What is underpinning?

UK Home Improvement

What is Underpinning?

While every property is different, one thing that always remains the same is the importance of a home’s foundations. 

Your home’s foundations provide the entire structure with a solid base that provides integral support for the entire building. They help to transfer the house’s heavy load to the rock and soil that are below the property and ensure there is sufficient bearing capacity so that it does not collapse. 

Without proper foundations in place, you cannot be sure that the property is safe to reside in and the structural integrity of the building will be called into question. 

In some instances, it may be necessary to underpin the property’s foundations to reinforce its structure, especially if there has been some subsidence in the land.

But what is underpinning and why is it important?

In this article, we’ll answer the question ‘what is underpinning a house?’ outlining everything you need to know about the process. Read on to find out more…

What Is Underpinning?

If a house’s original foundations have become damaged, weakened or are no longer strong enough to support the weight of a property, underpinning may be required.

In short, underpinning is the construction process of reinforcing foundations that are no longer properly supporting a house. 

Usually, this is as a result of a change in soil structure or subsidence. 

Underpinning can be done in a variety of ways including reinforcing the existing foundation, enhancing the strength of the soil or extending the foundation’s range so that the property’s load is more evenly distributed across a larger surface area. 

When Is Underpinning Required?

As mentioned above, underpinning is a construction technique that is used to strengthen a house’s foundations. With this in mind, it is typically required when a property has suffered from subsidence either recently or in the past. This can occur for a number of reasons but is most prominent if soil conditions have deteriorated. 

For example, if the soil that is supporting the foundation has changed due to subsidence, expansion, contraction or as a result of damaged plumbing then underpinning may be necessary. Furthermore, it may also be required if the soil’s properties were not properly assessed or understood when the original foundation was built meaning it was inadequate from the start. 

Other reasons why underpinning may be needed include:

  • The structure of the building has changed as a result of renovations 
  • New construction (such as an extension) nearby has meant soil needs to be excavated and therefore foundations are impacted 
  • The capacity of existing foundations needs to be increased 
  • If a natural disaster, like an earthquake, flood or drought, has occurred and has damaged the structure of the building or its foundations in some way

Different Types Of Soil

As we’ve highlighted in the section above, one of the biggest factors that impacts whether underpinning is required or not is soil. 

There are a whole host of different types of soil around, and the unique specifications of each type may suggest whether or not a building is more likely to be prone to structural changes. 

For insurance, some soil types may be more susceptible to dry or wet weather. Soil types that are likely to change their conditions due to the weather are known as ‘reactive’ soil types. 

Considering this, it’s always worthwhile understanding what type of soil is under your home and whether or not is more or less likely to have subsidence occur. 

Let’s take a look at the different soil classifications available: 

  • Class A 0-10mm: Known as acceptable soil, it is mostly common in sandy or rocky areas and offers little or no ground movement when moisture changes.
  • Class S 10-20mm: Satisfactory soil, it is slightly reactive to clay sites and offers only slight ground movement from expected moisture changes.
  • Class M/MD 20-40mm: Moderate soil, it is moderately reactive to clay or silt sites and moderate ground movement can be expected as a result of moisture changes.
  • Class H1 / H1-D 40-60mm: A highly reactive soil type, you can experience high-ground movement when the moisture changes.    
  • Class H2 / H2-D 60-75mm: Another highly reactive type, like the classification above you will experience high-ground movement from moisture changes.
  • Class E /E-D – An extremely reactive type, you can expect extreme ground movements when moisture is present.
  • Class P: Known as ‘problem’ sites, these are the types of areas where the most ground movement is expected. It includes soft soils such as clay, silt or loose sand and this classification is used in sites with abnormal moisture conditions or when it is not appropriate to classify the soil in any other way.

Methods Of Underpinning 

If underpinning is required, then there are various methods to choose from.

However, no matter what technique is chosen it is vital that the work is carried out to the highest quality. If not, it can lead to significant damage and, in extreme cases, even the collapse of the property. 

The most common underpinning methods include:

Mass Concrete Underpinning 

This is the most common type of underpinning you will come across and it is also sometimes referred to as the pit method. 

Mass concrete underpinning involves excavating the ground below the existing foundation in a controlled and consistent manner until you reach the stable soil depth that is most suitable for bearing capacity and the underpinning of the foundations. 

Beam And Base Underpinning 

If you require a more advanced method of underpinning, then beam and base underpinning may be the way to go.

It follows the same principle of mass concrete underpinning, but also uses reinforced concrete beams above, below or to replace the foundation footing. The beams are used to transfer the load of the building to the reinforced concrete bases. 

Mini-piled Underpinning 

Another underpinning method is mini-piled underpinning.

This technique screws hollow steel piles below existing foundations. The space between the steel frame will then be filled with concrete or grout which supports the soil. 

This is a particularly useful method if the soil is especially poor or there is limited access for equipment. It is also the most precise method which means it may be more expensive.


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