Guide 06 · Excavation
Trenching, service strikes and backfill
Two hazards define how a trench is run: collapse and service strike. Queensland's Work Health and Safety Regulation treats a trench 1.5 metres or deeper that a person enters as high-risk work requiring collapse protection and a safe work method statement. Locating live services before the bucket moves handles the second hazard.
Last reviewed September 2026 · General guidance, not a substitute for a site-specific SWMS
Why a metre and a half matters
A cubic metre of soil weighs well over a tonne. A collapse does not need a deep trench to be fatal — it needs enough material above chest height to prevent breathing.
Queensland regulation sets 1.5 metres as the threshold where a trench a person enters becomes high-risk construction work: collapse must be prevented by benching, battering the walls back to a safe angle, or installing shoring such as a trench shield, and the work requires a safe work method statement prepared before it starts.
Depth is not the only trigger. Ground conditions decide how a trench behaves: coastal sand on the Gold Coast will not stand vertically at any depth, while stiff clay holds until it dries, cracks and drops a slab of material without warning. Vibration from adjacent plant, rain on the spoil heap and previously disturbed ground all reduce stability further.
The controls that follow from that are unglamorous and non-negotiable: spoil set back from the edge, ladder access within reach, edge protection where people work nearby, and nobody in an unsupported trench regardless of how short the task looks.
Locating services: plans, then proof
Before You Dig Australia plans show approximate positions supplied by asset owners. They are the starting point, not the answer, and the gap between the two is where strikes happen.
Request the plans
Lodge the enquiry for the work area and read every asset owner's response, including the notes on accuracy and required clearances.
Locate on the ground
Electronic locating traces conductive services and tracer wires, and the marks go on the surface in the standard colours before any machine arrives.
Expose by hand or vacuum
Every crossing is proved visually. Non-conductive assets — older poly water, PVC conduit, fibre without tracer — are invisible to a locator and only a physical dig confirms them.
Work to the clearance
Asset owners nominate no-go zones around high-pressure gas, transmission cable and trunk water. Those distances dictate the method, and in some cases the alignment moves instead.
A strike on a live electrical cable or a gas main endangers the operator, the neighbourhood and the program. The cost of exposing a crossing by hand is a fraction of the cost of hitting it.
Bedding, overlay and backfill: the part that shows up later
A pipe is only as good as what surrounds it. Backfill decides two things: whether the pipe survives the load above it, and whether the surface holds its level in five years.
| Zone | Purpose | What goes wrong |
|---|---|---|
| Bedding | Uniform support along the pipe's full length at the designed grade | Pipe laid on rock or lumps of clay, creating point loads and future cracks |
| Haunching | Material worked in under the pipe's sides to resist deformation | Voids left beneath the barrel, allowing the pipe to flatten under load |
| Overlay | A protective layer above the pipe before general fill returns | Rubble tipped straight onto the pipe, damaging it during compaction |
| General backfill | Reinstating the trench in layers, compacted to suit the surface above | Deep uncompacted lifts that settle into a trench-shaped depression |
| Surface | Concrete, asphalt, pavers or turf restored to the original standard | Thin patch over poor fill, cracking within a season |
Material and compaction requirements come from AS/NZS 3500, the pipe manufacturer's specification and the authority's standard drawings. Trafficked areas add compaction testing to the hold points.
Questions
What documents should a contractor give me before digging?
A safe work method statement for the excavation, the Before You Dig response, plant and operator competency records, insurance certificates, and a traffic management plan where the work touches a road reserve.
Can a trench be left open overnight?
Only when it is secured: covered or fenced, signed, and protected from rain-induced collapse. On residential sites with children or pets we plate or backfill rather than fence.
How close can you dig to a tree I want to keep?
Structural roots determine the answer, and protected or significant trees carry council conditions. Where the alignment must pass close, hand excavation or boring under the root zone preserves the tree.
Do you guarantee the reinstatement?
The quote names the finish, the area and the standard, and the work is compacted in layers to meet it. Settlement caused by our backfill is ours to rectify.
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