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What is subsidence and what causes subsidence?

Subsidence refers to the downward movement of soil mass underneath foundations resulting in the structure above, gradually or suddenly, sinking into the ground. It is often seen as being a relatively infrequent issue affecting the built environment, however, when subsidence affects a property or infrastructure, it can be a costly and time-consuming task to find a suitable solution.

In this blog, we look in detail at the many causes of subsidence to illustrate how commercial buildings and structures such as warehouses, bridges, roads and railway tracks can be at risk of suffering from subsidence. The information is taken from our white paper ‘Treating subsidence problems for commercial and industrial buildings and transportation infrastructure with geopolymer solutions’.

If you have a project that is affected by the issue or would like to find out more about the topic, the white paper contains useful insights into the subject matter and discusses the options available for finding suitable methods to treat subsidence.

What are the main causes of subsidence?

Subsidence is manifested by a downward movement of a building foundation for reasons other than the weight of the building. Subsidence can occur for a variety of reasons, such as swelling or shrinkage of clay soil due to seasonal changes in its water content or washing away of soils by leaking drains.

Subsidence can occur in localised areas or be more widespread across all the foundations on an affected site. The variation in subsidence experienced will depend on the underlying cause of the ground instability. When looking at the reasons for subsidence, the ground itself is a very good place to start.

Subsidence and shrinking of clay soils

If the soil beneath a building or structure is made of clay it is more likely to suffer from subsidence. This is due to the cohesive nature of clay soil which means it will swell and shrink as its moisture content rises or falls. If it is dehydrated, for example, during long periods of sustained hot weather, then it will shrink causing the sinking of the building or structure into the ground. Often such movements are not uniform over the entire footprint of the buildings which results in the development of cracks in the buildings, and if unchecked, it this can also lead to structural instability. When clay soil gets saturated it can also expand, causing the ground to rise rather than fall below the foundation. Not all clays react in the same way to moisture, and they can range from high, medium, or low plasticity in relation to their capacity to shrink or swell.

Subsidence and swelling of clay soils

Swelling of clay soils as they absorb moisture is not classed as subsidence and is known as ‘ground heave’. This can apply upwards pressure to foundations and cause sections of a building or structure to rise to a higher level than unaffected areas. Like subsidence, heave will also induce differential movement between different sections of the foundation thus inducing cracking and eventual instability of structure if unchecked.

Subsidence and effect of tree roots

Tree roots can absorb moisture from the clay soils, also causing shrinkage, and are a very common cause of subsidence, with some trees such as oak, willow, sycamore, ash and poplar being more prone to causing problems than others. If trees are the cause of subsidence, it is more likely to only affect the foundations in localised areas within reach of the tree roots rather than become a site-wide issue.

Subsidence and defective drains

Water flow is another common factor causing subsidence. This is manifested by water eroding and washing away the soil under the building foundations. The running water can come from several sources with the main cause being defective drains with heavy rainfall and flooding making the effects worse. Typically, non-cohesive soils, such as sand, gravel and silt are more likely to be shifted by the moving water and cause subsidence to occur.

Subsidence and man-made effects

Other factors that could cause the downward movement of the foundations, also known as movement due to loading, which aren’t classified as subsidence, are based on foundation loadings that were not included in the original design of the structure. These include vibration due to nearby passing traffic, unsupported excavation near existing foundations and increased loading due to refurbishment works. Movement due to loading is also referred to as settlement.

With industrial premises, changes in processes and equipment can increase the load on the existing foundations. Whilst the suitability of the existing ground to bear the proposed increased load will be assessed, when dealing with existing foundations there is a risk that not all areas remain at the required specification. Areas of ground beneath may not be strong enough to support the increase in weight causing settlement where previously there were no signs of any issues.

If I have a project suffering from subsidence how can I find out the treatment options available?

If you would like more information on subsidence and the treatment options available, you can download our Mainmark white paper ‘Treating subsidence problems for commercial and industrial buildings and transportation infrastructure with geopolymer solutions’.

The white paper covers topics such as:

  • The impact of subsidence on commercial buildings and infrastructure
  • Available remedial solutions for subsidence related issues
  • What are geopolymers?
  • Application and benefits of geopolymer solutions for commercial and industrial buildings and transportation infrastructure

If you would like to speak to someone about subsidence, please contact our experienced team of ground engineers who will be happy to discuss the best solutions to fit your specific project circumstances and requirements.

Hossein Khansari
Dr Hossein Khansari is a Technical Lead at Mainmark in the United Kingdom. He holds a PhD in Geotechnical Engineering and possesses extensive expertise in the field, boasting over 25 years of valuable experience within the geotechnical industry.
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