How to prevent a trench collapse

A fatal trench collapse in wet soil illustrates potential causes and prevention of trench digging incidents.

The incident occurred during excavation of a 3m deep and 1.2m wide water diversion trench, in wet ground, using an excavator, reports Buildsafe SA.

The work had not been benched, battered or shored. A surveyor entered the trench to obtain survey data, although he had been instructed by the supervisor not to do so.

Soon after entering the trench a section of the lower face collapsed, pinning the surveyor by his legs against the trench wall. Within seconds the top two thirds of the face also collapsed covering the surveyor with soil and rocks and resulting in his fatality.

Trench collapse causes

• The sides of the trench had been dug, approx.15 degrees from vertical, with no benching or battering.
• The trench had been dug by a new excavator operator whilst the supervisor was away from the site.
• The supervisor returned and saw the trench was incorrectly dug and ordered the operator to stand down.
• The supervisor instructed the surveyor not to enter the trench, but his instructions were ignored.
• Responsibilities for inspection of excavations had not been clearly designated. The Work Permit process had also not been fully applied.
• The job safety and environmental analysis (JSEA) for the excavation included a requirement to bench the sides of the trench.

Trench collapse prevention

• Onsite safety training programme must continuously reinforce the importance of hazard recognition.
• Competency of plant operators must be assessed before being allowed to operate mobile plant.
• Risk control measures contained in job safety and environmental analysis (JSEA) must be followed.
• Compliance with Work Permit systems must be regularly monitored.
• Regular inspections must be carried out of trenching and excavation work.
• Supervisors must acknowledge and carry out their roles and responsibilities including following disciplinary procedures for employees not following recommendations.
• When new risks are identified, work must not proceed until a review of the risk assessment is completed and all relevant risk control measures are in place.

Trench construction laws

OHS Act , 8 (2)(b) Employer to take steps to eliminate hazards, Section 14 General Duties of Employees at work – Every employee has a duty to (a) take reasonable care of themselves (c) obey health and safety rules.

Construction Regulations; CR 7 Risk assessment; (4) ensure employees are informed, instructed and trained by a competent person regarding hazards and related work procedures.

CR 6 Supervision of construction work. The contractor should appoint competent employees to supervise construction work; (2) may appoint competent persons to assist the construction supervisor.

Sources; Buildsafe SA.

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