In August 2019, two workers were injured after a trench collapse at a construction site. Early enquiries indicate the two workers were installing footings in a trench next to a recently completed storm water pipeline.
For reasons unknown, the earth around the pipeline collapsed during the trench work.
Investigations are continuing.
Preventing a similar incident
Excavation work usually involves the removal of soil or rock from a site to form an open face, hole or cavity using tools, machinery or explosives. Specific excavation hazards include:
- ground collapse involving the fall or dislodgement of earth or rock
- previous disturbance of the ground including previous excavations
- instability of adjoining structures caused by the excavation
- underground essential services - including gas, water, sewerage, telecommunications, electricity, chemicals and fuel or refrigerant in pipes or lines.
Ground collapse is one of the primary risks which need to be controlled in excavation work as it can happen quickly and without warning, giving workers virtually no time to escape, especially if the collapse is extensive. The collapse of any un-shored (unsupported) section of trench may be caused by a variety of factors including the type of ground, any previous backfill, ground water, rain, and loading applied to the ground.
The person conducting the business or undertaking (PCBU) or the principal contractor (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. Effective risk management starts with a commitment to health and safety from those who manage the project, including company officers.
Managing work health and safety risks is an ongoing process. Risk management involves four steps:
- Identify hazards - find out what could cause harm.
- Assess risks - understand the nature of the harm that could be caused by the hazard, how serious the harm could be and the likelihood of it happening.
- Control risks - implement the most effective control measure that is reasonably practicable in the circumstances.
- Review control measures - ensure they are working as planned.
When assessing the risks associated with excavation work, consider the following;
- the local site conditions; including access to the excavation, any previously disturbed or weakened ground, any previous backfill, ground slope, adjacent buildings and structures, water courses (including underground) and trees
- the depth of the excavation
- the geotechnical engineer's report, including any information on; stability and safety of the trench excavation, any shoring or trench support requirements, the soil properties ground conditions, presence of ground water, effect of exposure to the elements
- unauthorised access.
Once the risks have been assessed, the next step is to control the risks. These control measures are ranked from the highest level of protection and reliability to the lowest (hierarchy of control). The WHS Regulation 2011 includes specific obligations for a PCBU to manage the risks associated with excavation work, including trenches.
Health and safety risks must be managed to eliminate the hazard. You must always aim to eliminate a hazard, which is the most effective control. If this is not reasonably practicable, you must minimise the risk by one or a combination of the following:
- Implementing engineering controls – examples of engineering control measures that minimise the risk of ground collapse include;
- Benching – this is a method of preventing collapse by excavating the sides of an excavation to create a series of steps to reduce the wall height of the excavation
- Battering – this is where the excavated face is cut back to a safe predetermined slope to ensure stability.
- Shoring – temporary support for trenches to prevent the movement of soil and therefore ground collapse. Shoring boxes and other trench support systems will protect workers who are between the shields of the shoring box from trench collapse. Shoring is a common method of ground support in trench excavation where unstable ground conditions, such as soft or wet ground including sand, silt or soft moist clay are often encountered. Types of shoring include hydraulic systems, steel sheet piling and steel trench sheeting.
- Administrative controls – if risk remains, it must be minimised by implementing administrative controls. For example;
- implement and maintain a safe system of work for installing footings in a trench
- installing warning signs near the excavation
- establishing exclusion zones
- Personal protective equipment (PPE) - any remaining risk must be minimised with suitable personal protective equipment. For example;
- provision of hard hats, steel cap boots, high visibility vests.
Administrative control measures and PPE rely on human behaviour and supervision and used on their own tend to be the least effective in minimising risks.
From July 2012 to June 2018, there were 359 accepted workers' compensation claims involving pits, trenches or excavations. Almost half (45%) of these claims resulted in a serious injury (involving 5 days or more off work).
From July 2012 to August 2019, OIR was notified of 254 incidents involving workers or bystanders either being injured, or at risk of injury relating to excavations and trenches.
In the same period, 623 statutory notices were issued addressing the risk of trenches and excavations across all industries. Approximately 80% of the notices issued were in the construction industry.
Prosecutions and compliance
In 2017, a company was fined $75,000 after a truck driver sustained fractures to his ribs, back and pelvis and other serious injuries. The company was involved in the excavation of a trench to lay sewer pipes. Another trench was located adjacent to the trench excavation work being carried out. The spoil was along half the length of the north side of the trench. The truck driver was instructed to pass a tool from a worker inside the trench to the excavator operator on the side of the trench. As he approached the excavator, the trench collapsed engulfing him in the spoil and side of the trench.
In 2016, a company was fined $30,000 after a worker was seriously injured when the trench he was working in collapsed. The trench was over 4m deep but the company had supplied a trench box that was only 2.4m deep and did not cover the lower section of the wall. The ground it was resting on collapsed and the worker sustained serious injuries including a fractured pelvis.
- Excavation work Code of Practice 2013 (PDF, 2.61 MB)
- How to manage work health and safety risks Code of Practice 2011 (PDF, 1.02 MB)
- Work health and safety consultation, co-operation and co-ordination Code of Practice 2011 (PDF, 0.47 MB)
- Excavations hazard