Excavation SWMS Australia: Trenching, Hazards and Controls

Trenching and excavation kill workers in Australia every year, almost always in ways that were foreseeable and controllable: a trench wall collapses onto someone working below, or a machine strikes a live service. This guide explains when a Safe Work Method Statement is legally required for excavation work under the model WHS Regulations, and what a genuinely protective excavation SWMS has to cover, ground collapse, locating services, plant and people, atmospheres, water and access. Requirements can vary between states and territories, so always check the rules that apply where you work.

When does excavation work need a SWMS?

The clearest trigger is depth: under the model WHS Regulations, construction work involving a trench or shaft deeper than 1.5 metres is high-risk construction work, so it needs a SWMS before it starts. But depth is not the only trigger, and a shallow excavation can still require a SWMS, for example where the work is on or near underground services, on or near energised overhead electrical lines, in or near a confined space, in an area with a contaminated or flammable atmosphere, or where there is movement of powered mobile plant. Each of those is its own high-risk construction work category. So the question is not only "how deep?" but "does this excavation involve any high-risk category?", and most real excavation jobs hit more than one.

What a SWMS must contain

Under the WHS Regulations a SWMS must identify the high-risk construction work, specify the hazards and risks, describe the control measures, and describe how those controls will be implemented, monitored and reviewed. It must be developed in consultation with the workers, prepared before work starts, kept readily accessible, and followed, if the work stops matching the SWMS, work stops until it is corrected. For excavation, the SWMS has to be built around the actual ground and services on this site, not a generic trenching description, because the controls that keep a trench safe depend entirely on the conditions.

Ground collapse, the risk that defines trenching

The hazard that makes trenching so dangerous is the one people underestimate: the collapse of the excavation face. A cubic metre of soil weighs well over a tonne, enough to crush or asphyxiate a person, and collapses happen suddenly and without warning, often in trenches that "looked fine". The SWMS must set out how the ground is controlled, chosen for the actual soil and depth: benching (cutting steps), battering (laying the walls back to a safe angle for the soil type), or shoring and shielding with hydraulic shores or a trench box. As a rule, no one enters an unsupported trench 1.5 metres deep or more, and for deeper or complex excavations a competent person, often a geotechnical engineer, assesses the ground and specifies the support. Previous excavation, backfilled ground, nearby loads (plant, spoil, structures), vibration and water all change the risk and belong in the assessment.

Locate services before you dig, BYDA and potholing

Striking a buried gas main, water main, electrical cable or optical fibre, or contacting overhead powerlines with a boom or tipper, is a leading cause of serious excavation incidents. The SWMS should set out the service-location process before any machine breaks ground: lodge a Before You Dig Australia (BYDA) enquiry and obtain the asset plans, mark up the site, use cable and pipe locators to confirm positions, and then pothole, expose services by hand digging or non-destructive (hydro or vacuum) excavation before the excavator goes near them. Plans are indicative, not exact; the only way to know where a service is, is to expose it. The SWMS should also carry a clear plan for what to do if a service is struck, for a gas strike, an electrical strike, or a water strike, because the first minutes matter.

Overhead services and powered mobile plant

Overhead powerlines are struck by excavator booms, tipper trays and cranes far more often than people expect. The SWMS must identify overhead lines, apply the exclusion and no-go zones set by your regulator or the asset owner, and use spotters and physical controls (goal posts, tiger tails) where plant works near lines. On the ground, excavators and other powered mobile plant working alongside people on foot is a hazard in its own right: the SWMS should separate plant and pedestrians, control the slew, swing and reversing zones, use a trained spotter or traffic controller where needed, and address quick-hitch safety, ground bearing capacity and the risk of the machine overturning near the edge of the excavation.

Spoil, edges and the load on the trench wall

Spoil and materials placed too close to the edge add load to the trench wall and are a frequent cause of collapse. The SWMS should require spoil and stockpiles to be set back from the edge, commonly at least the depth of the excavation, or a distance set by the competent person, and keep plant, materials and traffic away from the zone that surcharges the wall. Barricades, plates, lighting and public exclusion around an open excavation protect people on foot and belong in the method too.

Atmospheres and water

Deep excavations, pits and connections into sewers or drains can become confined spaces or develop hazardous atmospheres, oxygen deficiency, or flammable or toxic gases collecting at the bottom. Where that risk exists, the SWMS has to treat it as such: atmospheric testing before and during entry, ventilation, and the confined-space controls that go with it. Water is the other changing hazard: rain or groundwater entering a trench undermines the walls and adds weight, so dewatering, monitoring and a rule to get people out when water rises are part of the method.

Access, egress and staying out of the trench

People need a safe way into and out of the excavation, typically a secured ladder within a short distance (commonly around nine metres) of anyone working in the trench, so no one is ever far from a way out. The best control, though, is not being in the trench at all: where the work can be done from outside, using long-reach tools, or trenchless methods, the SWMS should favour keeping people out of the excavation entirely.

Competence, plant and the wider record

The SWMS should set out who is competent to do the work and operate the plant, and reflect that only trained operators run the excavator and other machines. As a business, the SWMS is one document among many: plant inspection and maintenance records, operator competencies and licences, service-location records, worker inductions, insurances and, where soil may be contaminated or contain asbestos, the relevant assessments all need to be current and produceable. Complys keeps the SWMS and the associated worker, plant and contractor records organised against the work, and flags what is expiring, which is what contractor management software and construction compliance software are for.

Consult, monitor and review

Develop the SWMS with the crew, induct every worker into it and record their sign-on. Because excavation conditions change as you dig and as the weather turns, monitor the excavation and review the SWMS whenever the ground, the water, the method, the plant or the services picture changes. Water entering a trench, a change in soil, a service found where the plans said there was none, or the excavation going deeper than planned are all review triggers, and if the work stops matching the SWMS, the work stops until it is fixed.

Excavation SWMS checklist

• Whether the work is high-risk (trench/shaft over 1.5m, near services, confined space, atmosphere, or mobile plant)
• Ground control for the actual conditions: benching, battering, shoring or shielding, with a competent-person assessment where needed
• BYDA enquiry, service plans, locators and potholing before machine digging
• A strike plan for gas, electrical and water services
• Overhead services kept clear, with exclusion zones and spotters for plant
• Separation of powered mobile plant and people on foot; slew, reversing and quick-hitch controls
• Spoil and material set back from the edge; public barricading of the open hole
• Atmosphere testing and confined-space controls where relevant
• Dewatering and a rule to exit when water rises
• Safe access and egress, and keeping people out of the trench where possible
• Competent workers and plant operators; plant inspection records
• Worker consultation, induction and recorded sign-ons
• Monitoring changing conditions and clear review triggers

Keep your excavation SWMS with the project rather than in a ute or an inbox: Complys keeps the SWMS and the associated worker, plant and contractor records organised against the work. See also SWMS software Australia, how to write a SWMS, and construction compliance software. You can create and manage your SWMS free for 30 days.