The trench rule is 1.5 metres, and it triggers two separate duties

What excavation work actually covers

Broader than digging a hole. The Code’s definition is work to make an excavation or to fill or partly fill an excavation, and it notes the work commonly includes work involving the removal of soil or rock from a site to form an open face, hole or cavity, including trenches, shafts and tunnels. Backfilling is excavation work. So is partial backfilling.

The exclusions are worth knowing because they are narrow and specific. The requirements do not apply to a mine, a bore to which a relevant water law applies, or a trench used as a place of interment. That last one is not a curiosity: it means a grave is carved out of this Code by name, and everything else that looks like a trench is not.

Excavation work is also construction work

This is the structural point people miss, and the Code states it plainly: Work connected with an excavation is a type of ‘construction work’. The consequence follows immediately, that when carrying out excavation work, the requirements relating to construction work must also be complied with.

So the excavation duties sit on top of the construction duties rather than replacing them. Anyone treating an excavation as a self-contained job with its own rulebook is reading half the obligations.

The first 1.5 metres: a SWMS

Construction work carried out in or near a shaft or trench with an excavated depth of greater than 1.5 metres, or a tunnel, is high risk construction work, for which a Safe Work Method Statement (SWMS) must be prepared.

Note greater than, and note in or near. The trigger is not only working down in the trench; work beside one deep enough counts. And a site commonly has several kinds of high risk construction work at once. The Code allows for that: where different types occur together, one SWMS may be prepared to cover any high risk construction work activities being carried out at the workplace, or a separate SWMS for each. One document covering the lot is permitted, provided it actually covers the lot.

The second 1.5 metres: secure it, and support every side

A separate regulation, WHS Regulation 306, attaches at the same depth and does different work. A PCBU proposing to excavate a trench at least 1.5 metres deep must ensure, so far as is reasonably practicable, that the work area is secured from unauthorised entry including inadvertent entry.

Then the part that decides whether people live: the PCBU must minimise the risk to any person arising from the collapse of the trench by ensuring that all sides of the trench are adequately supported, by one or more of:

• shoring by shielding or other comparable means
• benching, or
• battering

All sides. Not the side you are working against. The Code adds that combining methods may be most effective, and that In built-up areas or streets the excavation may have to be fully or partly sheeted or supported to prevent collapse due to localised vehicle movement, which is the traffic passing a few metres away doing what traffic does to saturated ground.

Why the slope that held yesterday proves nothing

Benching and battering are the methods that work without positive ground support, and the Code is careful about what they depend on. It describes a safe excavated slope as one where the slope does not flatten when left for a considerable period, there is no movement of material down the slope, and the toe of the slope remains in the same place.

And then the conditional that carries the risk. If work is planned without shoring, the continuing safety of the excavation will depend on the conditions arising during construction. Where conditions are not as expected or change, for example different soils or heavy rain and flooding, action should be taken immediately to protect workers, other persons and property, including suspending work until the ground is stable or providing positive ground support.

That is the whole argument against experience as a control. A batter angle that held through a dry week is a statement about last week’s ground. Rain overnight, a different soil horizon two metres along, a truck parked where there was none, and the assumption underneath the batter has quietly changed.

Where this meets confined spaces

Our explainer on what counts as a confined space notes that a trench is not a confined space on the risk of structural collapse alone, only if it may contain harmful airborne contaminants. This is the other half of that: collapse is governed here, in the excavation duties, with its own threshold and its own controls. A deep trench with bad air is both, and needs both sets of controls, not whichever one the crew is trained in.