CROSS Safety Report
This report is over 2 years old
When reviewing a consulting engineer’s proposal to underpin a party wall, the reporter noticed that there was insufficient vertical load to maintain stability under lateral load from the retained height of soil.
Key Learning Outcomes
For civil and structural design engineers:
It is important to recognise and know the boundaries of your expertise and work within the limits of your competence
For the construction team and client:
Safety critical works such as underpinning should be assessed and designed by a suitably qualified and experienced engineer
Consider appointing a competent temporary works coordinator (TWC) on site who should be able to ensure all temporary works are carefully considered and planned
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A reporter received an appointment from an adjoining owner’s party wall surveyor to comment on proposed basement and other works to an adjacent site. The consulting engineer for this site proposed to underpin the party wall to a depth of 3.8m using 325mm thick mass concrete. A simple glance at the details showed that there was insufficient vertical load to maintain stability under lateral load from the retained height of soil.
The drawings showed no requirement for temporary works and the site was so restricted that propping would have been impractical unless the ground and basement floors were designed to be built in a properly thought out sequence.
The consultant engineer designing the scheme demonstrated little understanding of the stability issues for the underpinning and, according to the reporter, was swayed by the architect into specifying the proposal to save space in the proposed basement.
Following several months of deadlock (the party wall award was not forthcoming until the reporter was satisfied with proposals), the scheme was radically revised and the stability issues were properly addressed, but the reporter is concerned at an apparent lack of understanding regarding stability of underpinning and the designer/contractor interface so far as responsibility for safety is concerned.
Expert Panel Comments
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The party wall surveyor was correct to call in an engineer because if the original scheme had progressed, a major collapse would have been likely with possible loss of life. The report highlights a lack of competency and failure to understand the processes of structural design (i.e. risk management) as well as the obligation on the designer to demonstrate how a scheme is to be safely constructed. Small projects can exhibit substantial risks. General comments are the same as for report 196 which discussed undermining shallow foundations.
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