Subsidence

Underpinning types

Underpinning is a process whereby additional support is laid below ground level to strengthen a building’s existing foundations. This may be required for a variety of reasons, perhaps because the original foundations are not strong or stable enough, or because the original purpose of the structure has changed.

subsidence home wall cracked

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It might become necessary to undertake underpinning if the conditions or characteristics of the soil beneath a building's foundations have changed, or that they were not correctly identified during the initial construction.

Underpinning might be needed if the development of nearby buildings leads the soil to be weakened around the existing structure's foundations. Or if it is more economical to use and strengthen an existing structure's foundations for a development, rather than building new foundations from scratch.

Depth and breadth

To underpin a property, you need to extend foundations in depth and/or breadth so that it either rests on soil that can provide better support or simply redistributes the weight over a larger surface area. Underpinning will often involve the use of micro-piles or jet grouting to achieve this.

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 method", which dates back a hundred years and has barely changed since. It involves digging box-shaped voids beneath a structure's existing foundations and filling them with concrete in a specified order for a "foundation below a foundation" 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

A refinement of the mass concrete method is the "beam and base method", where a reinforced concrete beam is 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.

Mini - piled

"Mini-piled" 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.

Underpinned insurance

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.


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