Underpinning, in a nutshell, is installing new foundations underneath existing foundations when the existing foundations are not able to provide the support that is required from the overlying building.
That support may be an existing condition that has been present for some time or it may be the building is being remodelled so the loads on the foundations differ from the original in either magnitude or position. Underpinning can take many forms, each with their own benefits and drawbacks either technically or on cost and programme.
As leading engineers in the field, Structures-made-Easy have extensive experience of underpinning. Our previous experience includes extending the foundations downwards to minimise the impact of an adjacent tree on cracking within a house, or creating a new basement beneath an existing house, or widening foundations so they can carry new loads from a new extension to a home.
The three examples above all adopted the most common form of underpinning which is new concrete pad or strip footings, either mass concrete or reinforced. This involves excavating beneath a short length of the existing foundation, perhaps 1m long. Temporarily, the wall above can span over this unsupported 1m length. A new deeper and /or wider concrete footing will then be cast and connected to the overlying existing footing so the existing footing bears onto the new one. That 1m length of wall now has a new, deeper footing. This process is repeated in a hit and miss manner along the length of the wall in those 1m long sections until the whole length of the wall has a new deeper and/or wider foundation.
The process is the same for creating a new basement but the underpins will be a combined new concrete wall and foundation to the full depth of the new basement.
Where we are simply extending a foundation downwards to reach stronger ground or beyond the influence of a tree, the concrete underpinning will not need the steel reinforcement bars that are often used in concrete. For creating a new basement, though, those bars are required to help the new concrete work as a water-tight retaining wall.
The primary benefit of this form of underpinning is it is a simple and cost-effective alternative to deeper, piled foundations or complex ground beams. Often the works can be carried out from outside the building, so disturbance to the internal space is reduced.
The risks with underpinning are relatively small. Careful thought is needed by the structural engineer to ensure cracking is not created in other areas of the building by the process. Particular care is also required during the construction works. Whilst it is a simple task, it does require an experienced contractor to manage the key tasks and timings. After installation, some further minor cracking often occurs, possibly in other areas of the house, as the building adjusts to its new supports.
If you would like to discuss further please feel free to contact us using our email support@structures-made-easy.co.uk or call us on 0207 965 7376
Case studies
Brent 01 London NW10
Domestic ground floor side extension to outrigger of a terrace house.
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