Underpinning in Ontario: Methods, OHSA Requirements & Adjacent Property Rights

What Is Underpinning?

Underpinning is the structural process of extending or replacing the foundation of an existing building to reach a deeper, more competent bearing stratum. The goal is to transfer the building's load away from the original shallow foundation to a new foundation element that is either deeper or more adequately sized for the applied loads.

In Toronto and across the GTA, underpinning is one of the most common structural interventions — driven by the enormous number of older homes with shallow rubble-stone or concrete strip footings at depths of 0.6–1.2 m, and the widespread desire to lower basements from unlivable 5-foot headroom to code-compliant ceiling heights. The same technique also appears on commercial sites where adjacent excavations must pass below neighbouring building foundations without causing settlement.

When Is Underpinning Required?

• Lowering a basement floor: The most common reason in Toronto residential — converting a low-ceiling basement to usable living space requires excavating below the existing footing level, which undercuts the footing and requires underpinning before excavation can proceed
• Adjacent excavation: When constructing a new building, adding a below-grade level, or installing utilities next to an existing building whose footings are at or above the planned excavation depth
• Foundation deterioration: Aging rubble-stone or unreinforced concrete footings may have deteriorated to the point where underpinning to new concrete is necessary to restore structural integrity
• Settlement correction: When differential settlement has caused structural distress, underpinning can transfer load to more stable bearing and arrest ongoing movement
• Building addition load increase: Adding storeys to an existing building may exceed the bearing capacity of shallow footings, requiring underpinning to increase bearing area or depth

Pit (Mass Concrete) Underpinning

The pit method is the most common underpinning technique for Toronto residential basement lowering. The process is highly sequential and must follow the approved engineer's underpinning plan:

1. Pin sequence design: The engineer divides the perimeter foundation into numbered segments (pins), no more than 25–30% of the total foundation length active at any time, to prevent overloading of the unexcavated sections
2. Excavate a pit: Workers hand-excavate a pit under one designated foundation segment — typically 1.0–1.5 m wide by 1.0–1.5 m long — down to the new bearing depth
3. Place concrete: A new mass concrete underpinning pad is formed and poured, filling the void. The concrete must cure to sufficient strength before the next adjacent pin is started
4. Repeat in sequence: The process continues around the perimeter until all pins are complete
5. Dry pack the top: Once all pads are cured, the gap between the top of each pad and the underside of the existing footing is filled with dry-pack mortar to achieve full bearing contact

Pile Underpinning

Where access permits larger equipment, pile underpinning transfers the existing foundation load to new piles driven, pressed, or drilled adjacent to the footing:

• Mini-piles (pushed piles): Short steel sections are hydraulically pressed into the ground using the weight of the structure itself as reaction — useful in confined spaces and where vibration is a concern
• Helical piles: Factory-made steel shafts with helical plates are screwed into the ground using a torque motor, reaching competent bearing stratum. Particularly suited to soft or expansive soils
• Needle beam system: A horizontal structural steel needle is threaded through the existing wall above the footing and bears on the new piles, transferring load without excavating directly beneath the footing

Micropile Underpinning

Micropiles are small-diameter (75–250 mm) drilled piles that can reach deep, competent bearing strata through difficult soils and in highly restricted access conditions. In Toronto's urban fabric — party-wall row houses, low-clearance basements, live buildings requiring continued occupancy — micropiles offer significant advantages:

• Equipment can operate in low headroom (as little as 2.0 m clearance for some rigs)
• Very low vibration — suitable adjacent to vibration-sensitive buildings and infrastructure
• High load capacity relative to diameter — a 150 mm diameter micropile can carry 300–800 kN in suitable bearing
• Grout-injected construction ensures intimate contact and load transfer

Micropile design follows FHWA-NHI-05-039 guidelines and must be stamped by a licensed P.Eng. Pile load testing (at minimum compression, and sometimes tension) is typically specified for the first few piles on a project to verify design assumptions.

Choosing the Right Method

Building Permits & OBC Requirements

Underpinning of any existing building in Ontario requires a building permit from the municipal building department. The application must include:

• Stamped structural drawings showing the underpinning design (pin layout, cross-sections, dimensions, concrete specifications) prepared by a licensed Ontario P.Eng.
• Stamped excavation support drawings (shoring design) for the underpinning pits wherever the excavation geometry or adjacent conditions require engineered support
• Geotechnical soils report confirming bearing capacity at the proposed new footing depth
• A survey confirming setback distances to adjacent structures

The OBC does not specify underpinning design methods in detail — the design engineer works to NBCC 2020 loading requirements, CSA A23.3 (for concrete elements), and established geotechnical engineering practice. The municipality's building inspector will inspect the work at the stages specified in the approved drawings.

OHSA Reg 213/91 & Worker Safety

OHSA Regulation 213/91 (Construction Projects) governs worker safety during underpinning operations. Key requirements for employers (contractors) include:

• An engineered shoring design (P.Eng.-stamped) is required whenever the excavation is adjacent to a structure or the site conditions require project-specific support
• Workers in excavations must have safe means of entry and exit arranged in accordance with the current regulation and the site layout
• No worker may enter an underpinning pit while an adjacent pit is being excavated (to prevent simultaneous loading of adjacent segments)
• The underpinning sequence must be filed with the project health and safety documents and followed strictly
• A competent person must inspect the excavation walls at the start of each shift and after any event that may have affected stability

OHSA orders are enforced by Ministry of Labour, Immigration, Training and Skills Development (MLITSD) inspectors. Violations can result in stop-work orders, fines, and in serious cases, charges against the employer and supervisor under the OHSA.

Adjacent Property Rights in Ontario

Ontario law places a significant duty on anyone undertaking excavation that may affect adjacent properties. Liability exposure can arise under several legal theories depending on the facts, so owners and contractors should obtain project-specific legal advice rather than rely on a single generalized rule.

Practically, this means:

• Pre-construction survey: The excavating party should commission an independent structural condition survey of all adjacent buildings before work begins, to establish a baseline. Without this, any pre-existing cracks become disputed claims
• Notification: Ontario work next to neighbouring structures should include advance notice to adjacent owners, documented pre-construction surveys, and a clear communication plan whenever excavation or underpinning could affect neighbouring foundations
• Support obligation: Excavating parties must design their shoring and underpinning to provide continuous lateral and vertical support to adjacent foundations throughout construction
• Settlement monitoring: Survey prisms, crack gauges, and tiltmeters should be installed on adjacent structures before work begins, with trigger levels that halt construction if movement exceeds thresholds

Adjacent property owners who are concerned about a neighbour's proposed excavation should retain a P.Eng. to review the construction drawings and, if necessary, seek an injunction restraining work that does not meet professional standards.

Pre-Construction Survey & Monitoring

A pre-construction condition survey documents the existing condition of all structures within the project-specific zone of influence of the planned excavation. The survey should include:

• Photographic record of all existing cracks, settlement, and distress in adjacent structures
• Measurement of existing crack widths
• Documentation of existing settlement and any out-of-plumb conditions
• Baseline readings for all settlement monitoring instruments

During construction, monitoring data are reviewed against engineer-specified trigger levels. At "Yellow" alert levels, the contractor modifies work practices. At "Red" alert levels, work stops and the engineer re-evaluates the situation before proceeding.