The following is an introduction to tree related shrink-swell (subsidence or heave) and relates to existing low-rise commercial and domestic properties.
The six most common natural ground stability hazards are;
- swelling clay,
- ground dissolution,
- running sand,
- collapsible, and
Mining related subsidence is man-made.
Trees and Shrink-Swell
There are five primary factors to be considered when assessing the likelihood of trees causing or contributing to shrink-swell (subsidence or heave). These factors are;
- superficial geology,
- tree species,
- tree height,
- distance of the tree from the building, and
- depth of foundations.
Soil, Superficial Deposits and Bedrock Geology
Soil is comprised of minerals, organic matter, air, water and multiple organisms. It covers the landscape and forms a thin layer of material over the superficial deposits and the bedrock geology. It is a medium for plant growth; a means of water storage, supply and purification; a modifier of the atmosphere and a habitat for organisms. From a geological point of view, most soils in the UK are less than 1.0m thick although they may be thicker where downslope accumulation has occurred.
Superficial deposits are unconsolidated materials less than 2.588 million years old that are typically laid down by processes of water, wind or ice. They may also include residual deposits (as a result of extended chemical and physical weathering).
In general, superficial deposits (formerly known as ‘drift’ geology) form a discontinuous layer over the bedrock geology. Locally, bedrock appears at the surface (either beneath the soil or exposed as rock) because over the last 12,000 years (since the last ice age in the UK), some superficial deposits have been removed by erosion (and man’s activities).
Bedrock geology (formerly known as ‘solid’ geology) is a term used for the main mass of rocks forming the Earth and present everywhere, whether exposed at the surface in outcrops or concealed beneath soil, superficial deposits or water.
What is a shrink-swell?
Shrink-swell occurs as a result of changes in the moisture content of the superficial deposits. The changes in moisture content cause a change in volume, resulting in either shrinkage or swelling.
Shrinkage can cause differential settlement in buildings and structures whilst swelling can cause lifting.
Photo 1: Differential subsidence due to the downslope side of house moving on shrinkable deposit. Image: Chris Page
Factors affecting shrink-swell
Moisture content can and does vary naturally with the seasons and hence clay rich deposits will naturally shrink and swell during the course of the year.
Moisture content can also be influenced by a wide range of other factors, including but not limited to:
- the presence of trees,
- the management of the trees,
- installation of new drainage systems,
- leaks in existing drainage systems,
- new hard landscaping,
- unusually high or low seasonal rainfall,
- prolonged high temperatures.
Photo 2: Damage caused by shrinkage of London clay deposits. Image: Peter Kelsey.
Trees including Shrubs and Hedgerows
Trees including shrubs and hedgerows extract moisture from the ground and on clay rich deposits this can cause shrinkage in volume. The extent to which a tree extracts moisture from the ground is determined by the species of the tree and the zone of influence. The zone of influence defines the lateral extent from which a tree extracts moisture.
Table 1. Water demand and mature height of selected trees
|Water Demand||Species||Mature Height|
|Oak – English||20|
|Poplar – Lombardy||25|
|Willow – Weeping||16|
|Chestnut – Horse||20|
|Maple – Norway||18|
The zone of influence of a tree is determined by the water demand and the mature height of the tree (see table 1).
Table 2. The zone of influence of a tree.
|Water demand||Zone of influence|
|High||1.25 x mature height|
|Moderate||0.75 x mature height|
|Low||0.5 x mature height|
Foundations are designed to have an adequate load capacity for a built structure with limited settlement.
Trees, Shrink-Swell and Existing Buildings – a Summary
Trees can cause or contribute to shrink-swell in the presence of swelling clays, when the foundations of an existing building are inadequate. Conversely, when the foundations are adequate, regardless of the presence of swelling clays or the presence of trees, the risk shrink-swell is insignificant.
National House Building Council Chapter 4.2 Building near trees 2014 British Geological Survey Note Ground shrinking and swelling – May 2012 A British Geological Survey Natural Ground Stability Report – March 2018