Complex Logistical Constraints and their Technical Impact on A Deep Excavation Design, Ottawa, Ontario, Canada-Paper

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The paper describes the design and construction of temporary excavation support for Zones 12 and 13a of Ottawa’s Confederation Line Extension cut-and-cover tunnel. Excavations reached 12 m deep through fills, sands, and glacial soils. Tight property boundaries, traffic staging, existing utilities crossing the alignment at steep angles, and limits on groundwater drawdown drove the design.<br /><br />Zone 12 used internally braced soldier-pile-and-lagging walls. Zone 13a required a watertight secant-pile system to limit groundwater impacts on nearby sensitive clay. Where utilities prevented secant-pile installation, the team developed a combined solution using secant piles, micropiles, pipe piles, lagging, reinforced-concrete diaphragm slabs, and utility support beams. The slabs and bracing transferred loads around irregular openings while maintaining excavation stability. A carefully staged sequence also enabled the shared bulkhead to be removed and the zones connected without losing support for the utilities or excavation.<br /><br />Construction began before the final design was issued, so the contractor and designers incorporated field-driven changes as work progressed. Vacuum excavation exposed utilities before piling; monitoring and staged load transfers protected them. Field observations led to refinements, including shotcrete sealing between micropiles and changes to the bracing-and-slab sequence.<br /><br />The work was completed successfully, with no visible water infiltration or utility damage. The authors conclude that logistical constraints can govern excavation design as strongly as technical requirements, so they should be identified early. Continuous designer–contractor collaboration and adaptable sequencing were essential to achieving a safe, constructible solution.

Keywords

excavation support

temporary support

cut-and-cover tunnel

soldier pile wall

secant pile wall

groundwater control

utility protection

micropiles

construction sequencing

Ottawa Confederation Line