The causes of subsidence

By Vincent Brady
Contact us if you need a subsidence engineer

There are many causes of subsidence. Thankfully, most of them are covered in both residential and commercial insurance policies. The damage caused by subsidence can be further reaching than first thought and become a considerable source of stress for the owners or occupants of a property. This is why correctly identifying the problem and taking the appropriate course of action is so important, and why using a professionally trained subsidence engineer is crucial.

Types of ground movement

There are several types of ground movement that will commonly cause damage commonly attributed to subsidence. They include

  • Landslip – This is when the ground underneath or around the property moves due to sloping ground.
  • Heave – This is the term used when the ground underneath the foundations of a property expands.
  • Subsidence – This is when the ground beneath the foundations of a property sinks, causing the foundation of the property to shift with it.
  • Settlement – This most commonly happens within a decade of a building being completed and is due to the weight of the building compressing the ground underneath the foundations.

Identifying which type of ground movement has caused the damage to the property is one of the first steps. This ensures the next steps are the correct ones.

Causes of subsidence

It’s important for a professional subsidence engineer to correctly identify the cause of any subsidence. One of the most common causes for the ground shrinking is a reduction in the amount of moisture in the earth. This can be caused by a prolonged dry spell or by trees and plants extracting all the moisture from the ground. Below we’ll look at the different causes a little more closely.

Poorly prepared ground

While nearly every new property will experience some settling in the years after its completion, this can be considerably worse when combined with other factors. These other factors can include poor ground quality, insufficient compacting of made-up ground material, the use of inappropriate materials by builders before the property is built, or even just the way the ground was prepared prior to building commencing.

When any of the above are combined with normal settling, it can lead to significant damage to the property, including floors and slabs. It’s not uncommon for newer properties to experience this type of movement on outdoor slabs, as these aren’t technically considered part of the foundation, and less attention might be paid to the fill used underneath outdoor areas.

Shrinking clay

This is the single most common cause of subsidence. Clay contains approximately 30% water. This means the higher the clay content in the soil, the more prone to this type of subsidence a property will be. Typically, shrinkage can occur when nearby vegetation saps the water out of the earth, particularly during long dry spells.

As the moisture level in the clay reduces, the clay shrinks and the reduction in volume leads to the foundations subsiding. This can also be caused by large trees, especially older trees with larger roots. There are various solutions to this, from removal, thinning, or topping of the tree, to ways of stopping root encroachment. However, a professional should always be consulted, as a bad choice can lead to soil becoming over-saturated and result in heave.

A sustained escape of water

This can happen when a pipe leaks or bursts underground. This can go undetected for a while, which leads to a sustained period of escaping water into the soil. This can have several effects. It can soften the soil, making it less capable of supporting the building. It can also wash away fine particle of the underlying soil, which reduces the volume of the soil, which ultimately causes subsidence.

Historic mining

When underground mines (usually long abandoned) collapse they can cause subsidence in any building above them.

Organic landfill

Decomposing organic landfill loses volume as it passes through its various stages of decomposition. Its loss of volume and resilience causes the soil to compact further, creating the same downward movement of any buildings above the area.

Solution features

This cause of subsidence happens when the ground below a property is primarily limestone or chalk. An escape of water can create an underground cavern that ultimately collapses. An early sign of this is the appearance of a small hole at surface level.

Other problems mistaken for subsidence

There are other problems that can cause damage that can be mistaken for subsidence. Below we’ll highlight some of the common causes. These include cracking, thermal movement, lintel failure and sulphate damage.

As soon as a property starts to exhibit cracking, people often suspect subsidence. However, cracking can sometimes just be a part of the settlement process or due to the weight of materials used and doesn’t automatically mean there are structural problems. A lot of settlement movement happens in a uniform way, whereas subsidence will see different parts of the property moving at different rates.

Sulphate damage (the chemical reaction between cement paste in slabs and the sulphates in the filling underneath) are sometimes mistaken for heave, as the damage is very similar. A lintel above a window or door can fail, leading to the brickwork above sagging. This sagging causes diagonal cracks that can be mistaken for subsidence. The difference is usually noticeable because of the position and pattern of the cracks.

The type of repairs that will be needed

When subsidence does strike there are a number of important facts you should use a qualified subsidence engineer to establish before beginning repairs. Depending on the severity and cause of the subsidence, underpinning can be required. This is followed by making sure all repairs being made are effective and long lasting, including those designed to stabilise the building and those designed to repair the superstructure.

Why Banrach are your perfect partner

With nearly two decades experience providing realistic solutions to property owners, we are well versed in all areas of consulting engineering. We understand the importance of providing cost-effective solutions in a professional way. Just because subsidence strikes, everything is not lost. Contact us if you need a subsidence engineer