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675 Collapse of unsupported trench

Report ID: 675

Published: Newsletter 56 - October 2019

Report Overview

A worker was trapped within a partially collapsed excavation which was unsupported and not suitably benched.

Report Content

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.

The reporter lists the following causes:

  • 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 geotechnical investigation and 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.
  • Design review: 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: Site inspections did not identify potential hazards for the works.
  • Supervision: Supervision on site allowed or instructed the worker to enter an unsupported trench.

Comments

The first report in this Newsletter 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, and below is a list of just a few CROSS reports on this topic.

128 House collapse
214 Need for licensed builders
368 Potentially dangerous excavation
576 Worker trapped in excavation
800 Retaining wall excavation collapse
AUS-6 Lift pit excavation hazards

There is valuable industry guidance on the support of excavations, including 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 addressed 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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View other CROSS reports published in Newsletter 56


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