Excavation and trenching WHS duties for residential builders

What it is

Excavation and trenching covers any work that removes soil or rock from the ground, including service trenches, footings, pool excavations, retaining wall cuts and bulk site cuts on residential jobs. The model Work Health and Safety (WHS) Regulations treat excavation as one of the highest risk activities on a construction site because cave-ins, falls into trenches and contact with underground services injure and kill workers across AU every year.

The Safe Work Australia model Excavation Work Code of Practice is the practical reference and the model WHS Regulations set the legal duty.

When excavation work becomes High Risk Construction Work

The model WHS Regulations (regulation 291 in the model and the equivalent reg in each state) list 18 categories of High Risk Construction Work (HRCW). Excavation triggers HRCW status in three common ways on a residential site:

• Work in a trench or shaft with an excavated depth greater than 1.5 metres
• Work in or near a tunnel
• Work involving the disturbance of an asbestos containing material

Once the work is HRCW, the principal contractor or PCBU directly engaging the workers must prepare a Safe Work Method Statement (SWMS) before the work starts. The SWMS has to be site specific and task specific. Generic excavation SWMS templates that do not address the actual ground conditions, trench depth and access do not meet the duty.

Controls for a safe excavation

The hierarchy of controls drives the choice of method. For trenches at or above 1.5 metres deep the practical options are:

Battering

Cutting the trench walls back to a safe angle so the soil cannot collapse on a worker. The required angle depends on soil class. Reactive clay and loose sand need a flatter batter than stable rock.

Benching

Stepping the walls in a series of horizontal steps. Useful where space allows and the depth runs past three metres.

Shoring

Installing shields, trench boxes, hydraulic struts or sheet piles to hold the walls in place. The only option where space, services or boundaries prevent battering or benching. Shoring needs to be designed by a competent person, often a structural engineer, for any trench beyond standard manufactured trench shield dimensions.

A combination of these methods is normal. A 2.4 metre service trench in residential reactive clay might use a hydraulic strut shoring system at the base with a one metre batter back at the top.

Other duties triggered by excavation work

Underground services

Dial Before You Dig (now Before You Dig Australia) plans must be obtained before any ground penetration. The PCBU has to verify the location of services on site, normally by potholing or non-destructive digging within the tolerance zone.

Edge protection and access

Any excavation deep enough that a person could fall into it needs edge protection or barricading. Safe access in and out has to be planned. A typical control is a ladder secured at the working face with no more than 7.5 metres horizontal travel.

Inspection

The Code of Practice expects daily checks of the excavation before work starts and after any event that could affect stability. Heavy rain, vibration from plant or freeze-thaw cycles all change ground conditions.

Spoil placement

Excavated material has to be stored at least the depth of the trench away from the edge to prevent surcharge loading from collapsing the wall.

Documentation that needs to be kept

A residential builder running an excavation needs the following on the job:

• Current Before You Dig Australia plans
• Site-specific SWMS signed by every worker doing the HRCW
• Shoring design or trench shield certification documents
• Pre-start and ongoing inspection records
• Plant pre-start records for any excavator or trencher
• Operator high risk work licence copies where the plant requires one

The SWMS must be available to a WHS inspector on request. If the work changes (different soil, different depth, different method), the SWMS has to be reviewed and updated before the change takes effect.

Where residential builders get caught

Three failure modes show up regularly in AU prosecutions:

• Treating a 1.6 metre trench as "not really" HRCW because it is only just over the threshold. The Regulations do not work that way. Once the depth passes 1.5 metres the duties apply.
• Using a generic excavation SWMS that does not name the soil class, the chosen control method or the access arrangements for the actual site.
• Allowing a worker into the trench to "just grab something" without the shoring in place. Most cave-in fatalities happen during these short unprotected entries.

Penalties under the model WHS Act for failing to comply with a duty (a Category 2 offence) reach $1.5 million for a body corporate.