Shoring Systems & Underpinning for Ontario Construction Projects

What Is Construction Shoring?

Construction shoring is a system of structural elements — piles, walls, braces, anchors, or lagging — used to laterally support soil and prevent collapse during an excavation. It protects workers in the excavation, adjacent buildings, utilities, and the public from the consequences of soil failure. Shoring is classified as temporary works: engineered structures built to facilitate construction and removed (or abandoned in place) once permanent structure is complete.

Underpinning is a related but distinct operation: the permanent deepening or strengthening of an existing foundation. Where shoring keeps earth out of an excavation, underpinning transfers load from a shallow existing footing to a new, deeper bearing element — typically when adjacent construction, increased building loads, or foundation settlement requires intervention.

Ontario Regulatory Framework: OHSA 213/91 & OBC

Two overlapping Ontario regulatory regimes govern excavation and shoring: the Occupational Health and Safety Act (OHSA) and its Construction Projects Regulation (O. Reg. 213/91), and the Ontario Building Code (O. Reg. 332/12).

OHSA Regulation 213/91 — Key Provisions

• Section 228: Excavations must be sloped, shored, or otherwise protected against collapse where worker access and soil conditions require a protected excavation.
• Section 234: Shoring for deeper or more complex excavations, especially near structures or roadways, must be designed and sealed by a P.Eng. before excavation begins.
• Section 230: Where excavation could affect the support zone of an adjacent footing, the constructor must retain a P.Eng. to assess the impact on that structure.
• Section 236: Ground anchors (tie-backs) require P.Eng. design, sealed drawings, and installation supervision.

Ontario Building Code Requirements

The OBC requires excavation adjacent to buildings be carried out without reducing the bearing capacity of existing foundations. Building permit applications for excavation near existing structures typically require sealed shoring drawings from a P.Eng. together with the supporting geotechnical information needed for the municipality's review.

Types of Shoring Systems

Soldier Pile & Timber Lagging

The most common shoring system in Toronto's urban core. Wide-flange or H-pile sections are drilled or driven at 1.5–2.5 m centres; horizontal timber or concrete lagging spans between piles as excavation lifts proceed downward. Soldier pile walls are economical and fast, but not watertight — they perform best in cohesive soils (clay, glacial till) where groundwater infiltration is not a concern. One or more rows of tie-back ground anchors are installed for deeper excavations requiring reduced internal bracing.

Secant & Tangent Pile Walls

Secant pile walls consist of overlapping drilled concrete piles (alternating "primary" and "secondary" piles) forming a near-watertight wall. Tangent walls use touching (non-overlapping) piles where water infiltration is less critical. Both systems outperform soldier pile for groundwater control and limiting ground movement adjacent to sensitive buildings — making them the preferred choice for condominium basement construction in Toronto's downtown core.

Sheet Piling

Interlocking steel sheet piles are driven to form a continuous wall. Fast and economical for shallow utility excavations and temporary cofferdams. Sheet piling is less common in downtown Toronto due to vibration concerns and difficult soil (boulders in Halton Till or Elgin Till), but is frequently used in softer lacustrine deposits and for waterfront and bridge work.

Soil Nailing

Passive steel reinforcing bars are drilled and grouted into existing soil at downward angles as excavation proceeds from top of cut to bottom. A shotcrete facing wall ties the nails together. Soil nailing is economical for cut slopes and retaining walls in competent cohesive soils — common in GTA residential development and landscaping projects.

Raker & Cross-Lot Bracing

Internal bracing — diagonal rakers bearing on the excavation floor slab, or horizontal cross-lot struts spanning between opposite walls — is used where ground anchors cannot be installed due to adjacent property constraints or geometry. Common in Toronto for municipal utility excavations in street rights-of-way.

Underpinning Methods

Mass Concrete (Traditional Pit) Underpinning

The existing foundation is divided into alternating sections (pins). Each is excavated below the footing in a controlled sequence to prevent simultaneous undermining of the full footing length. Each pit is poured with mass concrete and allowed to cure before the next section begins. This method is reliable, labour-intensive, and suited for modest underpinning depths of 1–3 m in residential construction.

Helical Screw Piles & Micro-Piles

For deeper underpinning or restricted access, helical screw piles or small-diameter drilled micro-piles are installed through or alongside existing footings and connected by a reinforced bracket. Micro-piles can be installed with compact equipment in low-headroom conditions (inside occupied basements), making them the standard solution for underpinning in Ontario's existing building stock without requiring demolition or extensive shoring.

Jet Grouting & Ground Improvement

High-pressure cement grout is injected into native soil to produce monolithic soil-cement columns, improving bearing capacity and reducing permeability in place. Used for underpinning and void-filling under existing foundations without excavation, and for ground improvement ahead of excavation in areas sensitive to settlement.

Shoring Design Process

A complete Ontario shoring design involves:

1. Geotechnical Investigation: Drilled boreholes, SPT/CPT testing, and lab analysis to characterize soil profile, shear strength (s_u for clay, φ for granular), and groundwater levels.
2. Geometric Assessment: Excavation extents, adjacent foundation depths and types, utility locations, water table, and surcharge loads from adjacent structures, equipment, and traffic.
3. System Selection: Based on soil type, excavation depth, groundwater, access constraints, adjacent structure sensitivity, and project schedule.
4. Structural Analysis: Earth pressure distribution (Rankine, Coulomb, or finite element), pile sizing (CSA S16 for steel, CSA A23.3 for concrete piles), anchor sizing, connection design, and stability checks.
5. Sealed Drawings & Specifications: P.Eng.-sealed drawings and written instructions provided to the constructor before excavation begins, per OHSA 213/91 s.234.
6. Field Review & Monitoring: The engineer observes excavation at each lift, reviews inclinometer and settlement monitoring data, and issues written field review letters confirming conformance or directing changes.

Excavation Adjacent to Existing Buildings

Excavating near existing buildings in Toronto's dense urban fabric requires careful planning. The standard of practice involves four key elements:

• Pre-construction condition survey: Photographic and video documentation of the adjacent building's existing cracks, settlement, and distress before excavation — essential for insurance and dispute resolution.
• Settlement analysis and limits: Engineering analysis establishes project-specific allowable total and differential settlement criteria for the adjacent structure.
• Continuous monitoring: Settlement monuments, crack gauges, and inclinometers monitored at defined intervals throughout excavation. Engineer defines "alert" and "action" trigger levels.
• Response plan: Pre-approved protocol if trigger levels are exceeded — slow excavation rate, install additional bracing, grout soil, or halt and reassess.

Ontario courts have consistently held both the constructor and the engineer-of-record responsible for damage to adjacent properties caused by excavation. Properly designed shoring, documented pre-construction surveys, and continuous monitoring records are the primary means of defence in any third-party damage claim.