CROSS Safety Report
Lack of control on site when underpinning
This report is over 2 years old
The concern is in relation to the sequencing of construction and the lack of control on site from competent persons during underpinning works.
Key Learning Outcomes
Quality control and competent supervision on site can help to ensure that underpinning works are carried out in accordance with the design
Having a competent temporary works designer/adviser in place to supply an engineered solution can ensure all temporary works are carefully considered and planned
Verification of temporary works erection by a competent person who can oversee and coordinate the whole process can also ensure the works are installed correctly
The design of underpinning works to basements should be carried out by a suitably qualified and experienced engineer
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The concern is in relation to the sequencing of construction and the lack of control on site from competent persons. The reporter says that a Victorian building with four storeys and a basement was being converted to a small hotel. As part of this work the basement level was being lowered by approximately 1m. The construction sequence adopted was to excavate the soil from the inside to the founding level across the whole basement, before underpinning the walls.
The footings for the retaining walls were on fill material, as was the backfill, so in the temporary case there was no lateral restraint to the toe of the walls and there was inadequate vertical support under the base of the wall. To make matters worse fill material was being stored close to the basement retaining walls at ground floor level, thus adding a substantial surcharge load.
The contractors had to be persuaded to add some temporary propping. A structural engineer was employed to carry out a design but had no remit to inspect the works on site. Their design did not take account of the additional retained height of fill against the walls in the permanent condition, nor did it suggest a safe method of construction.
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There have been several reports to CROSS of failures during building alteration and some have resulted in total collapse. Alteration of old buildings carries risks of structural movement and failure that are often underestimated by owners, developers and building contractors. This report highlights yet again that critical strength and stability states can easily occur during construction.
These are particularly dangerous in exposing construction workers carrying out alterations to severe risk. The report also highlights the dangerous interface in transition to a finished design where no competent party is in charge of stability during construction. Carrying out alterations in basements is complex and often entrusted to builders who may not fully appreciate the dangers. In this case, a proper design risk assessment could at least have highlighted the dangers of sideways footing movement and required a staged process of underpinning and restraint.
In this case, a proper design risk assessment could at least have highlighted the dangers of sideways footing movement and required a staged process of underpinning and restraint.
In a safety-critical industry that nonetheless permits unlicensed parties to operate and permits restricted briefs which prevent structural engineers from obtaining a full appreciation of the project, this situation will arise. There is an argument to say that under the Institution of Structural Engineers (IStructE) code of conduct, in order to ensure the safety of others, any engaged structural engineer must make enquiries to determine the appropriateness of his contribution to the project as a whole and must pass on sufficient information to the client or others on any assumptions made e.g. construction sequence.
Perhaps engineers should simply refuse to take on such works unless they have the full design and supervision brief. Indeed, there are cases of engineers being prosecuted by the Health and Safety Executive (HSE) for site failures. There is a need to encourage clients to engage qualified and competent structural engineers and give them an adequate brief.