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CROSS Safety Report

Undermining shallow foundations

Report ID: 196 Published: 1 July 2010 Region: CROSS-UK

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Overview

A reporter raises concerns about the tendency for existing shallow footings to be exposed and possibly undermined when adding extensions to older properties.

Key Learning Outcomes

For homeowners:

  • Competent builders should be appointed who have the relevant experience for carrying out the required works

  • Building control bodies can assist by providing advice at the start of the project with local knowledge of construction, and if there are doubts about the competence of the builder

Full Report

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A reporter’s firm has come across several similar problems when constructing extensions to older properties. The problem is a tendency for existing shallow footings to be exposed and possibly undermined during the process of reducing levels for the proposed new floor. This appears to occur more regularly on sites underlain with clay soil where a suspended floor is proposed for the extension.

In such a situation the reduced level for the slab may be more than 500mm below existing ground level which in many instances will expose shallow foundations. Such exposures and undermining can occur along significant foundation lengths (corresponding to the full extension width) with obvious dangers.

Expert Panel Comments

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Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.

Undermining existing foundations is a problem frequently encountered by building control bodies, and there are numerous examples of resultant substantial failures. The situation is predictable in older buildings and the designers should arrange for hand dug trial pits to determine the level of existing construction as part of the hazard elimination and risk reduction measures.

Excavation on a ’hit and miss’ principle may well be necessary, coupled with site visits at an early stage. This problem can also be addressed by employing competent builders and building control bodies can assist by providing advice at the start of the project with local knowledge of construction, and if there are doubts about the competence of the builder.

 CROSS is aware of incidents where foundations have been undermined leading to gross collapse of the wall overhead and death of those involved in the excavation.

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