Trench collapse injures workers

Two workers have been injured after a trench collapsed at a construction site in August 2019. The workers were installing footings in a trench next to a recently completed stormwater pipeline at the time, according to Workplace Health and Safety Queensland (WHS Queensland), which is investigating the incident. Ground collapse is one of the main risks in excavation work and can happen quickly and without warning, leaving workers virtually no time to escape, the regulator said. While they do not yet know what caused the collapse, WHS Queensland said the type of ground, previous backfill, ground water, rain and loading could all contribute to the failure of unsupported trenches.

To prevent collapse, persons conducting a business or undertaking (PCBUs) or principal contractors (PCs) must assess and manage the risks associated with workplace excavations before work starts — regardless of the trench’s depth. Here, PCBUs and PCs should reference the geotechnical engineer’s report, checking the soil properties and ground conditions, stability and safety of the intended trench excavation, whether any shoring or trench support is needed, whether any ground water is present and the effect of exposure to the elements. They should also consider risks associated with authorised versus unauthorised access to the site, ground slope, disturbed or weakened ground, any previous backfill, adjacent buildings and structures, water courses and trees.

Once identified, risks must be managed according to the hierarchy of control — eliminating the hazard where reasonably practicable. If hazards cannot be eliminated or substituted, PCBUs or PCs should reduce the risk by implementing engineering controls — such as benching, battering or shoring. Benching involves adding steps into the trench’s sides to reduce the wall height, while battering is cutting an excavated face back to a predetermined, safe slope that ensures stability. Shoring involves the temporary installation of hydraulic systems, steel sheet piling and steel trench sheeting that prevent the soil from moving and is commonly used to support sand, silt or clay-containing trenches.

Any additional risk must be managed with administrative controls — including a safe system of work for installing trench footings, adding warning signs near the excavation and establishing exclusion zones — and personal protective equipment, such as hard hats, steelcap boots and high-visibility vests.

Image credit: ©stock.adobe.com/au/Hanoi Photography