The second in our series of NBS Shortcuts examines the effects of tree-root systems on building design. Foundation design needs to take account of the water-absorbing capacity of nearby trees, as well as potential groundswell caused by trees that have been removed. This guide highlights the dangers to structures that may be caused by the proximity of certain species of tree.
Most trees in the UK have a significant radial root system, extending one to one-and-a-half 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 per cent of the root system; this undermines the tree's ability to absorb water and also leaves it unstable in high winds. Therefore, when laying service pipes, it is advisable to tunnel under the root system if possible, running the pipes under the middle of the tree (on plan) if necessary.
In general, 80-90 per cent of all tree roots are found in the top 600mm of soil and almost 99 per cent 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 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, such as oak, pine and fir, have significant central tap roots - the main, central roots from which the others spread - and, in most instances, even these extend downwards by only about 2m. 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. With around £400 million worth of tree-related insurance claims made every year in the UK, it is worth considering what precautions are needed to build near them.
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, such as roots exerting pressure on underground services and cracking drains, is quite a rare occurrence. The most important consideration for designers should be the secondary effects caused by changing ground conditions near a tree, caused by varying degrees of moisture removal.
Architects should check ground samples and liaise with the local authority and/or National House-Building Council (NHBC) guidance to see if they have any specific requirements to deal with local conditions. As a rule, for construction near trees, clay soils are more problematic than porous/sandy soils because of 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 clay soils to shrink. Each such action exerts a significant pressure on the foundation, causing cracking and subsidence. It should be borne in mind that around 60 per cent of all of the UK's housing is built on shrinkable clay.
It is important to remember, though, that removing trees will also affect ground conditions. Having recently removed a 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 that would otherwise have been removed swells clay soil, causing pressure on trenches and slab foundations.
Similarly, deciduous trees have a seasonal impact on ground moisture content, with winter rainfall rehydrating soil that has dried over summer, a time when dormant root action takes less water out of the ground. Given that mature elms, oaks, horse chestnuts, planes and ashes can draw up to 50,000l 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 being pushed into humps as the soil expansion exerts an upward pressure on the oor slab. Similarly, trench foundations can crack, with consequent movement affecting the structure above.
ZONES OF INFLUENCE All trees have radial 'zones of inuence' on buildings, that diminish the further away from the tree the construction takes place. As a rule, it is recommended that properties be built at least a distance equivalent to the tree's height away from that tree. Attempts to insert a root barrier around a construction (eg. a polypropylene or similar geomembrane) to dissuade root growth near a foundation often only cause roots to grow under and around it. Root barriers are also difficult to install, frequently compromising existing root growth, with the consequences described earlier. However, while using root barriers to block an existing tree's root system will tend not to work, installing these barriers to control a new tree's growth is more likely to be successful.
So first of all make an assessment of the ground conditions: the soil bearing, porosity, voids, root spread, etc.
Secondly, BS 5837 suggests that an aboriculturalist should be called in to identify the various trees on site, making note of all the species and locations. In reality, many building-control bodies do not need an arboricultural report on simple sites with common trees in non- problematic conditions, so it is worth discussing this early on the process. Also, be aware that with 150 million urban trees in Britain, a vegetation survey must include reference to conditions beyond the site boundary. Just because you can't see it from your site doesn't mean that it isn't going to affect what you build there. Of the seven million street trees in London alone, some 1 per cent are reputed to have been found liable for damage to properties.
The depth of the building's foundation that will be necessary to cope with soil shrinkage will depend on the type and proximity of the tree cover on and around the construction site. Bear in mind that the trees on a site might not have fully matured and it is advisable to consider their fully matured height when planning foundations.
Implementing a policy to prune or lop trees to prevent them from growing to full height is not a workable, long-term strategy for controlling the root network. A crown reduction of 70 per cent by volume (approximately 35 per cent height reduction) reduces the water draw by only a small amount and only during the year of pruning. In subsequent years the soil moisture levels return to normal. This is partly because pruning encourages shoot growth and hence larger leaves, which then make similar demands on the water absorption capacity of the root system.
The removal of trees altogether can leave significant rehydration problems in the soil. Where trees have been recently removed, foundation depths should still be designed to suit the height of the tree (see diagrams on p45). However, if the removed tree had not reached half the mature height, then the actual height can be used with NHBC guidelines to assess the foundation depth - provided that its height can be stated with some certainty.
The table above provides some indicative safe distances at which a standard 900mm or 750mm deep trench foundation is adequate. Where trees give rise to the need for foundation depths of up to and over 2.4m (where crossed in the table), then engineered foundation designs are recommended. The distance between trees and foundation should be identified on the table together with information about the relevant height of the tree, and the high, medium or low shrinkage nature of the soils, to enable you to read off the foundation depth. Where two or more trees influence the foundation, design and specify to the deepest requirements.
Specific conditions of trees and their ground conditions might result in an engineered solution less onerous than the ones quoted in the table. Also, exact surveys ought to reveal problematic conditions such as porous soils overlaying clay soil, underground drainage, water table height and other compensating or exacerbating factors.
The table above, however, is simply a rule-of-thumb guide to foundation depths, with a built-in contingency for certain unforeseen circumstances. Research into the actual, rather than perceived, tree action, the soil 'reaction' and the existing site conditions can be undertaken to provide real evidence of the site conditions, rather than the simplified relationship between vegetation proximity and foundation depth drafted here.
Essential reading: National House Building Council (2006) 'Building Near Trees', Chapter 4.2; BS 5837 (2005) 'Trees in Relation to Construction. Recommendations.'For further references, see the online version of this article at www. ajplus. co. uk. For more information on NBS Shortcuts visit www. thebuildingregs. com