Workplace Health and Safety Queensland released a safety alert to highlight the risk of trench collapse when a shoring box or any trench support system does not extend for the full depth of the trench.
The safety alert was issued following an incident last month where a worker was seriously injured when he was crushed by a ground collapse in a trench in Brisbane. A similar incident happened on the Gold Coast in 2016, injuring a worker.
The shoring boxes did not extend for the complete depth of the trench in both incidents.
“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,” Workplace Health and Safety Queensland stated in its safety alert.
The safety agency said shoring boxes should 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),” the safety alert states.