5.1 Excavation & Trenching Safety: Shoring, Battering & Benching

Excavation and trenching safety

A drainage trench can fail suddenly and without a useful warning. Soil type alone does not establish safety: water, surcharge, vibration, previous excavation, services, weather, and time all affect stability. The PCBU must eliminate or minimise risk so far as is reasonably practicable and use competent input where the ground-support decision requires it.

The 1.5 m rule is not a safe-depth rule

WorkSafe guidance says an excavation face more than 1.5 m deep should be shored so far as is reasonably practicable unless the face is cut back to a safe slope, the material is proven stable under all foreseeable conditions and no worker is exposed to danger, or shoring is impracticable and other precautions provide equivalent protection. A trench shallower than 1.5 m can still collapse, especially in fill, sand, wet ground, beside traffic, or near a service trench.

Notifiable excavation work includes a trench in which a person is required to work where the depth is more than 1.5 m and is greater than the horizontal width at the top. Notify WorkSafe through the prescribed process before carrying out notifiable work. Notification does not approve an unsafe method and does not replace the site risk controls.

Controls

Prefer eliminating entry: assemble from outside, use remote excavation or inspection, or redesign the sequence. Where entry is required, select shoring, shielding, benching, or battering for the soil and loading. A trench shield protects a person within it but does not necessarily prevent ground movement outside it. Hydraulic shores, timbering, and proprietary boxes must be installed and used to their design and manufacturer instructions.

Keep spoil, plant, and stored material at least 1 m from the excavation edge. This reduces surcharge and prevents material falling onto workers. Control vehicles and vibration. Use stop blocks, a spotter, and separation so an excavator cannot slew over or strike a person in the trench.

Provide safe access and egress appropriate to the excavation length, depth, and emergency plan. WorkSafe guidance does not establish a universal “ladder every 9 m” rule for all New Zealand drainage trenches. Secure ladders, extend them appropriately, and keep the route clear. Prevent falls at edges with barriers suitable for the exposure.

Water, atmosphere, and services

Divert surface water, dewater without undermining the face, and reassess after rain. Treat an excavation that could contain a hazardous atmosphere as a separate atmospheric risk; if it meets the confined-space criteria, apply the confined-space controls. Locate services using plans, utility information, detection, mark-out, and non-destructive exposure. Plans alone are not proof of exact location.

Inspection

A competent person inspects the excavation and support system before work, daily while occupied, and after events that can affect stability, including rain, vibration, ground movement, impact, or alteration. Record findings and stop entry if there is cracking, slumping, water inflow, support movement, loose material, or another changed condition.

Scenario

A 1.4 m trench in loose saturated fill beside a delivery route is not automatically safe because it is below 1.5 m. Vehicle surcharge, vibration, water, and poor material increase collapse risk. Move or isolate traffic, remove surcharge, control water, and have a competent person select protective measures before entry. Conversely, a 1.8 m excavation is not made safe merely by sending a notification; it still needs an engineered or otherwise demonstrably safe system.

The certifying drainlayer’s supervision includes checking that technical pressure to preserve grade never overrides worker safety. If shoring changes the available trench width, revise bedding and installation planning rather than removing protection.

Independent source-check exercise

For 5.1 Excavation & Trenching Safety: Shoring, Battering & Benching, practise answering from evidence rather than recall. Write the controlling source, its edition or effective date, its scope, and the exact paragraph, table, figure, or manufacturer instruction that supports the decision. Then state the site inputs that can change the result. In this section those inputs include Excavations deeper than 1.5 m generally require shoring unless safely battered, proven stable under foreseeable conditions without danger, or another permitted exception applies.; A shallower trench can still collapse and must be assessed.; Keep spoil, plant, and materials at least 1 m from the excavation edge unless an engineered system provides otherwise..

Build a four-column check: proposed condition, source requirement, evidence observed, and action. Test the answer with one changed assumption. A different pipe size, discharge-unit load, ground condition, licence status, authority, compliance path, or consent condition may produce a different result. If it does, explain why; if it does not, show which requirement remains controlling.

Finish with a field-verification plan. Name what must be inspected before concealment, what instrument or record demonstrates it, who has authority to accept a change, and what appears on the as-built or completion file. This exercise turns an open-book fact into certifying-level judgement. It also exposes accidental mixing of standards: if the source for a dimension differs from the source for the test or approval, either return to one complete compliance path or document the proposal as an Alternative Solution. Do not promote an example value into a national rule.