What is Underpinning?

Underpinning of damaged foundations using screw piles and brackets

 

Much like beam design, underpinning is an area of structural engineering that is rarely discussed outside of professional circles.

 

A major and often costly procedure, it pays to know the facts and what you are letting yourself in for should you go ahead with it.

 

In this short guide, we will look at underpinning in more detail, exploring why it may be required and the processes involved.

 

Underpinning Explained

 

Underpinning is the process of strengthening the foundations of a building.

 

It is usually required if the foundations of a building have deteriorated, usually the result of subsidence as caused by tree roots and damaged drains, or even natural disasters such as earthquakes.

 

The Process of Underpinning

 

Depending on the severity of the problem, there are typically four different techniques that can be implemented.  These are:

 

  • Soil strengthening
  • Mass Concrete Method
  • Beam and base method
  • Screw Piles and Brackets

 

Soil Strengthening – an alternative to more costly and complex underpinning procedures, soil strengthening is carried out if the soils beneath a building’s foundations are weakened or removed.

 

Also known as soil stabilisation, a specially formulated structural resin is injected into the ground, which effectively replaces the eroded soil.

 

Mass Concrete Method – during this procedure pits are dug beneath the existing foundations and filled with concrete, creating a secondary foundation layer beneath the primary foundations.  It is best suited to structures with shallow foundations.

 

Along with soil strengthening, the benefit of this type of procedure is that the building can still be used in the meantime.

 

Beam and Base Method – this is similar to the mass concrete method described above and involves reinforced concrete.

 

A more advanced form of underpinning, a reinforced concrete beam is placed below the current footing, which then transfers the building’s load onto strategically positioned concrete bases.

 

Screw Piles and Brackets – a relatively quick and cost-effective form of underpinning, screw piles and brackets are employed when more traditional forms of underpinning are unsuitable, usually due to the need for extensive excavation work.

 

In this procedure, a trench is dug around the affected area and screw piles are inserted deep into the ground, acting as anchor points.

 

Support brackets are then attached to the screw piles and positioned underneath the foundations, enabling the structure to be lifted back into a level position.

 

In What Circumstances is Underpinning Required?

 

There are a number of reasons why underpinning may be necessary:

 

  • The soil that supports the existing foundations of a building has eroded, possibly due to leaky drains, tree roots or changes in the weather. Such erosion can cause the foundations to shift or sink (see our blog on subsidence for more information);

 

  • If you plan on building an extra storey, underpinning may be required to increase the load capacity of the existing foundations;

 

  • The foundations may need to be re-stabilised if the structure has moved due to natural disasters such as floods or earthquakes.

 

Only a qualified building surveyor or structural engineer can determine if underpinning is required, so be sure to enlist the help of a professional if any of the conditions above apply to your property.

 

It is also a good idea to carry out a structural survey on a building you are thinking of buying to determine if underpinning has been carried out in the past, as sometimes buildings that have experienced foundation problems can be difficult to insure.

 

Are the foundations of your property in need of repair?  Contact HLN Engineering today for a no obligation quote.

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