Excavation hazards on construction projects
Learn about legal requirements and precautions to address hazards involving excavations on construction projects.
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Hazards involving excavations, in particular trenches, can lead to serious incidents involving workers at construction sites.
This resource does not replace the Occupational Health and Safety Act (OHSA) and its regulations and should not be used as or considered legal advice. Health and safety inspectors apply and enforce these laws based on the facts they find in the workplace.
Workers can be critically injured or die as a result of excavation cave-ins. Many cave-ins occur on small jobs of short duration, such as water, gas, electrical and sewer line connections as well as building foundation repair projects.
More excavation-related 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 O. Reg. 213/91 – Construction Projects
- removal of debris and excavated soil near excavation site [s. 232]
- preventing persons from falling into excavation [s. 233(4)]
- removal of excess water from excavations [s. 230]
- identification and locating of overhead power lines [s. 188] and underground services [s. 228]
- identification of soil types [s. 226] and applying controls as required such as: sloping, shoring or pre-fabricated, hydraulic or engineered systems [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) not performing work in a trench unless another worker is working above ground [s. 225]
- obtaining 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 [O. Reg. 213/91 s. 6 (a), (g) and (h)]
A Notice of Trench Work must be given in person or by phone or fax for a trench if section 6 of O. Reg. 213/91 – Construction Projects does not apply, but the project includes digging a trench more than 1.2 metres deep (47 inches) into which a worker may enter. [O. Reg. 213/91 s. 7]
Mark and locate utilities: Employers must ensure all gas, electrical and other services are located and marked in and 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. [O. Reg. 213/91 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. [O. Reg. 213/91
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. [O. Reg. 213/91 s. 226]
Wall stability: Strip the wall of a trench or excavation of any loose rock or other material, including ice, that may slide, roll or fall on a worker. [O. Reg. 213/91 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. [O. Reg. 213/91 s. 233]. If a drilling operation is underway, ensure that a restricted access zone has been established to prevent access by equipment [O. Reg. 213/91 s. 156.2 (2)(e)]
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. [O. Reg. 213/91
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. [O. Reg. 213/91
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:
- sloping which involves cutting back trench walls at an angle, inclined away from the excavation.
- 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.
- 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.
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. [O. Reg. 213/91 s. 240]
Some general duties of workplace parties
Employers have several duties and responsibilities under the OHSA and O. Reg. 213/91.
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 [O. Reg. 213/91 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)]
- video: trench digging and excavation safety
- trenching and excavation fact sheet from the Canadian Centre for Occupational Health and Safety
- Infrastructure Health & Safety Association resources:
- Construction Safety
- Infrastructure Health and Safety Association
- Workplace Safety and Insurance Board
- Ontario Sewer and Watermain Contractors Association
- Operating Engineers Training Institute of Ontario
- Canadian Standards Association (CSA): View CSA standards referenced in OHS regulations. Join “(CSA) Communities” for free access