Last Updated: 07/12/2017
The purpose of this safety alert is to highlight the risk of trench collapse when a shoring box, or any other trench support system, does not extend for the full depth of the trench. Shoring boxes are also known as trench shields or drag boxes.
In November 2017 a worker was seriously injured when he was crushed by ground collapse in a trench in Brisbane. He was kneeling down in an un-shored section of a trench one metre below the bottom edge of the shoring box (refer Photograph 1). The ground collapse was sudden and rapidly engulfed the lower part of his body.
A similar incident occurred on the Gold Coast in 2016, also causing serious injuries to the worker.
In both incidents the shoring box did not extend for the complete depth of the trench.
Photograph 1: Unprotected trench below shoring box.
The collapse of any un-shored section of trench can be caused by a variety of factors including the type of ground, ground water, rain, and loading applied to the ground. Where a trench has been excavated below a shoring box, the weight of the shoring box can increase the likelihood of the ground collapsing because the trench wall has a load applied within its zone of influence.
Shoring boxes are an effective way of preventing trench collapse but they will only protect workers who are between the shields of the shoring box.
Shoring boxes or any other trench support system should:
- extend for the complete depth of the trench
- be installed in accordance with the manufacturer's instructions.
There may be conditions where a shoring box can terminate above the base of the excavation such as where competent rock is encountered, but the particular case is to be assessed and signed off by a geotechnical engineer. Competent rock is rock that is structurally sufficient to prevent slippage into the trench (i.e. contains no unfavourably oriented defects such as cracks or joints).
Further information can be obtained from the following:
- Excavation work code of practice 2021 (PDF, 1.9 MB)
- Work Health and Safety Act 2011 - Sections 304 to 306 specify a number of requirements for excavations
- Checklist - planning for safe set-up and operations for excavation and trenching work (DOCX, 0.3 MB)