Calling in the Contractors

Reasons and Types of House Underpinning

5 September 2017 by Frank Elliott

House underpinning is a construction method employed to an already existing building or structure to strengthen its foundation and rest its load across a larger area.

Reasons for house underpinning

• The strength of the initial foundation has withered with time
• Construction of adjacent structures has had an impact on the soil structure supporting the foundation
• Natural catastrophes such as floods, earthquakes have caused the strength of the foundation to deteriorate
• The structure is to be put to other use than was intended
• The structure is targeted for increased load capacity by the addition of another storey; as well as a few other reasons

House underpinning methods

The most common house underpinning techniques are listed below.

Concrete Underpinning

This is loosely referred to as traditional underpinning, as it is the oldest technique employed over the years.

This method requires no machinery and strengthens the foundation manually by digging boxes under the foundation and filling them with concrete. The eventual result is a foundation built below the initial faulty one.

Its advantages include its simplicity and the low cost of labour. However, concrete underpinning is not the best to address a foundation collapsing due to poor ground conditions as the added weight of concrete can make the situation more critical.

Beam and Base Underpinning

In this advancement of the concrete underpinning method, a reinforced concrete beam is constructed underneath the current footings to support the original structure.

Trenches are dug along the wall of the building with a steel framework placed in the trenches and filled with concrete to create a reinforced base. Once the concrete has dried, the reinforced concrete beam transfers the building load to support the old foundations and prevent further damage.

This method provides access to plumbing for improvements and allows structural alterations in case of the need to change structural purpose.

Mini-piled underpinning

This method is employed in areas with restrictive access while also contending with dire soil conditions such as pollution. It transfers the structural load from the foundation to stable soil with minimal disturbance to the surroundings.

A hole is made underground through the existing foundation wall, with a needle beam cast through the wall to redistribute the load. This technique allows for greater depths to be reached with ease.

The various types of mini piles are listed below.

• Driven or jacked performed piles. these can be either precast concrete or steel-cased
• Bored and continuous flight auger (CFA) cast-in-place piles
• Screw piles
• Driven grouted steel piles
• Proprietary types of mini-piles such as CFA displacement piles

House underpinning cost

The actual cost implication of underpinning is dependent upon:

• Cause of the problem
• Profile of the structural footings
• Nature of the foundation soil
• Unforeseeable underground conditions
• Duration to complete the work
• Quantity of materials required

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