Safe Construction Trenches and Excavations

• Issued: May 2011
• Content last reviewed: May 2011
• PDF Version [ 617 Kb / 3 pages | Download Adobe Reader ]

Disclaimer: This resource has been prepared to help the workplace parties understand some of their obligations under the Occupational Health and Safety Act (OHSA) and regulations. It is not legal advice. It is not intended to replace the OHSA or the regulations. FOR FURTHER INFORMATION PLEASE SEE FULL DISCLAIMER

Construction trenching and excavation are inherently hazardous.

Cave-ins

A cave-in can suffocate you in less than three minutes — and even if you survive, your internal organs may be badly crushed. Most fatal cave-ins occur on small, short-duration jobs like water, gas, electrical and sewer connections.

Other hazards

• Falls into the trench or excavation
• Tripping over equipment
• Excavated material or other objects falling on workers
• Exposure to underground services or overhead electrical cables
• Unstable adjacent structures
• Mishandled or poorly placed materials
• Hazardous atmospheres (noxious gases/lack of oxygen)
• Toxic, irritating or flammable and explosive gases
• Incidents involving vehicles and other mobile equipment.

Plan before you dig

Obligations under the Occupational Health and Safety Act (OHSA)

• Remove debris and excavated soil near the excavation site. [O. Reg. 213/91, s. 233]
• Arrange to protect workers from falling into the excavation. [O. Reg. 213/91, s. 233(4)]
• Plan for removing water in the excavation. [O. Reg. 213/91, s. 230]
• Identify and “locate” overhead power lines and underground services. [O. Reg. 213/91, s. 228
• Know the types of soil and what sloping, shoring or shielding are required. [O. Reg. 213/91, s. 226]
• Prepare an emergency plan. [O. Reg. 213/91, s. 17 & 18]

Reasonable precautions under OHSA clause 25 (2) (h)

• Determine how workers will enter and exit the excavation.
• Know in advance what digging equipment and tools are needed.
• Consider the possibility of atmospheric hazards in the excavation.
• Be aware of nearby vehicles and mobile equipment.
• Note environmental conditions.

It’s the law!

Mark and locate services

Employers must ensure that all gas, electrical and other services in or near the area to be excavated are located and marked. If a potentially hazardous service cannot to be disconnected, the service owner shall be asked to supervise its uncovering during the excavation. [O. Reg. 213/91, s. 228]

Adjacent structures

Constructors must take suitable precautions to prevent damage to adjacent structures by involving a professional engineer who will specify in writing the precautions to be taken. [O. Reg. 213/91, s. 229]

Soil strength

Types of soil determine the strength and stability of excavation walls. Refer to section 226 of Ontario Regulation 213/91 (Construction Projects) to determine the soil you have encountered and protect the excavation walls from collapse accordingly. Tip: Inspect trenches and excavations following rain, melting snow, thawing earth, and overflow from adjacent streams, storm drains and sewers.

Wall stability

Strip the walls of a trench or excavation of any loose rock or other material that may slide, roll or fall on a worker. [O. Reg. 213/91, s. 232]

Maintain at least one metre (3 feet) from the upper edge of each wall clear of equipment, excavated soil, rock and construction material. Do not locate or operate a vehicle or machinery in a way that can affect the stability of a wall of an excavation. Tip: Keep heavy equipment, excavated soil or rock and construction material away from the edges of the trench or excavation. [O. Reg. 213/91, s. 233]

Work space

Maintain a clear work space of at least 450 millimetres (18 inches) between the wall of the excavation and any formwork or masonry or similar wall. [O. Reg. 213/91, s. 231]

Fall protection

If an excavation is more than 2.4 metres (8 feet) deep, provide a barrier at least 1.1 metres (42 inches) high at the top if it does not meet regulatory slope requirements. [O. Reg. 213/91, s. 233(4)]

Protect yourself

Unless the walls of a trench are sound and made of solid rock, never enter a trench deeper than 1.2 metres (4 feet) unless it is properly sloped, shored, or protected by a trench box.

Never work alone in a trench.

Protective systems

Three basic methods can protect workers against trench or excavation cave-ins and other hazards:

1. Sloping involves cutting back the trench wall at an angle inclined away from the excavation.
2. Shoring supports trench or excavation walls to prevent movement of soils, underground utilities, roadways and foundations — timber and hydraulic systems are most common. Both types must be designed by a professional engineer.
3. Prefabricated support systems (e.g., trench boxes and shields) can prevent soil cave-ins.

“Competent person”

Trenches and excavations must be inspected for hazards daily and as conditions change by a “competent person” (as defined by the Occupational Health and Safety Act) before workers enter them.

A “competent person”:

• has the knowledge, training and experience to organizes the work
• is familiar with the OHSA and the Construction Regulation, and
• can identify existing and potential health and safety hazards and/or unsafe working conditions.

For workers to work in an unsupported excavation that is not a trench and is not made in Type 4^{[ 1 ]} soil, a professional engineer must provide a written opinion that a worker will not be endangered and must inspect the excavation or appoint a competent worker to inspect it as frequently as the opinion specifies.

Entry and exit

Excavations require safe access and egress by using ladders, steps, ramps, or other safe means of entering or exiting. Trenches must be provided with ladders. Trench ladders must be placed within the area protected by the support system and be accessible in the event of a collapse. [O. Reg. 213/91, s. 240]

Inspector focus

When encountering trenches and excavations, Ministry inspectors will inspect the specific hazards identified in this fact sheet, making certain that employers, supervisors and workers engaged in trenching and excavation work comply with the Occupational Health and Safety Act and its regulations, particularly sections 222 through 242 of the Construction Regulation (O. Reg. 213/91).

More information

• Ministry of Labour, construction safety
• Infrastructure Health & Safety Association
• Workplace Safety & Insurance Board
• Ontario Sewer and Watermain Contactors Association
• Operating Engineers Training Institute of Ontario
• View CSA standards (view only) referenced in occupational health and safety legislation

Call toll-free

Call 1-877-202-0008 anytime to report workplace health and safety incidents or for general inquiries about health and safety at the workplace.

Safe workplaces mean productive workplaces