CROSS Safety Report
Collapse of unsupported deep trench excavation
This report is over 2 years old
A worker was trapped within a partially collapsed excavation which was unsupported and not suitably benched.
The reporter highlights a number of causes that may have led to the collapse.
Key Learning Outcomes
For construction professionals:
Acting on concerns and near miss incident reports can ensure work activities are carried out safely
It is good practice to carry out a risk assessment and method statement (RAMS) for all construction activities. This can ensure the sequencing of work activities such as trench excavation are considered and planned.
Consider appointing a competent Temporary Works Coordinator (TWC) on site who should be able to identify and address the risk of a potential trench collapse
Regular toolbox talks are a good way of engaging with work crews and highlighting any risks associated with work activities
In June 2019, the Temporary Works forum (TWf) published guidance on The safe management of temporary works - The basics for small and medium-sized enterprises (SMEs)
For civil and structural design engineers:
Consider how the design risks, such as the risk of water ingress into an excavation and the potential for trench collapse, can be effectively communicated to contractors on site
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A worker was trapped within a partially collapsed excavation whilst undertaking the construction of a ~10m long, 1m wide and 2m deep trench. The excavation was unsupported and was not suitably benched/sloped. A number of caused were identified by the reporter.
Geotechnical ground investigation
The geotechnical investigations identified ground water at ~1m deep. The bottom of the trench was designed to be below the ground water level identified from the investigation. The bottom of the trench was therefore within the ground water zone, indicating that the design did not properly consider this information.
Design risk assessment
The design risk assessment did not identify key risks for the construction team to mitigate. Namely, ingress of ground water and the potential for trench collapse.
The design risk assessment did not identify key risks for the construction team to mitigate. Namely, ingress of ground water and the potential for trench collapse
Temporary works requirements were not clearly identified and resolved. It was not physically possible to complete the works with a benched/sloped excavation. Therefore, it should have been identified that temporary works were required.
Plan of work
The plan of work did not adequately cover the full activities of the trench construction.
Close call reporting
Concerns were previously raised to the site management team when conducting similar work in a trench. These concerns were not recorded or acted upon.
Site inspections and supervision
Site inspections did not identify potential hazards for the works while supervision on site allowed or instructed the worker to enter an unsupported trench.
Expert Panel Comments
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The first report (report 858) in Newsletter 56 referred to ‘repeating the same mistakes’. There cannot be a better example of repeating the same mistakes than this case. It is of continued concern to hear of yet another unsupported trench collapse. All excavations are inherently dangerous and something as deep as 2m requires proper engineering consideration for assuring wall stability.
The worker is lucky to be alive as collapsing unsupported trenches have often had fatal consequences. There are countless news stories of collapsing excavations having fatal consequences, including a contractor being fined £2.6 million after an employee died when a trench he was working in collapsed on him. CROSS often receive reports on this topic and you can search for safety information on trench collapses on the CROSS website.
Industry guidance on the support of excavations
There is valuable industry guidance on the support of excavations, including the Health and Safety Executives (HSE’s) article on structural stability during excavations. This report also highlights the need for suitable temporary works procedures on construction sites that follow the principles of BS 5975 - Code of practice for temporary works procedures and the permissible stress design of falsework.
In June 2019, the Temporary Works forum (TWf) published guidance on The safe management of temporary works - The basics for small and medium-sized enterprises (SMEs). A competent Temporary Works Coordinator (TWC) should be able to identify and address the risk of a trench collapse. Those in the industry with the knowledge and contacts to do so, must help to inform workers and supervisors of the inherent dangers of trenches.
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