Trench Collapse Risk on a Boundary Sewer Connection
A crew is laying a 150 mm sewer line to a new dwelling. The trench runs 1.8 m deep along a side boundary, 1.2 m from an existing brick fence footing and within 2 m of a mature tree. The soil is a firm sandy clay, but recent rain has left the trench walls damp and a small slump has already occurred on one side overnight. The excavator has a 600 mm bucket, and the crew plans to use a 1.2 m aluminium trench shield. A worker wants to enter the trench to hand-grade the bedding. The site supervisor is off-site. There is no shoring certificate on file and the shield has not been inspected since the last job.
1. Assess whether entry is permissible and identify the controls required before anyone enters.
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Entry is not permissible as described. Key issues:
- Depth exceeds 1.5 m, triggering a requirement for a documented excavation risk assessment and appropriate support system.
- The soil is damp and has already slumped, indicating instability - a shield alone may not be adequate without assessment of the slump zone.
- Proximity to the fence footing and tree means load from structures and root disturbance increase collapse risk.
- The shield has not been inspected and there is no certificate or assessment.
- The supervisor is absent, so no competent person is overseeing the entry.
Controls before entry:
- Stop work and secure the area with barriers and signage.
- Have a competent person inspect the trench, shield, and adjacent structures; document the assessment.
- Inspect and certify the shield; reject if damaged.
- Install the shield with adequate overlap and secure it against movement; consider battering back the slumped side.
- Provide a means of escape within the trench, gas testing where required, and a spotter at the surface.
- Confirm no entry until these are in place and a competent supervisor is present.
2. Recommend how to complete the bedding work with minimal entry, and explain the trade-off.
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Options to reduce or eliminate entry:
- Use the excavator with a grading bucket to shape the bedding to grade, then verify with a laser or level from the surface.
- Use a remote or long-handled screed to finish the bedding.
- Pre-place bedding and laser-check before lowering pipe, then use pipe lasers to confirm grade.
- If entry is unavoidable, limit it to short durations with the shield fully installed, a spotter present, and the competent person on site.
The trade-off is precision versus safety: machine grading is faster and safer but may need more careful checking to meet the 1:60 grade and bedding depth. Hand work gives finer control but exposes workers to the highest risk. The defensible choice is to minimise entry, document the grade checks, and only allow short, supervised entry if the shield and assessment are in place.