In February 2018, a worker's foot and ankle were injured when part of a three metre high near vertical excavation face collapsed. The injured worker was part of a group building a boulder retaining wall. Four workers were gathered at the toe of the excavation when they heard a cracking noise and the excavation face collapsed. Three of them managed to jump out of the way but the fourth was buried up to his knees. Investigations are continuing.
Preventing a similar incident
Excavation failures are particularly dangerous because they can happen quickly, giving no chance for workers or others in the vicinity to escape, especially if the collapse is extensive.
The speed of an excavation collapse increases the associated risks and the consequences are significant as falling earth can bury or crush people. This can result in suffocation or serious crush injuries. Examples of excavation specific hazards include:
- underground services such as gas, water and sewerage
- the fall or dislodgement of earth or rock
- falls from one level to another
- falling objects
- the instability of adjoining structures caused by the excavation
- previous disturbance of the ground including previous excavation.
When assessing the risks associated with excavation work the PCBU should establish the location of underground services before directing or allowing excavation work to commence, and consider:
- local site conditions, including access, ground slope
- depth of the excavation
- soil properties, including variable soil types, stability, shear strength, cohesion, presence of ground water, effect of exposure to the elements
- fractures or faults in rocks
- any specialised plant or work methods required (e.g. ground support)
- the number of people involved
- local weather conditions
- the effect of mobile plant close to the excavation.
The PCBU must ensure that an emergency plan is prepared for the workplace.
If a worker must enter a trench where there is a risk of engulfment, control measures should be implemented regardless of the depth of the trench, such as:
- shoring by shielding or other comparable means (e.g. boxing)
Section 306 of the Work Health and Safety Regulation 2011 sets out requirements for trenches of at least 1.5 metres deep. Where shoring, benching or battering of the trench is not carried out, a geotechnical engineer must be engaged to assess the safety of the trench where persons are required to enter the trench. However, in the case of a bulk excavation (i.e. where only one excavated face poses a risk) the written advice of a geotechnical engineer may also be needed. The geotechnical engineer's report should include:
- details of the soil conditions
- any shoring or trench support requirements
- dewatering requirements
- any longer term effects on stability and safety of the excavation
- comment of the effects of rain
- minimum distances that mobile plant may approach the excavation
- limitations on the use of machinery or storage of equipment in the vicinity of the trench or excavations that may affect it's stability.
Any written advice should state the period of time to which it applies and may be subject to a condition that specified natural occurrences may create a risk of collapse.
Since, 2012, there have been 358 accepted workers' compensation claims for injuries involving pits and trenches. Workplace Health and Safety Queensland has issued 157 improvement notices, 116 prohibition notices, 2 electrical safety protection notices and 2 infringement notices associated with excavations and trenches. Sixty-nine per cent of these were to construction industry workplaces.
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.
Also in 2017, a company was fined $40,000 when a 15 year old first year apprentice plumber fractured his leg. The apprentice was clearing soil from around underground power cables to expose them for further trench digging works to be carried out by an excavator. The cables were 500mm below the surface and he was instructed to finish the job using hand tools to remove the remaining soil. The work was in the near vicinity of an open trench that was approximately 2.6 metres deep and 2 metres wide with unstable and unsupported sides. He had finished clearing soil from around the cables and moved back toward and into the larger trench to move away from the approaching excavator. As he moved into the larger trench a large piece of concrete, approximately 400kg, fell from the side of the trench onto his leg.
In 2016 a company was fined $30,000 after a worker was seriously injured when the trench he was working in collapsed. The worker was working in a trench that was over 4 metres deep. The company had supplied a trench box that was only 2.4 metres deep and did not cover the lower section of the trench wall. The ground it was resting on collapsed and the worker sustained serious injuries including a fractured pelvis.
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