Subsidence

Trial Holes

Soil & Root Sampling

Subsidence

What is subsidence?

Subsidence is the downward movement of a surface, causing the ground beneath the foundations of a building to become unstable.

Common reasons for subsidence are:

• The shrinking of soil beneath a property

• Previous mining activity on the site

• Ground vibrations

Soil shrinkage can be caused by a number of factors, such as water leaking into the soil under the property, as this can wash away the soil from the foundations. This is most common in soils with a high sand or gravel content.

Clay soils are particularly vulnerable to shrinking, and as the soil shrinks, it pulls the foundations which may result in structural movement. Clay soil shrinkage can also occur following a dry spell of weather.

Tree roots surrounding a property can significantly influence the likelihood of subsidence occurring – as they can cause the shrinkage or reduction of the soil beneath the foundations by extracting water. Tree roots removing moisture from the soil is the most common reason for subsidence in properties.

What are the usual indications of subsidence?

There are many noticeable signs on the inside and outside of a house which will signal you towards the existence of subsidence.

The usual indications of subsidence are:

• Cracks in the walls, ceilings and outside brickwork

• Expanding of existing cracks

• Cracks appearing after a long phase of dry weather

• Rippling of wallpaper that isn’t caused by damp

• Sticking of doors and windows.

 It is essential to seek advice as soon as you notice signs of subsidence, as the sooner it is identified and diagnosed the easier it will be to rectify. Solving subsidence can be a lengthy process, with many cases having to be monitored for up to 12 months, making it even more important to get help as soon as possible.

Trial Holes

A trial pit (also known as a test pit) is a type of intrusive ground investigation that is used as a means of determining the condition of the ground, typically before beginning construction works.

Trial pits are commonly used to investigate shallow ground conditions to develop an understanding of the profile of soils within the ground. They can be particularly useful where buried structures, variable conditions or contamination is suspected or needs further investigation.

They can be excavated by hand or using a backhoe excavator, generally to a depth of up to 3.5-4.5 m. If a trial pit is deeper than 1.2 m and is intended to be entered by people, it should be made secure against the possibility of structural collapse.

Trial pits (or trenches) can be more cost-effective than boreholes but they cannot reach the same depth. Trial pits can also excavated relatively quickly.

Soil & Root Sampling

The abundance of shrinkable clay soils results in an increased risk of tree-related subsidence damage to buildings. Establishing the cause of subsidence often requires a multidisciplinary approach.

Trees and/or shrubs that have the potential to affect the soil moisture content under the foundations of a building or structure. 

We are able to extract soil samples and obtain root samples for the purposes of subsidence investigation which are then sent for laboratory analysis to establish, definitively, which tree or trees within a group of trees are implicated in the damage.

Initially a surveyor or structural engineer will conduct a programme of monitoring to determine whether the movement of a structure is seasonal. If this is the case, a soil moisture deficit brought about by the uptake of water by vegetation is, generally, assumed to be the cause. At this point an arboricultural survey is often undertaken to identify any trees or shrubs that have the potential to affect the soil moisture content under the foundations of the building/structure.

Further investigation, such as root identification through microscopy or DNA analysis, may be required to establish which tree or trees within a group of trees are implicated in the damage. This is particularly relevant in cases where trees, owned by a third party, are potentially the cause. Once the trees causing the damage have been identified, recommendations for tree management can be made and remedial action undertaken.

 It is important to state that tree-related subsidence can only occur on shrinkable clay soils. Therefore, soil analysis is an important part of a subsidence investigation, particularly when tree removal is the preferred option.