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Understanding Subsidence

(July 2, 2020)

What causes Subsidence? 

Think Jelly and marbles in a jar!

Cohesive soil – Clay – Think jelly – Very fine platelets: 10,000 clay particles make up the thickness of a 10p coin (2mm) and the molecular forces hold water within their structure (cohesive)

Non Cohesive soil – Granular soil; Gravel and Sand – Think marbles in a jar or footballs in a box…. (RICS)

So with the above in mind, let’s take a look at some of the common causes of subsidence, 

Leaking Drains. If the drains are defective this will allow erosion and washing away of the fines (marbles in a jar), or softening of the soil (jelly). A CCTV survey will provide you with details of the condition of the drains, and allow you to assess if drainage is causing/ contributing to subsidence at the property.

Trees. Clay with a high moisture content can dry out as a result of nearby vegetation sapping the water, particularly during long, hot summers. This means shrubs and trees that are growing close to a building can cause clay shrinkage subsidence. For example, an oak tree can absorb up to 1000 litres per day! If the property is built on granular soils, then these tend not to be affected by these problems. 

Inadequate foundations. This generally tends to be a problem with older properties, and may only affect part of the structure, if, for example, extensions have been built at a later date. Things to look out for are shallow foundations, that may be built on shrinkable clay or on loose poor made ground or Strip foundations that are too narrow to support imposed loads, resulting in differential movement. 

 

Identifying Subsidence

Subsidence is not without warning signs,

The most obvious sign is cracks that are wider at the top and follow a diagonal, stepped tapering pattern. They often appear around windows and doors. 

Properties built on clay soils are more susceptible to subsidence. Each different type of clay has different shrink/swell properties. The purer the clay the more likely it is to be affected by seasonal volume changes due to moisture content. If subsidence is suspected, a site investigation would confirm the soil type and if clay shrinkage is an issue. 

Extensions and bays can rotate away from a property because of subsidence. Very often the foundations are different from the main structure. Subsidence related damage usually appears as tapering cracks at or near the junction of the projection. 

 

Is it subsidence or is it settlement? 

Subsidence is the process by which land or building sink to a lower level. It is unconnected with the weight of a building and will occur whether there is a building there or not. 

Settlement is the process of the slow sinking of a building or the ground. The downward movement is due to the application of the superimposed load (weight) from the building. 

It would usually occur in the early years following the construction of a building. 

 

What’s the opposite problem to Subsidence?

Heave. This is the upward movement of the ground usually associated with the expansion of clay soils which swell when wet. It is the opposite to subsidence, because in this situation the ground rises up, but it can still cause structural damage. Cracking associated with heave is generally diagonal, tapered, stepped cracking but is wider at the bottom, whereas subsidence related cracks are wider at the top. 

It is often caused by the removal of trees prior to the construction of a building. Before trees are removed an assessment should be made, as a rule of thumb is if the tree is the same age of the building there is no risk of heave, if the tree is older than the adjacent building, eg the building was constructed on a desiccated soil, there is a risk of heave. 

 

Subsidence and Mining

Mining subsidence is caused when the ground beneath or near a property’s foundations has been weakened by mining works. If mining works were carried out directly beneath a building’s foundations it can cause catastrophic damage, such as collapse. 

 

Subsidence Surveys

A Surveyor or Structural Engineer can undertake a subsidence survey. This involves a visual inspection to look at cracks in the walls, sticking doors and windows, sloping floors, and any other obvious signs of subsidence. They will then assess, in line with BRE guidelines,  if the damage is due to structural movement or normal property movement. 

Sometimes, subsidence isn’t always obvious, and diagnosing can take a while, and may involve a period of monitoring. In addition to this, it may be necessary to complete geographical and drain surveys, to ascertain what is causing the subsidence, eg: leaking drain or unstable soil. 

A big part of a surveyor’s/ Structural Engineer’s assessment is the homeowner. They know the property best and are best equipped to answer questions such as, when the crack first appeared, and if it has changed. They may also be aware if neighbouring properties have issues, can confirm any significant events, eg: new construction to the home/surrounding area, tree removals, drain defects etc. 

 

Websites for reference 

British Geological Survey – View maps – Geology of Britain

RICS  – The Royal Institute of Chartered Surveyors, is a professional organisation that promotes and enforces the highest standards in the valuation, management, and development of land, real estate, construction, and infrastructure.

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