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Excavation Hazards

Safe At Work Ontario
  • Issued: July 17, 2015
  • Content last reviewed: July 2015

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.


Hazards involving excavations, in particular trenches, can lead to serious incidents involving workers at construction sites.


Workers can be critically injured or die in cave-ins. Many cave-ins occur on small jobs of short duration, such as water, gas, electrical and sewer line connections.

Employers, supervisors and workers must follow the requirements set out in the Occupational Health and Safety Act (OHSA) and Regulations for Construction Projects.

More hazards

  • falls into trenches or excavations
  • tripping over equipment, debris and spoil
  • 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 atmosphere (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 Construction Regulations

  • remove debris and excavated soil near excavation site [s. 232]
  • arrange to protect workers from falling into excavation [s. 233(4)]
  • plan for removing water in excavation [s. 230]
  • identify and locate overhead power lines [s. 188) and underground services [s. 228]
  • know soil types [s. 226] and what sloping, shoring or pre-fabricated, hydraulic or engineer systems are required [s. 234 to 242]
  • notification requirements [s. 6 (a), (g) and (h)]
  • requirements for when support system must be engineered [s. 235(2) and s. 236]
  • prepare emergency plan [s. 17 and 18]
  • worker(s)shall not perform work in trench unless another worker is working above ground [s. 225]
  • obtain utility locations before digging [s. 228]

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

  • determine how workers will enter and exit excavation
  • know in advance what excavation equipment and tools are needed
  • consider possibility of environmental hazards (rain) in excavation
  • be aware of nearby vehicles and mobile equipment causing soil to vibrate
  • provide training to workers

It’s the law!


A constructor is required to complete a Notice of Project when:

  • a project exceeds $50,000 or
  • excavation is planned for a trench that a worker may enter and that trench:
    • is more than 300 metres long or
    • more than 1.2 metres deep (47 inches) and more than 30 metres (98 feet) long or
    • is required by regulation to be designed by a professional engineer [Construction Reg. s. 6 (a), (g) and (h)]

A Notice of Trench must be given in person or by phone or fax for a trench if section 6 of Construction Regulation does not apply, and the project includes digging a trench more than 1.2 metres deep (47 inches) and may be entered by a worker. [Construction Reg. s. 7]

Mark and locate utilities: Employers must ensure all gas, electrical and other services are located or marked in or near the area to be excavated. If a service poses a hazard, it must be shut off and disconnected before the excavation activity begins. If a potentially hazardous service cannot be disconnected, the service owner must be asked to supervise the service’s uncovering during the excavation. [Construction Reg. s. 228]

Adjacent structures: Constructors must prevent damage to adjacent structures by engaging a professional engineer who must specify in writing the precautions to be taken. [Construction Reg. s. 229]

Soil strength: Determine the soil type to protect excavation walls from collapsing. This can be determined by doing things such as inspecting trenches and excavations following a rainfall, melting snow, thawing earth and overflows from nearby streams, storm drains and sewers. The soil type determines the strength and stability of the excavation walls. [Construction Reg. s. 226]

Wall stability: Strip the wall of a trench or excavation of any loose rock or other material that may slide, roll or fall on a worker. [Construction Reg. s. 232]

Equipment: Keep heavy equipment, excavated soil or rock and construction material at least one metre away from the upper edges of the trench or excavation. Do not position or operate a vehicle or machinery in a manner that could affect the wall’s stability. [Construction Reg. s. 233]

Work space: Maintain a clear work space of at least 450 millimetres (18 inches) between the wall of an excavation and any formwork, masonry or similar wall. [Construction Reg. s. 231]

Fall protection: Provide a barrier at least 1.1 metres (42 inches) high at the top if an excavation does not meet regulatory slope requirements and is more than 2.4 metres (eight feet) deep. [Construction Reg. s. 233(4)]

Protect yourself: Never enter a trench deeper than 1.2 metres (47 inches) unless the walls are sound, made of solid rock, properly sloped, shored or protected by a trench box. Never work alone in a trench.

Protective systems: Workers must be protected against trench or excavation cave-ins and other hazards using three basic methods:

  1. Sloping which involves cutting back trench walls at an angle, inclined away from the excavation.
  2. Shoring which helps support trench and excavation walls to prevent movement of soil, underground utilities, roadways and foundations. Timber and hydraulic systems are the most commonly used supports to shore up walls. Both types must be designed by a professional engineer.
  3. Prefabricated support systems (for example, trench boxes and shields) which can prevent soil cave-ins.

"Competent person": Trenches and excavations must be inspected daily for hazards, and when conditions change, before workers enter them. This must be done by a "competent person", as defined by the OHSA.

A "competent person":

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

Entry and exit: Provide safe access and egress for workers at excavations by means of ladders, steps, ramps, or other safe methods of entering or exiting. Trenches must have ladders placed in the area protected by the support system and be accessible in the event of a collapse. [Construction Reg. s. 240]

Some general duties of workplace parties


Employers have a number of duties and responsibilities under the OHSA and Construction Regulations.

Some examples of employers’ duties:

  • provide information, instruction and supervision to workers to protect their health and safety, including on safe work policies and procedures specific to the workplace and type of work the workers will perform [OHSA s. 25(2)(a)]
  • ensure equipment operators and signallers are competent workers [Construction Reg. s. 96 and 106]
  • take every precaution reasonable in the circumstances for the protection of workers [OHSA s. 25(2)(h)]
  • ensure prescribed measures and procedures are carried out in the workplace [OHSA s. 25(1)(c)]
  • ensure equipment, materials and protective devices required by the regulations are provided and maintained in good condition [OHSA s. 25(1)(a) and (b)]
  • provide assistance to, and co-operate with, the workplace’s Joint Health and Safety Committee and/or a health and safety representative [OHSA s. 9(29) and 8(9)]
  • prepare and review, at least annually, a written occupational health and safety policy for the workplace, and develop and maintain a program to implement that policy [OHSA s. 25(2)(j)]
  • post a copy of the OHSA in the workplace [OHSA s. 25(2)(i)]


Some examples of supervisors’ duties:

  • ensure workers comply with the OHSA and its regulations [OHSA s. 27]
  • ensure any equipment, protective device or clothing required by the employer is used and/or worn by workers [OHSA s. 27(1)(b)]
  • advise workers of any potential or actual health or safety dangers known by the supervisor [OHSA s. 27(2)(a)]
  • where prescribed, provide workers with written instructions about measures and procedures to be taken for workers’ protection [OHSA s. 27(2)(b)]
  • take every precaution reasonable in the circumstances for the protection of workers [OHSA s. 27(2)(c)]


Some examples of workers' duties:

  • wear appropriate personal protective equipment [OHSA s. 28(1)(b)]
  • use or operate equipment in a safe manner [OHSA s. 28(2)(b)]
  • report any defects in equipment to your supervisor or employer [OHSA s. 28(1)(c)]
  • work in compliance with the OHSA and its regulations [OHSA s. 28(1)(a)]
  • report any known workplace hazards or OHSA violations to your supervisor or employer [OHSA s. 28(1)(d)]
  • know your OHSA rights, including the right to refuse unsafe work [OHSA s. 43(3)(a), (b) and (c)]

More information

Call toll-free

Call 1-877-202-0008 any time to report critical injuries, fatalities or work refusals. Monday to Friday 8:30 a.m. to 5 p.m. for general inquiries about workplace health and safety

Always call 911 in an emergency.

Disclaimer: This web resource has been prepared to assist the workplace parties in understanding some of their obligations under the Occupational Health and Safety Act (OHSA) and the regulations. It is not intended to replace the OHSA or the regulations and reference should always be made to the official version of the legislation.

It is the responsibility of the workplace parties to ensure compliance with the legislation. This web resource does not constitute legal advice. If you require assistance with respect to the interpretation of the legislation and its potential application in specific circumstances, please contact your legal counsel.

While this web resource will also be available to Ministry of Labour inspectors, they will apply and enforce the OHSA and its regulations based on the facts as they may find them in the workplace. This web resource does not affect their enforcement discretion in any way.