News & Insights
Trench Collapses & Injury Risk to Workers
Two workers have been severely injured in recent trench collapses in South Australia in July 2021. In two separate incidents, workers were engulfed, one to waist height, the other to shoulder height, and trapped for approximately 30 minutes. One worker received serious injuries to their internal organs and spine, and the other was left with bruising and severe muscle strain.
A trench collapse occurs when the walls of an excavation fail to contain the pressure of the tonnes of soil pressing upon it. It is essential to implement the appropriate equipment and safety processes when excavating and working in a trench.
Potential Health & Safety Risks
There are known hazards and risks associated with trench construction and excavation. These include trench collapses, cave-ins and side wall deterioration, of these known risks, trench collapse is the most common.
There are also some hidden risks of trench construction and excavation such as:
- Inhalation of toxic fumes.
In the event of a trench collapse, generally severe injuries are obtained or result in a fatality.
Potential injuries to workers during a collapse include broken bones, such as to the extremities, spine and ribs, damage to internal organs, and even death due to asphyxiation or crushing.
Construction workers exposed to and who work in trenches are at risk of the dangers of trench collapse. A person conducting a business or undertaking who proposes to excavate a trench at least 1.5 metres deep must ensure, so far as is reasonable, that the work area is secured from unauthorised access, including inadvertent entry.
In securing the trench or excavation, you must consider:
- The risks to health and safety that could arise from unauthorised access to the work area
- Likelihood of unauthorised access occurring.
The requirement aims to protect other workers on site who may be at risk by restricting access to the excavation area. It applies in addition to the duty that the person with management or control of the construction site has to ensure that the site is secured from unauthorised access from members of the public, for example when the site is near schools, parks, shops or other public places.
There are many contributing factors that can lead to trench collapse, some of these are:
- Trench walls not supported by shoring or trench boxes
- Trenches having been excavated on previously disturbed soil
- Vibration of the ground around the trench area by vehicles operating too closely
- Unsafe distance between spoil pile and the edge of the trench
- Dried trench walls that make trenches weak and subsequent heavy rainfall.
Effective Risk Reduction Controls
Elimination of health and safety risks is the most effective control for trench collapses. If elimination is not practical in a given situation, the risk can be mitigated by one, or a combination of the following:
Implementing engineering controls:
Examples of engineering control measures that minimise the risk of ground collapse are:
This is a method of preventing collapse by cutting the sides of an excavation to create a series of steps to reduce the wall height of the excavation.
This is where the excavated face is cut back to a safe, predetermined slope to ensure stability.
This is a temporary support for trenches to prevent the movement of earth, soil and rock, and to avert ground collapse. Shoring boxes and other trench support systems protect workers who are between the shields of the shoring box from trench collapse.
Shoring is a convention method of ground support for excavations where unstable ground conditions, such as soft or wet ground including sand, silt or soft moist clay are often encountered.
Types of shoring include hydraulic systems, steel sheet piling and steel trench sheeting.
Excavated Material & Loads Near Excavations
Mechanical plant, vehicles and storage of materials including excavated material or other heavy loads must not be situated in the ‘zone of influence’ of an excavation unless the ground support system installed has been designed by an expert, for example a geotechnical engineer, to support such loads.
The zone of influence depends on the ground conditions. It is the zone in which there may be an influence on the excavation including possible ground collapse.
There are further controls available to reduce the risk of harm to workers from trench collapses. Provision of safe, stable and appropriate access and egress points from trenches and excavations is essential for worker safety.
Readily available and appropriate emergency plans and equipment could prevent harm from occurring in the future, especially if it is constantly referred back to on-site.
Soil classification and design needs to be conducted by an expert, such as a geotechnical engineer, and if risks still remain after these actions, administrative controls can be used to minimise and raise awareness on the issue.
Administrative controls include:
- Ensure a ‘dial before you dig’ has been obtained and existing services have been identified and marked
- Implementing and maintaining a safe system of work for installing footings in a trench
- Installing warning signs near the excavation
- Establishing exclusion zones
- Conducting routine inspections
- On-site supervision.
Any remaining risk must be minimised with suitable Personal Protective Equipment (PPE) that are required on-site, such as hard hats, steel capped boots, high visibility clothing (hi-vis), safety goggles, hearing protection and more.
Administrative control measures and PPE rely on supervision and worker conduct and awareness, and are lower on the hierarchy of control.
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