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What is Subsidence?

Updated: Nov 23, 2023

Subsidence is a geological process where the ground level sinks or lowers. This can occur naturally due to various reasons such as soil shrinkage, earthquakes, and land erosion. However, it can also be caused by human activities such as mining, excessive groundwater pumping and construction.

However, in the south east of England, the main cause of subsidence is clay shrinkage caused by tree roots (root induce clay shrinkage) extracting moisture from the soil, and escape of water from leaking below ground services such as drainage pipes or a cold water main, which can lead to erosion of the finer particles in the soil or compaction of the soil and cause the ground level to sink.

This region is particularly susceptible to subsidence because the underlying geology to a large part of the area is composed of London Clay. London Clay is susceptible to shrinkage when tree roots extract moisture from the soil (root induced clay shrinkage), or leaking below ground services cause soft sediments or sand and gravel soils to compress or eroded. As the ground sinks, buildings and infrastructure on the surface can be damaged, leading to expensive repairs and even safety hazards. To prevent subsidence, it is important to manage vegetation or below ground services depending on the cause.

The Oxford English dictionary's definition of subsidence is "the gradual caving in or sinking area of land". However, the important definition is the one defined by insurance companies as subsidence is normally covered by the building insurance policy and insurers have a different definition. Insurers view subsidence as the downward movement of the ground on which the building is founded for reasons unconnected to the loading of the ground from the building.

A building built on virgin ground suffers a downward movement owing to consolidation of the ground under the weight of the building. This movement is known as settlement and occurs in the early life of the property and is not covered under the buildings insurance policy. Although some minor damage may occur it is not normally a serious issue and any cracking to the superstructure (the house above ground level) that has occurred from settlement can normally be repaired without the necessity for mitigation works.

Subsidence, as defined by insurers, is a different matter as there is normally a specific cause other than consolidation of the ground from the weight of the building. Typically, in the south east there are two major causes as noted above.

What is Root Induced Clay Shrinkage

Root-induced clay shrinkage is a process that occurs when the roots of trees and other vegetation extract water from the soil, causing the underlying clay to shrink and potentially leading to subsidence. This occurs in soils with high clay content, which are more prone to shrinking and swelling in response to changes in soil moisture levels.

As roots absorb water from the soil, the soil around the roots dries out and contracts, causing the soil to shrink. This can cause the ground surface to sink, creating small cracks or fissures in the soil. Over time, these cracks can widen and deepen, leading to more severe subsidence.

Root-induced clay shrinkage can be particularly problematic in areas with large trees, as they can extract large amounts of water from the soil. This can cause significant damage to buildings, roads, and other infrastructure, and can also impact the health of the trees themselves.

To prevent root-induced clay shrinkage, it is important to carefully manage the planting and maintenance of trees and other vegetation in areas with high clay content soils. This may involve using less water-intensive species or managing water usage in the surrounding area to ensure that the soil remains moist enough to prevent excessive shrinkage.

Additionally, buildings and infrastructure in areas prone to subsidence may need to be designed or reinforced to prevent damage.

In Surrey and the south east this type of subsidence has the potential to increase if climate change is not dealt with effectively.

Leaking Below Ground Services

Leaking below ground drains or cold water mains can cause subsidence. The subsidence is caused by the leaking water and saturating the soil, which can cause the soil to become unstable and compress, leading to sinking or settlement of the ground.

Leaking below ground drains or cold water main can cause subsidence in a few different ways. One way is that the leaking water can cause the soil to become saturated, reducing its ability to support the weight of the building above. This can lead to sinking or settlement of the ground, which can cause damage to the building's superstructure.

Additionally, the leaking water can cause erosion of the soil, which can cause voids or empty spaces to form beneath the foundation, leading to further subsidence.

It is important to note that not all leaking drains will cause subsidence, and other factors such as the soil type and the depth of the drain and distance from the property can also play a role. However, if you suspect that leaking drains are causing subsidence on your property, it is important to seek advice from a professional who can assess the situation and recommend appropriate repairs.

Other Causes

There are occasionally other causes such as long-term consolidation of made ground or even more rarely (in Surrey and the south east) collapse of underground workings such as mines. In these cases, underpinning of the foundations is sometimes required to stabilise the property.

Subsidence can affect the saleability of the property, mortgage lending and insurability but with the correct help these risks can be significantly reduced.

What are the most common signs of Subsidence?

When subsidence occurs, it can cause various signs and symptoms on the superstructure of a house, which is the part of the building above ground level. Some of the most common signs of subsidence in the superstructure of a house include:

1. Cracks in walls, floors, and ceilings: Subsidence can cause cracks to appear in plaster, brickwork, and other parts of the building. These cracks are often diagonal and can be wider at the top than the bottom.

2. Doors and windows sticking: As the building shifts, doors and windows may become harder to open and close smoothly.

3. Leaning or tilting walls: Subsidence can cause walls to tilt or lean, particularly at corners or where walls join.

4. Rippling wallpaper: When subsidence causes cracks in walls, it can also cause wallpaper or paint to ripple or wrinkle.

5. Uneven floors: As the building sinks, it can cause floors to slope or become uneven.

6. Cracks in external brickwork: Subsidence can cause cracks to appear in the exterior brickwork of the building.

It is important to note that not all cracks are caused by subsidence. Other factors, such as settlement or thermal movement, can also cause cracks to appear. However, if you notice any of these signs, it is important to seek advice from an engineer or building surveyor who can assess the cause of the damage and recommend appropriate repairs

What is the best way to cure for subsidence?

The best way to cure subsidence will depend on the underlying cause of the problem. However, some common methods of subsidence treatment include:

Underpinning: This involves digging beneath the affected area of the foundation and installing new supports or piles to stabilize the foundation and prevent further sinking.

Grouting: This involves injecting a special grout mixture beneath the foundation to fill any voids and improve the soil's stability.

Soil stabilization: This involves improving the soil's stability through various methods, such as compaction, chemical treatment, or the installation of geotextile membranes.

Drainage improvement: If subsidence is caused by excessive water in the soil, improving drainage can help to reduce the water level and stabilize the soil.

Tree removal: If subsidence is caused by root-induced clay shrinkage, removing trees or other vegetation with large roots may help to prevent further subsidence.

The best subsidence treatment will depend on the specific circumstances of the property and the underlying cause of the subsidence. It is important to seek advice from an engineer or building surveyor who specialises in subsidence and can assess the cause of the subsidence and recommend appropriate treatment options.

Author: Tony Waring is a Chartered Building Engineer (C.Build E MCAB), an Associate of the Royal Institute of Chartered Surveyors (AssocRICS) and a member of the Faculty of Party Wall Surveyors (MFPWS).  He has over 30 years of surveying experience and 20 years as an engineer.

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