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 upon the facts they find in a workplace.

Overview

Underpinning is the process of supporting or strengthening the foundation of an existing house, building or similar structure to increase the capabilities of an existing foundation. This is accomplished by either:

  • reinforcing the existing foundation
  • strengthening the soil by introducing an expanding filler
  • extending the foundation so that the load is distributed over a greater surface area

Underpinning is done for a few different reasons, including:

  • an additional storey is being added to the structure
  • to increase the ceiling height in a basement
  • the soil supporting the existing foundation has changed and needs to be reinforced

This page includes information about possible ways to comply with the requirements in O. Reg. 213/91: Construction Projects, the construction regulation under the Occupational Health and Safety Act (OHSA), so that health and safety hazards can be mitigated while performing underpinning work during:

  • renovations to homes
  • industrial, institutional and commercial building
  • foundation repair

The recommendations set forth in this document are not an exhaustive representation of the relevant duties that may apply during underpinning and excavating.

Hazards

Hazards present during underpinning operations can lead to critical or fatal injuries for workers.

Collapses

Workers can be injured because of a structural collapse. A structural collapse can occur with little to no warning, leaving workers with limited time to react or escape a dangerous environment.

A structural collapse typically occurs on home renovation projects and building foundation repair projects where a professional engineer licensed to practice in Ontario did not develop the project specific design, procedures and sequencing. Or, if they were developed, they were not followed.

Other underpinning hazards

More examples of underpinning-related hazards, include:

  • 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
  • access and egress related issues

How to achieve compliance

Employers, supervisors and workers must follow the requirements set out in:

Under the OHSA, workplace parties all have general duties. Employer duties include the duty to take every precaution reasonable in the circumstances to protect workers from exposure to hazards, including those related to underpinning work.

Employers must also meet the requirements related to underpinning work in O. Reg. 213/91: Construction Projects.

Reasonable precautions

Employers should take the following precautions and comply with the following duties to ensure the health and safety of workers is protected:

  • have an engineered drawing and procedure developed to follow for underpinning sequencing
  • ensure that any building or structure can support any loading that may be applied to it
  • develop procedures for entering and exiting that protect the health and safety of workers know in advance what excavation equipment and tools are needed
  • develop measures and procedures for addressing risks posed by environmental hazards be aware of nearby vehicles and mobile equipment causing soil to vibrate
  • provide training to workers

Requirements under the OHSA

Employers must comply with all OHSA requirements that are relevant to the health and safety of workers during an underpinning project. Under clause 25(2)(h), an employer must take every precaution necessary for the protection of a worker. With respect to underpinning, this duty could include:

  • having an engineered drawing and procedure developed to follow for underpinning sequencing
  • developing procedures for entering and exiting that protect the health and safety of workers
  • knowing in advance what excavation equipment and tools are needed
  • developing measures and procedures for addressing risks posed by environmental hazards
  • being aware of nearby vehicles and mobile equipment causing soil to vibrate and implementing necessary measures and procedures to address the hazard

Other applicable OHSA duties employers must comply with include:

  • ensuring that any building or structure can support any loading that may be applied to it
  • ensuring that workers are properly trained for the hazards in the work

Requirements under the construction regulation

O. Reg. 213/91: Construction Projects has the following requirements that could apply to an underpinning operation.

Notifications

A constructor must complete a Notice of Project when:

  • the cost of a project exceeds $50,000
  • the work is the erection or structural alteration of a building more than two storeys or more than 7.5 metres high
  • the regulation requires that part of the permanent or temporary work be designed by a professional engineer

Engineering requirements

Constructors and employers must ensure that an engineer licensed to practice in Ontario designed:

  • all temporary and permanent structures
  • the alterations to permanent or temporary structures

These structures must be designed and constructed to resist all of the loads and forces they are likely to be subjected to [O. Reg. 213/91 s. 31(1)(a)].

Structural bracing

Constructors and employers must ensure that all temporary and permanent structures are braced to prevent any movement that may affect stability or lead to failure or collapse [O. Reg. 213/91 s. 31(1)(b)]. Since underpinning involves the removal of existing structural support for buildings, bracing is critical in this type of work. A professional engineer should design bracing design and methodology.

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].

Employers must also ensure that the location of any overhead power lines have been taken into consideration  as measures and procedures must be developed if it will be possible for equipment to encroach on minimum distances to overhead conductors [O. Reg. 213/91 s.188].

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 s. 229].

Soil strength

Soil strength is important for underpinning work. Soils strength will determine:

  • the size and type of foundations
  • the methodology you need to use in accomplishing the work
  • the appropriate support system to protect excavation walls from collapsing

The soil type determines the strength and stability of the excavation walls. [O. Reg. 213/91 s. 226 and 238] The strength of the soil and the stability of excavation walls can be compromised by:

  • rain
  • melting snow
  • freeze/thaw cycles
  • water runoff from other sources

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)]

Ensure that equipment operators and signallers are competent workers and have been trained accordingly [O. Reg. 213/91 s. 96 and 106].

Workspace

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

Fall protection

Provide a barrier at least 1.1 metres (42 inches) high at the top if an excavation is not sloped per regulatory slope requirements and is more than 2.4 metres (eight feet) deep [O. Reg. 213/91 s. 233(4)]. If a drilling operation is underway, ensure that a restricted access zone has been established and that workers required within proximity to the hole are using appropriate fall protection [O. Reg. 213/91 s. 156.2 (2)(e and f)].

Protect yourself

Never enter a trench deeper than 1.2 metres (47 inches) unless the walls are:

  • sound
  • made of solid rock
  • properly sloped
  • adequately shored
  • protected by a trench box

Never work alone in a trench [O. Reg. 213/91 s 225].

Protective systems

Workers must be protected against trench or excavation cave-ins and other hazards using three basic methods [O. Reg. 213/91 s 234 to 242]:

  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.
  3. Prefabricated support systems (for example, trench boxes and shields) which can prevent soil cave-ins.

Competent person

Trenches and excavations should be inspected daily for hazards, and when conditions change, before workers enter them. This inspection must be done by a supervisor or other "competent person", as defined by the OHSA. The term “competent person” is defined in the OHSA as a person who:

  • is qualified because of knowledge, training and experience to organize the work and its performance
  • is familiar with this Act and the regulations that apply to the work
  • has knowledge of any potential or actual danger to health or safety in the workplace

Entry and exit

Provide safe access and egress for workers at underpinning projects by means of ladders, steps, ramps, or other safe methods of entering or exiting [O. Reg. 213/91 s.70]. Trenches must have ladders placed in the area protected by the support system [O. Reg. 213/91 s.240].

Health and safety resources

Contact us

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

Always call 911 in an emergency.

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 upon the facts they find in a workplace.