What is underpinning

Underpinning is a process whereby additional support is laid below ground level to strengthen a building’s existing foundations. 

To underpin a property, you need to extend foundations in depth and/or breadth so that they either rest on soil that can provide better support or simply redistribute the weight of the building over a larger surface area.

Though underpinning house foundations is often seen as a last resort when tackling subsidence these days, there are occasions where you have no other option. The technologies involved in the underpinning process have developed a great deal in recent years, and there are now a number of different underpinning methods and techniques available.

This article helps you understand what the various options for underpinning foundations methods are so that you can discuss them with your surveyor or structural engineer before making a choice.

Soil strengthening

Where soils are weakened, or extracted, a process of soil strengthening may be used instead of underpinning, which involves injecting a special type of grouting or structural resin foam into the earth. Foam underpinning foundations can usually be done quite quickly and without you needing to move out of your home.

Mass concrete method

The traditional type of underpinning has historically been the mass concrete or mass pour method, which dates back a hundred years and has barely changed since. It involves digging box-shaped voids to a specified depth beneath a structure’s existing foundations and filling them with concrete in a specified order for a foundation-below-the-foundations effect.

This process is usually used when the property’s existing foundations are shallow but can be implemented even when they are particularly deep. Heavy machinery is not usually needed, labour costs can often be kept down and sometimes the structure can continue to be used while the work is carried out.

Beam and base method

The beam and base method is a refinement of the mass concrete method and involves a reinforced concrete beam being built either above, below or to replace the current footing. This beam redirects the weight of the structure onto several mass concrete bases placed at certain strategic positions, the placement and size of which depend on the ground conditions and the size of the affected property.

The type of construction used to build the beam will depend on the type of building and where loads are applied. Where there is a danger of clay soils expanding after the measures are implemented, anti-heave precautions will usually be incorporated into the design of the underpinning works.

Screw piles and brackets

Underpinning with screw piles and brackets can be employed where traditional underpinning is too impractical, owing to the need to excavate to an excessive depth, or where there’s no space to get a piling rig close to the property. This underpinning method can be carried out much more quickly than the traditional mass pour techniques, so costs and inconvenience can be reduced.

Mini-pile

Mini-pile underpinning methods are useful where ground conditions are variable or where access to the foundations is limited, and allows the loads to be transferred down to stable grounds at depths usually greater than five metres. The technical installation of mini-piles is more involved than mass concrete, requiring specialist machinery and skills.

Pile and beam underpinning

Pile and beam underpinning can be carried out in conjunction with mini-piling processes. It involves the installation of mini-piles to either side of an affected wall, then a subterranean pocket of brickwork is removed so a pre-fabricated steel cage can be installed to span across the two piles. Once the cage is inspected by Building Control, it can be concreted to complete the underpinning.

Cantilever pile and beam underpinning uses a similar approach to pile and beam underpinning, only all of the works are undertaken from the property’s exterior. In this way, the removal of internal floors and fixtures is not required so the costs and inconvenience can be substantially reduced. This method can also be carried out in conjunction with screw piling.

Piled raft underpinning is a system used when the whole property has been affected by subsidence. All floors need to be removed and mini-piles need to be constructed, before pockets of brickwork can be taken out for the reinforcement to be placed under an engineer’s instruction. After inspection, the modifications are concreted to complete the underpinning.