Worker seriously injured in trench collapse

Possible control measures to prevent similar incidents

A person conducting a business or undertaking (PCBU) that commissions a construction project is the principal contractor (PC) for that project. A PC is also a PCBU and must also comply with the duties of a PCBU. Each construction project will have a PC who has specific duties under the Work Health and Safety Regulation 2011, in addition to their duties as a PCBU. The specific requirements for excavations, including trenches, are located in the Work Health and Safety Regulation 2011, Division 3 – Excavation work.

The PCBU or the PC must manage the risks associated with excavations at the workplace, regardless of its depth. Before starting any excavation work, the risks associated with this kind of task must be managed. Controls can include, but not limited to, the following examples.

Benching and battering

If excavation work is planned to be carried out without positive ground support (shoring or trench boxes), the continuing safety of the excavation will depend on the conditions arising during construction.

Benching is the creation of a series of steps in the vertical wall of an excavation to reduce the wall height and ensure stability. Benching prevents collapse by excavating the sides of an excavation to form one or more horizontal levels or steps with vertical surfaces between levels. Benches should be wide enough to stabilise the slopes and to prevent material from the top falling down to the working area. They should also be sloped to reduce the possibility of water scouring. The size and type of earthmoving machinery to be used, and related haul routes, should be considered when designing the face slopes and widths of benches.

Battering is where the wall of an excavation is sloped back to a predetermined angle to ensure stability. Battering prevents ground collapse by cutting the excavated face back to a safe slope. When benching or battering the walls of an excavation, an angle of repose of 45 degrees should not be exceeded unless designed by a competent person and certified in writing.

Shoring

Shoring and all support systems should be removed in a way that protects workers from ground collapse, structural collapse or being struck by structural members. Before removal begins, temporary structural members may need to be installed to ensure worker safety.

When removing shoring, the support system should be extracted or dismantled in reverse order to its installation. Persons performing the work in the excavation should not work outside the protection of the ground support system. No part of a ground support system should be removed until the trench is ready for final backfill and compaction.

Trench boxes

Trench boxes can be permanently installed or portable and designed to move along as work progresses. Trench boxes are designed and constructed to withstand the earth pressures of particular trench depths and ground types. They incorporate specific lifting points for installation and removal.

Trench boxes differ from shoring as shoring is designed to prevent collapse; while trench boxes are only designed to protect workers if a collapse occurs. Used correctly, trench boxes can provide a safe workspace for workers needing to enter an excavation. Trench boxes should be maintained or they may fail unexpectedly, particularly if they have been abused or misused.

Trench boxes should not be subjected to loads exceeding those which the system was designed to withstand. Earth pressures are reduced when correct benching and battering practices are used. Trench boxes should be stored and transported in accordance with the manufacturer’s instructions.

Trench 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.

Figure 1 Example of a trench box

System of work

Develop a safe system of work for managing the risks associated with excavations. This could include, but is not limited to:

• if high risk construction work is to be carried out, the Work Health and Safety Regulation 2011 requires that a safe work method statement (SWMS) is prepared before the proposed work starts. All duty holders involved in a high-risk work activity must make sure the work is carried out in accordance with the SWMS.
• providing information, training and instruction to workers and others at the workplace associated with excavations for example; the nature and condition of the ground and working environment, weather conditions, static and dynamic loads near the excavation.
• following the manufacturer’s instructions for the installation, use, removal and maintenance of shields and boxes
• PCBU’s must also consult with workers whose health and safety is likely to be affected (consultation must be regular and ongoing). Further guidance is available in the Work health and safety consultation, co-operation and co-ordination Code of Practice 2021 (PDF, 0.54 MB)
• consultation with all relevant parties before work starts. Sometimes a PCBU may share responsibility for a health and safety matter with other PCBU’s who are involved in the same activities or who share the same workplace. In these situations, the PCBUs must exchange information to find out who is doing what and work together in a cooperative and coordinated way so that all risks are eliminated or minimised as far as reasonably practicable.
• conducting an inspection of the work area prior to commencing a task to ensure that it is safe as the condition of soil surrounding excavations can change quickly due to the soil drying out, changes in the water table or water saturation of the soil.

Depending on the task, any remaining risk must be minimised with suitable personal protective equipment. For example; hard hats, gloves, protective footwear, eye protection and high visibility clothing.

The control measures you put in place should be reviewed regularly to make sure they work as planned.