Ground collapse can occur quickly and without warning, giving a worker very little time to escape.

Protective systems must be designed by a competent person in accordance with the Temporary Works Procedure (linked below).

Where excavations are over 1.5m in depth or there is potential for collapse, suitable protective systems must be in place to prevent collapse and protect workers.

Protective systems must be capable of withstanding all loads that could reasonably be expected to be applied or transmitted to the system.

Where excavations and trenches may affect the stability of adjacent structures, support systems must be provided to ensure the stability of the structure and protect the workers in the excavation.

Typical systems used to prevent the collapse of an excavation include benching, battering, shoring and using shields.

Benching

Benching is the creation of a series of steps in the vertical wall of an excavation to reduce the wall height and ensure stability.

Benching is a method of preventing collapse by excavating the sides of an excavation to form one or more horizontal levels of steps with vertical surfaces between levels.

Excavation Work Code of Practice 2013 - Queensland

In Victoria, no vertical side is to exceed 1 meter.

Victorian Compliance Code – Excavation 2019

Battering

Battering is where the wall of an excavation is sloped back to a predetermined angle to ensure stability.  Battering prevents ground collapse by cutting the excavated face back to a safe slope.  Battering should start from the bottom of the excavation.  In some circumstances, it may be appropriate to use a combination of benching and battering.

Battering only

A combination of battering and benching

*Pictures from Excavation Work Code of Practice 2013 - Queensland

When benching or battering the walls of an excavation, the angle of repose must not exceed 45 degrees unless designed by a geotechnical engineer and certified in writing.

The image below shows a bucket (engineered and certified) used at the Goulburn-Murray Water Connections Project by a local Subcontractor to increase efficiency and ensure trench compliance with benching and battering.

Shoring

Shoring is a positive ground support system that can be used when the location or depth of an excavation makes battering or benching impracticable. Shoring systems must be designed by a competent person such as a Geotechnical Engineer and be deemed suitable for the area it is being used on.

Some common types of shoring are:

• Hydraulic systems
• Steel sheeting piling
• Steel trench sheeting
• Prefabricated concrete panel, and
• Ground anchors

Ground anchors for sheet piling

Excavation Work Model Code of Practice. Safe Work Australia (2018)

Shields (also known as trench or shoring boxes)

A shield is a structure which can withstand the forces imposed by ground collapse and to protect workers within it.

They are designed and constructed to withstand the earth pressures and incorporate specific lifting points for installation and removal

Typical Shoring Box

Excavation Work Code of Practice 2013 - Queensland

Shields differ from shoring as shoring is designed to prevent collapse, while shielding is designed to protect workers if a collapse occurs.

Ground conditions within the excavation area must be considered when selecting protective systems to ensure they are suitable and will effectively control the risk of collapse.

Excavations are considered a form of temporary works.  This means the Temporary Works Procedure applies to all excavations and we must consider the design requirements and checks that must be in place prior to excavating.

Refer to GMR 7 for further information on controlling temporary works.

If an excavation is cutting into stable rock and is not at risk of collapse, A Geotechnical Engineer may determine controls such as benching or battering the face of an excavation unnecessary.

Tunnelling Design Requirements

All underground excavation works will be designed with primary (temporary) support controls identified and documented within the IFC (issued for construction) design drawings / specifications.

The design will set out the parameters and constraints that must be incorporated as part of the excavation and support activity.  These requirements will be reflected in the Permit to Tunnel (PTT).

Excavation and tunnelling support requirements will vary due to the uniqueness of each design.  These requirements, along with the construction methodology will be determined in conjunction with the construction team and Designer.

The time or curing period required prior to access being granted into these areas will be defined within the PTT.  This will include shotcrete curing strength or duration of curing and testing regimes for the support classification. During the curing period, physical barriers are to be used to prevent unauthorised access to the area.  

Accessing areas under unsupported ground is not permitted.