Underpinning is a process whereby additional support is laid below ground level to strengthen a building’s existing foundations.
Though underpinning 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 methods and techniques available.
This article helps you understand what the various underpinning options are so that you can discuss them with your surveyor or structural engineer before making a choice.
Depth and breadth
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.
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 into the earth. No matter what type of underpinning a building needs to undergo, the process is likely to be disruptive, time consuming and expensive. It is also likely to lead to problems finding underpinned property insurance down the line.
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.
Need insurance with underpinning?Get an insurance quote!
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 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.
No matter what type of underpinning you have had, you are likely to have difficulty getting underpinned house insurance. If you have had your property underpinned in the past, underpinning insurance is likely to be a much greater risk than insurance for a comparable property that has not had its foundations reinforced.
Insurance for underpinned houses can still be obtained from a specialist insurance broker, though it might require a little research to find. With HomeProtect you can get a competitive online quote for underpinned property insurance based on the risk you face today and not during the underpinning process.
Very Helpful, easy to arrange home insurance, particularly for a property that had some higher risks indicated. Would recommend. Wednesday, 11 October 2017