Trenching Safety: Understanding the Zone of Influence Around an Excavation
Trenching Safety: Understanding the Zone of Influence Around an Excavation
Trenching is a routine part of earthmoving and civil construction, but it is also an activity where seemingly small decisions can have serious consequences.
Where the excavator is positioned, where spoil is stockpiled, where trucks travel and even where other materials are placed can all affect the stability of a trench.
One area that operators and workers should understand is the zone of influence.
What Is the Zone of Influence?
The zone of influence is the area of ground surrounding an excavation that may be affected by additional loads or forces.
These loads can include:
excavators and other earthmoving equipment
trucks and vehicles
stockpiled spoil
pipes and construction materials
site buildings or structures
cranes and lifting operations
vibration from plant or nearby traffic
groundwater and changing ground conditions
The important thing to understand is that there isn’t one universal distance that defines the zone of influence on every excavation.
Its size will depend on factors including the soil and ground conditions, depth and shape of the excavation, groundwater, nearby structures and the loads being placed around it.
If additional loads are placed within the zone of influence, they can increase pressure on the sides of the excavation and contribute to ground movement or collapse.
Keep Spoil and Heavy Loads Away From the Edge
One of the simplest trenching controls is also one of the easiest to get wrong.
Don’t automatically pile excavated material directly beside the trench.
Spoil adds considerable weight to the surrounding ground. The larger the stockpile becomes, the greater the additional load being placed around the excavation.
Where practicable, excavated material should be placed outside the zone of influence.
The same principle applies to pallets, pipes, attachments, concrete products and other heavy materials.
If loads need to be located close to the excavation because of limited working space, the excavation and its ground support may need to be specifically designed to accommodate those loads.
Think About Where the Excavator Is Sitting
The excavator itself is a substantial load.
Operators should consider their position in relation to the excavation rather than simply working as close to the edge as possible.
The weight of the machine, movement of the tracks and vibration generated while operating can all influence the surrounding ground.
Particular care should also be taken when travelling alongside an open trench.
The question shouldn’t simply be:
“Am I far enough away that I won’t fall in?”
It should also be:
“Could the weight of my machine affect the stability of this excavation?”
Where plant needs to operate close to an excavation, appropriate controls and, where required, advice or design from a suitably qualified person should be in place.
Trucks Can Create the Same Problem
Excavators aren’t the only machines that matter.
A loaded truck can place a significant load on the ground surrounding a trench.
Site planning should therefore consider where trucks will:
approach the excavation
turn around
wait to be loaded
travel alongside the trench
tip or unload material.
Don’t establish a haul road beside an excavation simply because it is the most convenient route.
Traffic management and excavation safety need to be considered together.
Flagging and Barricading the Excavation
A trench should also be clearly identified and protected so workers, visitors and plant operators know where they can and cannot go.
The appropriate control will depend on the excavation and the risk.
Controls may include:
highly visible bunting or flagging
star pickets and parawebbing
temporary fencing
solid barricades
guardrails
signage
designated pedestrian routes
physical separation between plant and people.
Flagging or bunting should not automatically be treated as fall protection.
Its purpose may simply be to identify an exclusion area or warn people not to enter.
Where there is a genuine risk of someone falling into an excavation, a stronger physical control such as fencing, guardrails, covers or another suitable barrier may be required.
Don’t Put the Barricade Right on the Edge
Another common mistake is installing warning bunting immediately beside the trench.
That can mean someone has already entered the hazardous area before reaching the barrier.
Where flagging or barricading is being used to establish an exclusion area, it should be positioned an appropriate distance back from the excavation based on the site conditions and risk assessment.
In lower-risk situations, such as excavations in unpopulated greenfield areas, a setback barricade and signage may be suitable. Higher-risk areas may require physical edge protection or secure fencing.
Remember that the exclusion zone, fall-protection area and zone of influence aren’t necessarily the same thing.
Each is controlling a different risk.
Make the Exclusion Zone Obvious
Whatever system is used, everybody on site should understand what it means.
If yellow bunting means “do not enter”, workers shouldn’t have to guess whether they’re allowed to cross it.
Consider:
where authorised access points will be
how workers will safely enter and exit the trench
whether pedestrians need a separate route
whether plant needs a separate operating area
whether barriers can be seen at night or in poor weather
whether signs are required
whether members of the public could access the excavation.
A barricade is only useful if people understand why it is there.
Safe Access Into the Trench
Workers should never have to climb down the side of an excavation or use the excavator bucket as their normal means of access.
Where people need to enter an excavation, provide suitable access such as a secured ladder, stairs, ramp or another appropriate access system.
Access points should also be positioned so workers aren’t unnecessarily exposed to operating plant or unstable ground.
Emergency access and rescue arrangements should be considered before work begins, particularly where there is a risk of engulfment.
Watch for Changing Ground Conditions
An excavation that was stable yesterday isn’t automatically safe today.
Conditions can change because of:
rain
groundwater
drying or cracking soil
vibration
nearby excavation work
additional spoil
vehicles travelling nearby
damaged shoring
ground movement or slumping.
Open trenches and their controls should be regularly inspected, particularly after weather events or anything else that could affect their stability.
Operators are often in a good position to notice these changes.
Cracks forming near the edge, material falling from the trench wall, unexpected water or ground movement should never simply be ignored.
Deeper Trenches Bring Additional Requirements
Excavation requirements vary between Australian jurisdictions, so contractors need to check the rules that apply where they are working.
As a general national reference, excavation work in or near a shaft or trench with an excavated depth greater than 1.5 metres is classified as high-risk construction work under the model WHS framework and requires a Safe Work Method Statement.
Victoria also has specific requirements for high-risk construction work involving trenches deeper than 1.5 metres.
Depending on the work and jurisdiction, excavation notification requirements may also apply.
Before You Dig
Trench safety starts before the bucket enters the ground.
Before excavation begins, consider:
Underground services – obtain current service information and positively identify services where required.
Ground conditions – understand the soil, groundwater and potential for collapse.
Excavation design – determine whether battering, benching, shoring or a trench shield is required.
Zone of influence – determine where plant, spoil, vehicles and other loads can safely be positioned.
Traffic management – plan plant and vehicle movements around the excavation.
Fall protection – determine the appropriate fencing, barricades, guardrails or other controls.
Access and egress – provide safe ways for workers to enter and leave the excavation.
Emergency procedures – have appropriate procedures in place before work starts.
Communication – make sure operators, workers and subcontractors understand the excavation controls.
Ongoing inspection – continue checking the excavation as conditions change.
The Takeaway
The edge of the trench isn’t necessarily the edge of the danger area.
The zone of influence can extend beyond the excavation, and loads placed within that area can affect the stability of the ground and the excavation itself.
That means trench safety isn’t just about what happens inside the trench.
It’s also about what’s happening around it.
Think about where the excavator is positioned. Think about where trucks are travelling. Keep spoil and heavy materials away where practicable. Establish clear exclusion areas. Provide proper fall protection where required. And don’t assume that because an excavation looked safe when it was dug, it will remain that way.
A few extra minutes spent setting up an excavation properly can prevent a much bigger problem later.
For ready-to-use safety documentation, check out our SWMS and JSA templates, designed specifically for earthmoving and civil construction businesses to help make site safety paperwork simpler.
Important: This article provides general safety information only and is not a substitute for a site-specific risk assessment, SWMS, engineering or geotechnical advice, or the requirements of the relevant WHS/OHS regulator. Excavation requirements differ between Australian states and territories. Always check the requirements that apply to the work being undertaken.
Trenching Safety: Understanding the Zone of Influence Around an Excavation
Trenching Safety: Understanding the Zone of Influence Around an Excavation
Trenching Safety: Understanding the Zone of Influence Around an Excavation
Trenching is a routine part of earthmoving and civil construction, but it is also an activity where seemingly small decisions can have serious consequences.
Where the excavator is positioned, where spoil is stockpiled, where trucks travel and even where other materials are placed can all affect the stability of a trench.
One area that operators and workers should understand is the zone of influence.
What Is the Zone of Influence?
The zone of influence is the area of ground surrounding an excavation that may be affected by additional loads or forces.
These loads can include:
The important thing to understand is that there isn’t one universal distance that defines the zone of influence on every excavation.
Its size will depend on factors including the soil and ground conditions, depth and shape of the excavation, groundwater, nearby structures and the loads being placed around it.
If additional loads are placed within the zone of influence, they can increase pressure on the sides of the excavation and contribute to ground movement or collapse.
Keep Spoil and Heavy Loads Away From the Edge
One of the simplest trenching controls is also one of the easiest to get wrong.
Don’t automatically pile excavated material directly beside the trench.
Spoil adds considerable weight to the surrounding ground. The larger the stockpile becomes, the greater the additional load being placed around the excavation.
Where practicable, excavated material should be placed outside the zone of influence.
The same principle applies to pallets, pipes, attachments, concrete products and other heavy materials.
If loads need to be located close to the excavation because of limited working space, the excavation and its ground support may need to be specifically designed to accommodate those loads.
Think About Where the Excavator Is Sitting
The excavator itself is a substantial load.
Operators should consider their position in relation to the excavation rather than simply working as close to the edge as possible.
The weight of the machine, movement of the tracks and vibration generated while operating can all influence the surrounding ground.
Particular care should also be taken when travelling alongside an open trench.
The question shouldn’t simply be:
“Am I far enough away that I won’t fall in?”
It should also be:
“Could the weight of my machine affect the stability of this excavation?”
Where plant needs to operate close to an excavation, appropriate controls and, where required, advice or design from a suitably qualified person should be in place.
Trucks Can Create the Same Problem
Excavators aren’t the only machines that matter.
A loaded truck can place a significant load on the ground surrounding a trench.
Site planning should therefore consider where trucks will:
Don’t establish a haul road beside an excavation simply because it is the most convenient route.
Traffic management and excavation safety need to be considered together.
Flagging and Barricading the Excavation
A trench should also be clearly identified and protected so workers, visitors and plant operators know where they can and cannot go.
The appropriate control will depend on the excavation and the risk.
Controls may include:
Flagging or bunting should not automatically be treated as fall protection.
Its purpose may simply be to identify an exclusion area or warn people not to enter.
Where there is a genuine risk of someone falling into an excavation, a stronger physical control such as fencing, guardrails, covers or another suitable barrier may be required.
Don’t Put the Barricade Right on the Edge
Another common mistake is installing warning bunting immediately beside the trench.
That can mean someone has already entered the hazardous area before reaching the barrier.
Where flagging or barricading is being used to establish an exclusion area, it should be positioned an appropriate distance back from the excavation based on the site conditions and risk assessment.
In lower-risk situations, such as excavations in unpopulated greenfield areas, a setback barricade and signage may be suitable. Higher-risk areas may require physical edge protection or secure fencing.
Remember that the exclusion zone, fall-protection area and zone of influence aren’t necessarily the same thing.
Each is controlling a different risk.
Make the Exclusion Zone Obvious
Whatever system is used, everybody on site should understand what it means.
If yellow bunting means “do not enter”, workers shouldn’t have to guess whether they’re allowed to cross it.
Consider:
A barricade is only useful if people understand why it is there.
Safe Access Into the Trench
Workers should never have to climb down the side of an excavation or use the excavator bucket as their normal means of access.
Where people need to enter an excavation, provide suitable access such as a secured ladder, stairs, ramp or another appropriate access system.
Access points should also be positioned so workers aren’t unnecessarily exposed to operating plant or unstable ground.
Emergency access and rescue arrangements should be considered before work begins, particularly where there is a risk of engulfment.
Watch for Changing Ground Conditions
An excavation that was stable yesterday isn’t automatically safe today.
Conditions can change because of:
Open trenches and their controls should be regularly inspected, particularly after weather events or anything else that could affect their stability.
Operators are often in a good position to notice these changes.
Cracks forming near the edge, material falling from the trench wall, unexpected water or ground movement should never simply be ignored.
Deeper Trenches Bring Additional Requirements
Excavation requirements vary between Australian jurisdictions, so contractors need to check the rules that apply where they are working.
As a general national reference, excavation work in or near a shaft or trench with an excavated depth greater than 1.5 metres is classified as high-risk construction work under the model WHS framework and requires a Safe Work Method Statement.
Victoria also has specific requirements for high-risk construction work involving trenches deeper than 1.5 metres.
Depending on the work and jurisdiction, excavation notification requirements may also apply.
Before You Dig
Trench safety starts before the bucket enters the ground.
Before excavation begins, consider:
The Takeaway
The edge of the trench isn’t necessarily the edge of the danger area.
The zone of influence can extend beyond the excavation, and loads placed within that area can affect the stability of the ground and the excavation itself.
That means trench safety isn’t just about what happens inside the trench.
It’s also about what’s happening around it.
Think about where the excavator is positioned. Think about where trucks are travelling. Keep spoil and heavy materials away where practicable. Establish clear exclusion areas. Provide proper fall protection where required. And don’t assume that because an excavation looked safe when it was dug, it will remain that way.
A few extra minutes spent setting up an excavation properly can prevent a much bigger problem later.
For ready-to-use safety documentation, check out our SWMS and JSA templates, designed specifically for earthmoving and civil construction businesses to help make site safety paperwork simpler.
Important: This article provides general safety information only and is not a substitute for a site-specific risk assessment, SWMS, engineering or geotechnical advice, or the requirements of the relevant WHS/OHS regulator. Excavation requirements differ between Australian states and territories. Always check the requirements that apply to the work being undertaken.
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