When Does An Excavation Need Shoring?

In NSW, a trench 1.5 metres or deeper needs its sides supported by shoring, benching or battering, unless a geotechnical engineer has advised in writing that all sides are safe from collapse. A basement excavation goes further, because it needs shoring wherever a sloped cut won't fit inside the site or would undermine something next door.

So depth is only the start of the answer. The ground, the water, the loads near the edge and the neighbours all have a say, and a shallow cut in the wrong place can be more dangerous than a deep one in good rock.

The 1.5 metre rule

The rule comes from the NSW work health and safety regulation and SafeWork NSW's Code of Practice for excavation work. For a trench at least 1.5 metres deep, the person running the work has to minimise the risk of collapse by supporting all sides, using one or more of three controls.

Shoring holds the wall up with shields or a built support system.

Benching cuts the wall into a series of steps.

Battering slopes the wall back to a safe angle.

A trench or shaft deeper than 1.5 metres is also high risk construction work, so it needs a safe work method statement before anyone starts.

Shallower than 1.5 metres

The 1.5 metre figure isn't a safe depth. The code says that where there is a risk of engulfment when someone enters a trench, those controls should go in regardless of how deep it is.

Soil is heavier than people expect, and a person bent over in a trench 1.2 metres deep is working with their head below the top of it. Loose sand, wet ground or old fill can let go at a depth that looks harmless from the surface. On this one, call me before the trench is open, not after.

The written advice exception

A geotechnical engineer can advise in writing that the sides of a trench are safe from collapse, and that advice can take the place of shoring, benching and battering. The code expects it to state how long it applies, and it can carry conditions for events such as heavy rain.

It suits a trench cut in sound rock, for a set period. It isn't a blanket approval, and the moment the ground or the weather changes, so does the answer.

Why a basement is a different question

A trench is narrow and short-lived. A basement is wide, deep and open for months, with people and plant working at the bottom, so the question moves from the trench rule to how the whole site stays stable.

A batter is the cheapest way to hold a face, but it takes land. The safe angle comes out of the geotechnical report, and even a modest slope in soil runs a long way back from the bottom of the cut. On a Sydney site built boundary to boundary, the batter would finish in the neighbour's yard or the footpath. That is usually what decides it, and the wall types are compared in the [shoring and bulk excavation guide](/blog/shoring-bulk-excavation-developer-guide).

Rock changes things again. Sound sandstone can stand close to vertical, as long as the joints don't dip into the cut, and a face that looks perfect can still drop a wedge. We inspect those faces as they are uncovered and add rock bolts and shotcrete where the joints call for them.

Water in the ground

Water changes every answer above. A clay face that stands up well in a dry week can slump after rain, and a sand face below the water table will flow into the hole.

Excavations below the water table need a plan for that water before the digging starts, and the methods are set out in [dewatering and groundwater control in excavations](/blog/dewatering-and-groundwater-control-in-excavations). The material you stockpile counts too, because the code warns against piles of spoil holding back run-off at the edge.

Loads near the edge

The code calls the area around an excavation its zone of influence, and it depends on the ground conditions. Excavators, concrete trucks, material stacks and spoil heaps should stay out of it, unless the support system has been designed by a competent person to carry them.

That is why we ask the builder early where the concrete pump and the crane will stand. A shoring wall designed only for soil won't carry a truck that parks on top of it, and the sums are in [surcharge loads on retaining walls](/blog/surcharge-loads-on-retaining-walls).

Neighbouring footings

Any excavation that goes below the level of a neighbouring footing can affect that structure, retaining walls included. The code says it should be assessed by a competent person and held by a designed support system, and work must not start until steps are in place to prevent collapse.

That means shoring or [underpinning](/blog/underpinning-adjoining-structures-explained) comes before the excavator gets near the boundary. A dilapidation report records the neighbour's building before any of it happens.

How we approach it

We design the excavation as a sequence of stages rather than a finished hole, because the wall is often at its weakest halfway down. The permanent walls are designed under AS 4678, the temporary stages are checked against the same ground model, and the excavation procedure goes on the drawings.

A collapsed trench gives nobody time to react. That is why this one gets decided before the first bucket of soil comes out.

ACSES provides geotechnical engineering, including shoring and bulk excavation design, across Sydney. Talk to us about a project.

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