Design And Construction Of A Circular Secant Pile Shaft For Connection To A Deep Collector Sewer In Ottawa

ABSTRACT: Construction of Stage 2 of Ottawa’s Confederation Line LRT system required re-routing of a shallow sewer near Cleary Avenue and making a connection for it to the existing 1.5 m I.D. West Nepean Collector (WNC) sewer at about 13 m depth. A water-tight shaft needed to be constructed to make that connection due to ground conditions consisting of glacial till with intermittent thick water-bearing sand layers. An 8 m I.D. circular secant pile shaft was constructed to make that connection. Challenges for the design and construction of the shaft included having to span over and around the WNC without damaging it as well as sealing the ‘windows’ around the WNC against groundwater inflow and potential ground loss around the shaft. The WNC was constructed in the 1950’s and the condition of the pipe was of concern. The method of construction of the WNC was also unknown. Therefore, the pipe required monitoring for strains and vibration and was continuously inspected by internal video camera feed for the duration of secant pile drilling above the pipe and during excavation of the shaft. A pre-excavation grouting program of the windows around the WNC was also implemented through sleeves in the secants to address the groundwater inflow concerns. This case study provides details on the design, instrumentation, monitoring, construction, and performance of the shaft.