That Sinking Feeling

Trees, Shrink-Swell and Existing Buildings

We have had a surge in enquiries this month regarding shrink-swell (subsidence and heave) and existing low-rise commercial and domestic properties and therefore it seems appropriate to write on the subject.

 

The six most common natural ground stability hazards are:

  • swelling clay
  • landslip
  • ground dissolution
  • running sand
  • collapsible ground
  • compressible ground

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).

  • superficial geology
  • tree species
  • tree height
  • distance of the tree from the building
  • 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. Most soils in the UK are less than 1.0m thick.

Superficial deposits are unconsolidated materials less than 2.588 million years old that are typically laid down by processes of water, wind or ice. 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 is 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 because 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.

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 year. Moisture content can also be influenced by a wide range of other factors, including, but not limited to:

  • the presence and management of trees
  • installation of new drainage systems
  • leaks in existing drainage systems
  • unusually high or low seasonal rainfall
  • prolonged high temperatures

Trees and Moisture Extraction

The extent to which a tree extracts moisture from the ground is determined by the a) species of the tree and b) the zone of influence of the tree. The zone of influence is the lateral extent, in meters, from which a tree may extract moisture. For example, an oak may extract water from a 25.0m radius, an ash tree may extract water from a 17.25m radius and a birch tree may extract water from a 7.0m radius.

Foundations

Foundations are designed to have an adequate load capacity for a built structure with limited settlement.

In 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 of shrink-swell damage is insignificant.

References
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