Buildings on Clay

The effects of geology, climate and vegetation on heave and settlement

Derek Clarke and Joel Smethurst


	Trees are vital elements in the character of rural and urban architecture and are often wrongly blamed for causing movement in buildings.

The geology of many areas of the UK, particularly southern England, consists of clay materials. These occur either as shallow surface layers or as deeper geological formations many tens of metres thick. A characteristic of many clay soils is that they swell in volume when they get wet and reduce in volume as they dry. ‘Shrink-swell’, as this phenomenon is known, tends to occur near the ground surface and rarely more than five metres deep. Historic buildings are particularly susceptible to problems associated with seasonal movement because they were often built with shallow foundations which do not extend below the affected clay layers.

The magnitude and direction of shrink and swell displacements are affected by a variety of factors but are rarely more than 150mm in the horizontal and vertical directions, combined. Nevertheless, displacements of this scale can have serious impacts on some buildings and structures.

Claims submitted to the insurance industry show that trees are often cited as the cause of subsidence problems due to root penetration or the more extensive drying that occurs in the vicinity of the tree. However, to fully understand the causes of clay shrink-swell, it is necessary to appreciate the factors that contribute to changes in the water content of clays and to relate these to the context of the building and environs being investigated. For example, not all buildings resting on clay foundations are affected by clay shrink-swell. If the clay material remains moist throughout the year then its volume will not change. The same is true if the soil remains continually dry, as under a paved car park for example. Equally, movement does not necessarily cause any damage, and signs of building movement can be caused by many factors apart from clay behaviour, including foundation collapse, changes in building loadings, blockage of subsurface drains, and structural failure. If a building is showing signs of cracking, subsidence or other deformation, it is important to consider these factors even if the building is located in an area of known problems associated with clay soils.

Whatever the suspected cause, it is most important that expert advice is sought before taking any remedial action: a rapid response such as felling nearby trees may not solve the problem and may even make matters worse.

This article aims to provide some background on the location and types of clays that may cause problems, the mechanisms that cause soils to wet and dry, the likely scale and timing of the effects, and the possible long-term impact of soil drying caused by predicted climate change.

CLAY SOILS: TYPE AND DISTRIBUTION

Clay materials exist in many areas of the UK, but tend to be much more common in the south and east of England. Clay types that have a high shrinkage potential and are associated with damage to buildings are the London Clay, Gault Clay, Weald Clay and Lias Clay. This list is not exhaustive and there are other clays which have more limited shrink-swell characteristics.

The British Geological Survey maintains a useful database of the geology of the UK called ‘Britain beneath our feet’. This lists ‘shrinking and swelling clays’ as one of a number of geological hazards, and a map is provided showing ‘shrink-swell potential’ in categories of ‘low to nil’, ‘moderate’ and ‘significant’. This, it should be stressed, enables an overall assessment of the potential only, so although large areas of the London conurbation are built on the London Clay, for example, not all buildings in this area are at serious risk of movement.

FACTORS THAT MAY CAUSE CLAY TO SHRINK AND SWELL

A clay soil will not change its volume unless the water content changes. Structural damage may occur if the change in moisture content is relatively rapid (say over a few weeks or months) or if it affects only part of a building. Situations which can cause this include changes in subsurface drainage (such as a water leak from a burst or blocked pipe or drain), localised drying of a soil (due to plant growth or proximity of trees), or a sudden change in the water regime (due to the removal or pruning of a tree which was sheltering the soil and keeping it relatively dry, for example).

Perhaps surprisingly, it is the growth of vegetation not variations in rainfall which has the most significant impact on shrink-swell in the UK, as the winter months are on average only slightly wetter than the rest of the year, and heavy rain can occur in any month of the year. From May onwards, however, grass, trees and bushes, which were relatively dormant during winter, begin to grow, transpiring water through their pores and drawing water from the soil. Rising temperatures result in an increase in both transpiration and the evaporation of water from the soil and leaf surface. Termed ‘evapotranspiration’ (or ET for short) this moisture-loss is recorded in the same units as rainfall: millimetres. Data which can be obtained from the Meteorological Office Regional Evaporation Calculation System (MORECS) shows that rainfall and ET vary around the UK, but typically in central southern England, where clay soils predominate, the annual total rainfall is between 500mm and 800mm and evapotranspiration between 350mm and 500mm. The difference between these annual totals is the water that either runs off into rivers or soaks into groundwater such as the chalk aquifers.


	Figure 1: Typical daily rainfall in southern England: note that heavy rain can fall in any month (see July 2007, for example, when over 60mm of rain fell in one day; this was the same event that caused extensive flooding in southern England).

	Figure 2: Evapotranspiration from a grass surface in clay soil in southern England: by comparison with rainfall, evapotranspiration tends to follow a more predictable pattern each year, with only an occasional minor extreme, say 5mm in a day, which is low relative to many daily rainfalls.

The driest period usually occurs at the end of summer when ET has exceeded rainfall for several months. The wettest time of the year is generally springtime, although in clay soils, a sudden wet period following a dry summer may cause swelling to manifest itself at the end of the year in November or December.

The drying of clay soils tends to be a slow and progressive process. Grass typically extracts no more than 5mm of water per day from a soil, and trees and bushes rarely raise this to much more than 6mm per day. Also, when the soil dries, the amount of water available to be transpired decreases, causing the vegetation to become stressed and die back. This is often seen in hot summers when grasses turn brown, but the behaviour of trees with slightly deeper roots can be different. Insurance claims data shows that these very dry summers cause higher incidence of building damage.

The range of measured water contents in a clay soil beneath a grass surface and an oak tree is shown above. For the grass cover, drying at the surface reduces the water content from around 50 per cent by volume in springtime to 35 per cent in late summer. However, the rooting depth of grass means that the drying is usually limited to no more than one metre below ground level.

Under a large tree such as an oak drying can continue to three metres or more (Figure 4). This depth of drying is more extensive than under grass and is caused by deeper root penetration. In addition, light summer rain showers are
intercepted by the leaf canopy and re-evaporated, stopping water wetting the soil.


	Figure 3: Range of moisture change under grass

	Figure 4: Range of moisture change under an oak tree

The soil moisture deficit (SMD) is a measure of how dry the soil is. SMD is the amount of rainfall that would be needed to fully re-wet the soil profile. In a hot, dry summer a grass-covered clay soil may experience an SMD of 120mm. Under a deeper-rooted tree, SMD may be as high as 200–250mm. Individual trees close to part of a building can therefore cause a greater extent of soil drying and this, as mentioned above, is often responsible for building damage.

Biddle, in his definitive book Tree Root Damage to Buildings, gives an excellent overview of the role of trees in affecting buildings. He compared measurements of soil drying with incidents of building damage for a large number of trees on clay soils to understand the depth and extent of tree drying. Different tree types have different zones of influence.

Figure 5 shows measurements of the vertical soil displacements at 6m and 12m from a single oak tree in a grassy field. Biddle found that for poplar, oak, elm and willow, in 75 per cent of cases of building damage the trees were less than 12–15m from the building; for ash, horse chestnut, beech, sycamore, lime and plane, 75 per cent were less than 8–10m; and for birch, cherry, apple, cypress, 75 per cent were less than 5–8m. Since 2007 this has been incorporated into guidance from the National House Building Council as a series of distances that different types of tree should be kept from new or existing buildings of a given foundation depth. Biddle shows that the maximum SMD measured adjacent to a sample of 60 trees on clay soils during the summer of 1989 (a dry summer in south-east England) averaged about 180mm, with a range from 80–280mm.


	Figure 5: Vertical ground surface displacements adjacent to an oak tree on London Clay

	Figure 6: Possible effects of climate change in the 21st century: increasing evapotranspiration (upper lines 1961–1990, 2020s, 2050s and 2080s) and decreasing summer rainfall (lower lines)

INFLUENCE OF CLIMATE CHANGE ON EVAPOTRANSPIRATION FROM PLANTS

The UK Climate Impact Programme (UKCIP) has published scenarios of rainfall and climate for the rest of this century. Figure 5 shows the expected changes in evapotranspiration and rainfall for the Oxford-London area over four periods: 1961–1990, the 2020s, the 2050s and the 2080s. Warmer and drier summers are likely to cause a higher incidence of drought. Different types and species of plant are likely to react to this in different ways; some may be drought tolerant, others may struggle and die out in the warmer, drier parts of the UK. 2003 was notable for its extended hot, dry weather and the SMD was equivalent to a one-in-ten-year event. However, if we compare this with the likely future climate scenarios using the UKCIP Medium High CO2 emissions scenario (2002), an SMD of 155mm will occur in nine years in every ten by the end of the century. This is a big change in the frequency of dry summers, with the implication that building damage will occur far more frequently in the future.

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Author

DR DEREK CLARKE and DR JOEL SMETHURST, University of Southampton. DEREK CLARKE is a specialist in hydrology and the extraction of water from soil by plants and Vegetation. JOEL SMETHURST is a geotechnical engineer
with experience in measuring the movement of clay caused by seasonal changes in weather.

Further information

Structural