Project: Deep sewer main installation
Location: Meadowbrook, QLD
Sector: Civil Construction / Water & Sewer Infrastructure
Client: DDS Civil Earthworks
Services Provided: Shoring System Supply, Trench Safety Equipment, Delivery & Site Coordination
Equipment Used: Steel Trench Shoring Boxes, Davit Arm, Handrail System, Ladder Access, Lifting Chains
The Challenge
The sewer alignment required excavation to approximately 6 metres. Under Queensland WHS requirements, any trench 1.5 metres or deeper must be protected against collapse, so full-depth ground support was mandatory rather than optional.
Battering the trench back was not practical. A safe open-cut batter at 6 m would have needed a very wide excavation footprint. The alignment ran directly beside a live road and site fencing. The trench walls had to be supported vertically within a narrow corridor.
Depth also changed the access problem. Workers laying pipe at the base of a 6 m excavation needed a rated means of entry and exit, and a rescue method that did not depend on climbing out unaided. Open trench edges at the surface created a fall-from-height risk for crews working around the excavation.
The Solution
Legend Hire supplied a Steel Trench Shoring Box system, selected for the panel strength and rated wall capacity needed at 6 m depth.
The boxes were assembled at ground level, with spreaders fixed between the steel panels. DDS’s Kobelco excavators then lifted them into position on four-leg lifting chains. The shields were lowered progressively as excavation advanced, so the trench walls were supported before crews entered. Spreader positions were set to give clearance for the sewer pipe being laid through the box.
Legend Hire also supplied the surface and access safety equipment:
- Davit arm: a rated anchor point above the trench for controlled entry, egress and emergency retrieval of workers from the base of the excavation.
- Handrail System: mounted to the top of the shoring boxes to provide edge protection around the open trench.
- Ladder and Ladder Access Platform: a safe, fixed means of access into the shored zone.
This gave DDS a single package covering ground support, access, fall protection and rescue, with one supplier responsible for the whole system.
Engineering Considerations
Panel rating and depth: At 6 m, lateral earth pressure on the trench walls is the governing load. Steel panels were specified over aluminium for their higher rated capacity at depth.
Surcharge from plant and traffic: Excavators working at the trench edge, and the adjacent road corridor, add surcharge load to the trench walls. The system needed to be rated for these loads as well as the soil pressure.
Pipe clearance: Spreader heights were configured to keep a clear working zone for pipe laying and bedding at invert level.
Rescue at depth: At this depth, rescue has to be designed in. The davit arm gave crews a rated retrieval point directly above the work zone throughout the excavation.
Equipment used
- Steel Trench Shoring Boxes: high-capacity ground support for the 6 m sewer excavation
- Davit Arm: rated entry, egress and retrieval at depth
- Handrail System: edge protection at the trench surface
- Ladder & Ladder Access Platform: safe access into the shored zone
- Lifting Chains: rated rigging for placing and relocating shoring boxes
The Outcome
The sewer main was installed at depths of up to 6 m with trench walls fully supported throughout the works. Crews worked at invert level inside a protected zone, with rated access and rescue provision in place at all times.
The shoring system was installed and relocated with DDS’s existing excavators. No additional plant was mobilised.
Frequently Asked Questions
At 6 m, trenches typically need a rated shoring system such as steel trench shoring boxes or slide rail shoring, specified for the soil conditions and surcharge loads. Lighter aluminium systems are generally suited to shallower excavations.
Queensland WHS requirements state that trenches 1.5 m or deeper must be secured against collapse by shoring, benching or battering. Where site space rules out benching or battering, as in road corridors, shoring is the practical option.
A davit arm provides a rated anchor point above the trench for fall arrest, controlled entry and exit, and emergency retrieval. It allows a worker to be recovered from the base of a deep excavation without relying on ladders alone.
A steel trench shoring box is two steel side panels held apart by adjustable spreaders. It is lowered into an excavation to support the trench walls and create a protected work zone for pipe laying and service installation.
