Underpinning of existing foundations

Did you know that underpinning of foundations is notifiable work under the Building Regulations? You must make an application to the building control authority in all cases, without exception.

If the underpinning works are to be carried out adjacent or in close proximity to another existing building, you need to ensure your client complies with the requirements of the Party Wall Act 1996 by advising them to enter into a party wall agreement with the adjoining owner.

Where a building has been damaged by foundation movement and has required underpinning, there may also be a need to carry out structural repairs or strengthening work to the superstructure. Such works should be designed and specified by a structural engineer or a specialist structural repair company.

Mass concrete underpinning

The most common method is traditional mass concrete underpinning - a simple technique that involves excavating a segment of ground below the existing building foundation in controlled stages, to a depth where suitable bearing strata exists.

The excavation is then filled with concrete and allowed to cure before the next ‘pin’ is excavated. To transfer the building load safely to the new pin, a dry sand cement packing mortar is rammed in between the new and old foundation.

This method is low cost and suitable for shallow depth underpinning. Work can be carried out from one side of the wall and in areas of difficult and restricted access. It is suitable for heavy foundation loads and massive structures and for the formation of new cellars and basements beneath existing buildings.

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Points to remember

• The mass concrete technique cannot be adopted as a solution for all foundation failures. Other techniques will be required to overcome difficulties with groundwater, lose ground or fill.
• Ensure that the decision to use traditional mass concrete underpinning has been made by a competent person such as an experienced structural engineer.
• Where foundation movement has been caused by ground heave, (such as moisture recovery of clay soils following removal or decay of a tree) the underpinning scheme should incorporate appropriate anti-heave protection.
• Underpinning piles may require special design consideration and incorporate reinforcement over their entire length.
• Underpinning works are required to comply with the Construction, Design & Management Regulations (CDM). The Client and appointed contractor are required to enforce a suitable health and safety policy for the underpinning operations.
• The measures required to comply with the CDM Regulations may determine the nature of the proposed underpinning works.

The majority of reputable underpinning contractors offer a 12-year defects insurance guarantee that applies to underpinning, mini piling and foundation works. This is operated by the Association of Specialist Underpinning Contractors. A list of its members is available from www.asuc.org.uk

The free-to-download LABC Warranty Technical Manual has functional requirements and guidance that can help you get your foundation job right.

Further information

Approved document Part A: Structure

Also read: Foundations on clay soil