. Grouped into four main subject areas, the Shortcuts range from rainscreen cladding to fire protection, from lofts to lifts, and from LEDs to SUDS.

To order a copy of this book, please visit RIBA Bookshops externallink.

Copyright RIBA Publishing March 2009.

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