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Beware of the trees Our latest geotechnical1 article looks at building near trees, suggesting how recent failures could have been avoided

technical & practice

In the uk the shrinkage and swelling of clay soils, particularly when influenced by trees, is the single most common cause of foundation movements which damage domestic buildings.

Currently, the value of insurance claims for subsidence and heave damage to domestic properties amounts to almost £400 million per annum. While the problem primarily affects the older existing building stock, it is also a potential problem for new construction with implications for design, detailing and construction quality control to prevent it becoming a real problem. A fuller explanation is given in 'bre Digest 298'2 which has just been revised.

The influence of trees

Trees can damage buildings and building services by direct or indirect action. Direct action includes the growth of roots or the trunk lifting or distorting structures or services. Damage by direct action is relatively uncommon.

Guidance on safe distances from trees to avoid direct damage is given in bs 5837: 19913. Indirect action means, in particular, the problems associated with clay subsoil.

Trees can cause clay soils to shrink by drawing water through their roots, predominantly during spring and summer. This shrinkage results in both vertical and horizontal ground movements that, if transmitted to a building's foundations, can cause damage to the building structure.

The amount of shrinkage depends on the type of clay soil, the type and size of tree, and on climate. Long dry periods result in the greatest movements particularly when, as was the case in 1989/90, two dry summers are separated by a relatively dry winter. Trees growing under grass cover are forced to compete for their water and to extract water from greater depths than they might otherwise do.

The figure illustrates how the water content of a high volume-change- potential clay soil might vary with depth remote from and near to a large tree (from 'bre Digest 240'4). The left-hand drawing shows water-content profiles remote from the tree in open grass-covered ground, towards the ends of both summer and winter. Near the ground surface there can be relatively large changes in soil water content between summer and winter as a result of evaporation from the ground surface and transpiration by the grass. Such variations are normally confined to the top 1-1.5m of the ground, possibly less adjacent to buildings.

If there were a mature tree growing at the same location, then the water- content profiles might look more like those shown on the right-hand side of the figure. In this case the seasonal fluctuations in soil water content are both larger and extend to a greater depth. Soil volume changes and hence ground movements will be greater.

In addition, a persistent water deficit may develop as the tree grows - the toned area in the right of the figure. That is, the ground is unable to rehydrate completely over the winter months. The establishment of a persistent moisture deficit results in shrinkage of the soil and consequent subsidence of the ground.

Where a persistent deficit has been established, tree removal will result in rehydration of the ground. Water contents at depth will return to values close to their original ones, accompanied by soil swelling and ground heave.

In summary, it is possible to identify four types of ground or foundation movement associated with clay soils and vegetation:

normal seasonal movements, associated with evapo-transpiration from, say, a grass-covered surface

enhanced seasonal movements, associated with trees

long-term subsidence, as a persistent water deficit develops

long-term heave, as a persistent water deficit dissipates.

Of these, the first rarely gives rise to problems under normal circumstances. It is the others that can result in more intractable problems.

Different tree types cause problems of different extents, depending on how much water they extract from the soil mass and where they extract it from in relation to founding level.

The potential magnitude of a tree species' influence on the ground is often referred to as its 'water demand'. Trees like birches and holly tend to have low water demand whereas poplars, willows and oaks have high water demand. Tables indicating the mature tree height and the degree of water demand for many tree species encountered in the uk are given by nhbc5 and Zurich Municipal6.

Trees close to new foundations

The movements described above can result in damage to existing buildings and to problems of design, detailing and construction quality control for new buildings.

The investigation of damage to existing buildings is not considered in this article, but the topic is covered in 'bre Good Repair Guide 2'7.

For new construction, the basic principle is to take foundations to a depth where they are not significantly influenced by vegetation. bre has long advocated the use of pile-and-beam foundations for low-rise buildings on heavy clay sites near major vegetation. Concrete piles should be reinforced to well below the zone of soil where heave is occurring. However, many builders are unwilling to install such a pile-and-beam system, preferring to use a deep trench fill foundation instead.

The nhbc has produced widely-used tables which give different foundation depths according to the soil type, the foundation's proximity to trees and the tree's water demand. Atkinson8 and 'bre Digest 241'4 show typical foundation details used on sites where trees are a problem. As discussed in the recent revision to 'bre Digest 298'2, this approach can present problems, especially in swelling ground. Apart from construction disadvantages such as the large quantities of spoil to be removed, the risk of trench collapse and, of course, the cost of the large quantities of concrete required, there are also two technical reasons why deep trench-fill foundations are undesirable in swelling ground:

they may be subject to lateral movement and rotation owing to swelling pressures acting laterally on large areas of foundation

they may be subject to large uplift forces generated by swelling pressures acting vertically on the sides of the foundation.

Where trees are removed prior to construction, it is clear that swelling of the ground is a possibility. However, it is not always recognised that where deep trench-fill foundations are to be constructed, swelling may be initiated by the severing of tree roots within the building footprint even if trees are not being removed from the site.

Compressible or collapsible building materials are commonly used to reduce the pressures transmitted to foundations and suspended slabs. Some grades of expanded polystyrene can compress under modest load and are therefore commonly used to resist the build-up of excessive lateral pressure on the inside face of trench-fill foundations.

Collapsible honeycomb cardboard, designed to soften on wetting to such a degree that it collapsed under very light pressure, used to be in common use under domestic floor slabs. Over recent years the use of such products in the housing market has reduced because, under certain circumstances, they have been found to generate methane as they decompose. However, it is now recognised that the reactions required for methane generation are only likely to take place when the material is persistently waterlogged. Therefore it is highly unlikely that methane-generating conditions would apply under a ground-level domestic floor slab. Caution is still required for deep trench-fill applications.

Compressible or collapsible materials are also used to address the vulnerability of trench fill foundations to uplift forces in swelling ground. Their use on one face of the foundation as recommended by nhbc5 helps to reduce the uplift force transmitted to the foundation. But it is important that the trench faces are smooth and vertical. For internal foundation walls, if no compressible material is being supplied it would be prudent to supply a slip surface, such as polythene sheet, on both faces.

The bre has recently been involved with a number of cases of damage to new buildings where trench-fill foundations have failed due to lateral swelling of the soil within the building footprint. These failures have shown that provision of the correct quantity, grade and thickness of compressible or collapsible materials in the correct place is vital for the adequate performance of trench fill foundations.

Mike Crilly is project manager at the Centre for Ground Engineering and Remediation, bre. Tim Chapman is an associate with Arup Geotechnics.

References

1 Previous geotechnical articles were on ground investigations (aj 24.9.98), piles and retaining walls (aj 24.9.98), gravity retaining walls (aj 22.10.98), on why ground problems arise (aj 17/24.12.98) and on water in the ground (aj 11.3.99).

2 Low-rise building foundations: the influence of trees in clay soils. 'bre Digest 298. 1999'. From crc, tel 0171 505 6622.

3 bs 5837: 1991: Guide for trees in relation to construction.

4 Low-rise Buildings on Clay Soils: Part 1. 'bre Digest 240'. 1993. Low- rise Buildings on Clay Soils: Part 2. 'bre Digest 241'. 1990. From crc, tel 0171 505 6622.

5 Building Near Trees. National House-Building Council Standards, Chapter 4.2. 1997. nhbc, tel 01494 434477.

6 Foundations - Proximity of Trees in Clay Soils. Zurich Municipal Builders' Guidance Note 3A. 1996. Zurich Municipal, tel 01252 522000.

7 Damage to buildings caused by trees. bre Good Repair Guide 2. 1996. From crc, tel 0171 505 6622.

8 Structural foundations manual for low-rise buildings. M F Atkinson. e&fn Spon. 1993.

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