Preventing collapse of bulk excavations

Background

This safety alert replaces the 2011 alert titled ‘Ground support for excavations’ and provides updated information on this issue.

WorkSafe is aware of bulk excavations being carried out without adequate ground support. Some excavated sites have required complete backfilling and compacting as ground support could not be retrofitted without placing workers at risk from collapse of excavation walls.

As underground parking becomes more common in housing, small residential and commercial projects, principal contractors are required to undertake bulk excavations to build these facilities.

Due to site restrictions, vertical walls are often excavated close to site boundaries. The collapse of unsupported excavation walls puts workers and the public at risk of:

• being buried
• falling into the unprotected excavation
• drowning if the excavation floods; for example, as a result of heavy rainfall or damaged pipes.

The collapse of excavation walls may damage buried utility services, like gas or sewerage pipes, and undermine the structural integrity of:

• neighbouring buildings and structures permanent or
• site fencing
• excavation fall protection barriers
• footpaths and road ways.

Control measures

Workers and the public must be protected from the risks associated with the collapse of excavation walls, even where these walls are low (for instance, less than one metre). Risk controls must be implemented in accordance with the hierarchy of control set out in Part 5.1 of the OHS Regulations.

Before bulk excavation work starts, the employer must:

• engage suitably qualified persons (for example, experienced geotechnical and civil engineers) to assess ground conditions and determine the appropriate ground support or retention system for the site
• work with engineers to develop a safe system of work for the installation of the ground support system
• develop an emergency response plan (ERP) to deal with the risk of a person becoming engulfed by soil or other material.

When excavating, ensure:

• a competent person experienced in excavation work is supervising the work the engineer’s ground support design is followed
• safe work method statements (SWMS) are developed and followed for the excavation and ground support works
• (if ground support is to be progressively installed) workers never work ahead of the support or remove it prematurely
• workers are trained and aware of the SWMS and ERP
• the public is prevented from accessing the edge of the excavation or the construction site – ensure all perimeter fencing (including temporary fencing) is secured to prevent collapse.

While the excavation remains open the principal contractor should ensure the excavation and site security is inspected regularly by a competent person – and as soon as possible after any event that could affect the safety of the excavation, like a storm or ground slip.