Cofferdam & Water Diversion Engineering in Ontario: OHSA, MNRF Permits & Design Methods

OHSA Requirements for Cofferdams

OHSA Construction Regulation O.Reg 213/91 Part XI (Cofferdams and Caissons) governs temporary water-exclusion structures on Ontario construction sites:

• Section 216: Cofferdams must be designed by a P.Eng. The design must be on-site before construction of the cofferdam commences.
• Section 217: Cofferdams must be built in accordance with the P.Eng design. Changes to the design or field conditions that affect structural adequacy must be reviewed and approved by the P.Eng before implementation.
• Section 218: Where workers are required to work inside a cofferdam, means of quick escape must be provided. The constructor must ensure the cofferdam interior is inspected after freshets, storms, or any event that may have affected its structural integrity.
• Section 219: Pumping equipment for dewatering must be maintained and have a back-up in case of primary pump failure — a flooded cofferdam with workers inside is a fatality scenario.

Ontario Permit Requirements

Cofferdam projects in Ontario require permits from multiple authorities depending on project type and location:

Multi-permit cofferdam projects — which are typical for any work in an Ontario watercourse — often involve extended permitting lead times. MNRF, DFO, conservation authority, navigation, and dewatering reviews can run on different schedules, so early pre-application consultation with all permitting authorities is essential for project scheduling.

Cofferdam Types Used in Ontario

• Sheet pile cofferdams: The predominant type for Ontario urban bridge and waterfront construction. Cold-formed steel sheet piles (Larssen Z-profiles or U-profiles) are driven into the river or lake bed using vibratory or impact hammers. Interlocking clutches provide a watertight seal. Internal bracing struts or tiebacks resist the net water/soil pressure. Sheet piles can often be extracted and reused after the permanent structure is complete.
• Earthen cofferdams: Compacted earth berms placed in shallow, slow-moving water — typically used for culvert replacement or minor bridge work on small Ontario watercourses with minimal flow. Low cost but limited to very specific site conditions. Environmental impact on sediment and fish habitat can be significant and triggers DFO scrutiny.
• Cellular cofferdams: Interconnected circular or segmented cells of flat sheet pile filled with granular material (sand, gravel). Suitable for large-scale cofferdams resisting high hydrostatic head — used in dam construction, major bridge pier work, and port rehabilitation. The cell fill carries the lateral earth/water pressure in compression (the design relies on filled cell shear resistance), making the analysis more complex than braced sheet pile walls.
• Double-wall cofferdams: Two parallel rows of sheet pile with granular fill between them — used where single-wall cofferdams are structurally impractical due to depth, high water pressure, or limited bracing options. The fill provides both lateral resistance and stability.

Structural Design Considerations

Cofferdam structural design addresses the following primary loads and conditions:

• Hydrostatic water pressure: Net pressure is the difference between external water level and internal dewatered level. In tidal or fluctuating river conditions, the design water level must represent the design flood event (typically 1-in-100 year flood if the cofferdam will be in place through a flood season).
• Soil pressure: Where the cofferdam wall is embedded in soil, passive and active earth pressures on the embedded portion govern the required embedment depth and wall section modulus.
• Seepage and piping forces: Upward seepage under the cofferdam bottom can cause hydraulic heave (piping failure), especially in fine granular soils. The stability ratio against piping (ratio of submerged soil weight above the seepage path to the upward seepage pressure) must exceed 1.5–2.0.
• Ice loads in Ontario: For cofferdams in rivers subject to winter freeze and spring ice break-up, ice pressure on exposed cofferdam walls must be included in the design. CSA standards for ice loading apply to permanent structures; the same principles apply to temporary cofferdams in seasonal waterways.
• Construction live loads: Drilling equipment, cranes, concrete trucks, and material stockpiles inside or adjacent to the cofferdam impose significant loads that must be included in the design.

Dewatering & Adjacent Foundation Effects

Dewatering the cofferdam interior lowers the groundwater table in the surrounding soil. The shape and extent of the groundwater drawdown cone depends on soil permeability, pumping rate, and cofferdam seal effectiveness. Potential effects on adjacent structures include:

• Settlement of shallow foundations: Lowering the water table in compressible fine-grained soils (silt, soft clay) can cause consolidation settlement. Structures founded on shallow footings in the drawdown influence zone can settle differentially — cracking masonry, racking frames, damaging utilities.
• Loss of pile capacity: Timber piles used as foundations on older waterfront structures (common in southern Ontario heritage buildings) can decay when the water table is lowered and the pile's upper portion is no longer submerged. This is an irreversible and permanent consequence of dewatering adjacent to timber pile foundations.
• Mitigation through re-injection: Where groundwater drawdown near sensitive structures cannot be prevented by improved cofferdam seal, re-injection wells can actively inject water back into the ground adjacent to vulnerable foundations to maintain the water table level. This requires careful flow balance management.

Monitoring During Cofferdam Operation

Ontario OHSA Section 218 requires inspections after significant weather events. Best practice for P.Eng-managed cofferdam projects includes continuous or frequent monitoring of:

• External water level vs. internal water level (net head on the cofferdam walls)
• Cofferdam wall deflection (inclinometers in instrumented sheet piles)
• Piezometric heads around the cofferdam perimeter (shallow piezometers to detect seepage shortcutting the sheet pile seal)
• Settlement of adjacent structures and ground surface settlement points
• Cofferdam strut/waler loads (load cells in critical bracing elements in deep or complex cofferdams)

Monitoring trigger levels and action thresholds — the values at which construction must slow, stop, or the engineer must be notified — must be established in the P.Eng. design and communicated to the site superintendent before cofferdam dewatering begins.

Cofferdam Removal

Cofferdam removal is as critical as installation. The removal sequence must be engineer-designed because re-flooding the cofferdam interior too quickly can cause: flotation uplift on the new structure (if the permanent structure is not yet backfilled and has upward water pressure below it); structural damage to the newly-completed foundation if re-flooding creates hydraulic surge; and re-mobilization of sediment into the watercourse creating turbidity in excess of DFO permits.

Sheet pile extraction by vibratory hammer can cause settlement and vibration impacts on adjacent structures similar to pile driving. The extraction sequence and vibration monitoring thresholds must be included in the removal plan.