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What are the top three causes of subsidence in the UK for 2023?

 

The top three causes of subsidence that require repair in the UK in 2023 are:

  • Root induced clay shrinkage (60%)
  • Running water from leaking drains or mains water supply pipes (18%)
  • Poor ground, infill, and consolidation issues (18%)

These are based on the most up-to-date insurance data available from valid subsidence claims collated by the Association of British Insurers (ABI). The remaining 4% of claims cover ground heave, landslips, the appearance of sinkholes or subsidence related to mining issues.

The causes of subsidence are linked to the quality of the ground beneath the foundations. Subsidence happens when the support provided by that ground is no longer sufficient to hold the weight of the building, and part or all the foundations, and the building above sinks into the ground. If subsidence affects only a section of the building this can, depending on the scale of the effect, cause shearing forces to be set up within the structure and this can lead to large cracks appearing in the walls.

The top cause of subsidence, soil shrinkage, is due to the cohesive nature of clay soil which means it will expand and contract as its moisture content rises or falls. If it is dehydrated, by tree roots seeking out new sources of water for example, then it will shrink and, if it is underneath foundations, they, and the building above, will sink into the ground. Unusually hot summers will also present more instances of subsidence linked to the shrinkage of clay soils that are not directly related to the influence of tree roots.

Running water underneath foundations can wash away the soil, especially if it is non-cohesive, such as sand and gravel, causing subsidence as the foundations around the leak sink into the gap that has been created. The last of the three causes, where, for a variety of reasons, the ground beneath the foundations is not sufficient to bear the load of the foundations, reinforces the link between subsidence risk and ground stability.

How many properties across the UK could be affected by subsidence?

As subsidence is strongly influenced by the location of the property, different parts of the UK will be prone to different causes of subsidence and the numbers that could be affected will be influenced accordingly.

As an example, Wales and England have large areas of clay soil. This is not the case north of the border in Scotland. These large areas of clay soil will put those areas at a higher risk due to clay shrinkage during periods of prolonged hot weather. In the last hot summer of 2018, domestic subsidence claims totalled 23,000, whereas in 2021, where the summer was wetter and cooler than the average, there were only 15,000 claims for subsidence. It will be interesting to see what impact of the exceptionally hot summer of 2023 has on this year’s subsidence claims and the effect of climate change in the coming years.

Recent analysis by the British Geological Survey (BGS) has looked at the ‘shrink-swell subsidence susceptibility due to changes in climate’. According to its projections more than 3% of properties are likely to be affected by subsidence issues by 2030 and this could rise to nearly 11% by 2070.

As you might expect, those properties that are built in areas with clay-rich soil will be proportionally more affected. The analysis suggests that the number of properties in London likely to be affected by climate change ‘will rise from 20% in 1990 to 43% in 2030 and almost 3 times (57%) by 2070.’

Data taken from Dye and Durham’s Climate report estimates the number of properties exposed to medium to high risk of subsidence at more than 5.76 million. This rises to approximately 6.64 million properties by 2030 . However, the figures on subsidence are presented, they show that the risk is real for many properties across the country, and this looks likely to only increase over time.

What are the costs of subsidence to the UK economy?

Subsidence, of course, affects both domestic and commercial buildings, as well as the infrastructure of the UK and the BGS, calls it ‘one of the most damaging geo hazards in Britain today’. It estimates that the cost to the economy of subsidence over the past decade is around £3 billion.

These costs are made up of many different factors. The 23,000 insurance claims in 2018 cost £145m[1] to put right and there will be a knock-on effect of higher premiums and larger excesses for houses that have suffered from subsidence in the past. Subsidence can also reduce the value of the property by as much as 20%. The number of subsidence claims that are refused by insurance companies can be as high as 35%, therefore the actual cost of repairing all the instances of subsidence within the UK in 2018 is far higher than the £145m paid out by the insurance industry.

The cost to commercial buildings affected by subsidence can be substantial due to the disruption to normal business caused by the underpinning work required to rectify the problem. If this work requires parts, or all the premises to be closed, then the business losses will be higher still. This highlights the need for businesses to choose the optimum underpinning solution that causes as little disruption to normal business patterns as possible.

What are the best ways of fixing subsidence?

The best way of fixing subsidence is to employ a method that has been approved for the task by a qualified structural engineer and one that will do the job in a way that causes minimum disruption to the property or business concerned.

Traditional methods of underpinning include mass concrete where holes are dug beneath the foundations at selected points and sections of earth are replaced by concrete. This is done one section at a time and the concrete must set before moving on to the next section, making it a slow process. It is also only suitable for shallow foundations. Piling is a method that can be used for deeper foundations and is suitable for use in a wide range of soil types. It requires skilled operatives and heavy machinery to drive the piles into the ground that support the foundations and can be a very disruptive and lengthy process.

More modern methods of fixing the causes of ground subsidence are to inject special grouts or resins into the ground. This is done in a controlled process, with minimal disruption to the building or its occupants and, once ground below the affected foundations has been reinforced, further injection can raise the subsided parts of the building back to its original level.

Mainmark offer two modern methods to fix the problems caused by ground subsidence with minimal disruption and in a quick and cost-effective manner.

Our Teretek® engineered resin injection method delivers an expanding, environmentally inert, polyurethane resin mix into the ground through several carefully positioned small bore injection tunes. It strengthens the ground underneath the foundations, underpins the area and can, through the injection of more resin, raise the subsided section to bring it back into line with the rest of the building. The process, backed by our 20-year product warranty, is suitable for use across both domestic and commercial buildings and for infrastructure projects.

Mainmark’s JOG Computer Controlled grouting system can support and re-level buildings affected by subsidence through a unique multi-point cementitious grouting process. It uses computers to control the delivery of a cementitious grout through an array of pumps as part of a proprietary pumping system. The pumps deliver grout in small doses across a range of multiple injection points to carefully raise the building and will return it to the required level that it occupied before suffering the subsidence.

If you would like to learn more about the causes of subsidence and how to fix them or have a particular subsidence issue and would like some expert advice, please get in touch for our full support.

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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