PROJECT REPORT | TRENCHLESS


spacing and degrees of weathering. The upper bedrock surface locally consists of weathered and decomposed rock, transitioning to more competent material at depth. Overall rock quality was suitable for tunnelling but posed significant challenges for conventional excavation methods. Groundwater conditions were characterised by perched water accumulating on the relatively impermeable bedrock surface during wet weather. These conditions increased the complexity and risk associated with deep open-cut excavation and dewatering, particularly at the trench depths that would be required.


3. CONSTRUCTION METHODOLOGY The selected pipeline crossing extends approximately 137m (450 linear feet) beneath the Lincoln St and West 23rd Ave intersection. The installation consists of a 1828mm- diameter (72 inch) steel casing installed by slurry-type microtunnelling, housing a 762mm-diameter (30 inch) welded steel carrier pipe and a 610mm-diameter (24 inch) ductile iron carrier pipe. The 30-inch and 24-inch pipelines are for potable water and storm drainage, respectively, and were installed side by side, horizontally, in the casing/ utility tunnel. Depth of cover was governed by existing utilities, required vertical clearances, and connection elevations at the College Hill Storage Tank site, and ranged from 2.8m-7.6m (9ft -25ft) along the tunnel alignment. The resulting profile placed the alignment well within bedrock, requiring a construction method capable of maintaining precise line and grade under rock conditions while minimising surface impacts.


3.1 Open-cut pipeline construction Open-cut pipeline construction is the most common installation method for urban pipelines and is typically the first option evaluated due to contractor familiarity and perceived simplicity. For the College Hill under-crossing, however, the approach presented substantial technical, logistical, and community-related challenges, as follows: Required construction elements: Open-cut installation


would require large excavators, rock-hauling trucks, substantial support-of-excavation systems, continuous dewatering, and controlled blasting operations. Due to trench depths approaching 10.7m (35ft), support systems would likely have to include soldier pile and lagging systems socketed into bedrock. Blasting would need vibration monitoring, blast mats, permitting, and extensive safety controls. Surface area & excavated materials: The open-cut


footprint would cross the full width of the intersection and adjacent roadway approaches. Surface impacts would have included removal and reconstruction of pavement, curbs, sidewalks, ramp improvements, and landscaping, along with potential tree removal. Excavation quantities would have included bedrock and associated spoils requiring off-site disposal. Dewatering volumes would have been significant due to expected perched groundwater conditions, increasing handling and treatment requirements. Risks: Primary risks associated with open-cut


construction included blast-induced vibration damage to nearby residential structures and utilities (water and


MTBM telescopic can. July 2026 | 21


Table 1 - Evaluation Summary Criteria


Schedule


Contractor Availability Equipment Availability Public Impacts O&M Cost


Open Cut Slow Yes Yes


High Poor High


Blasting Slow Yes Yes


High Poor High


sewer predominately), public safety hazards, overhead power line conflicts, over-excavation, and support-of- excavation. Residential structures were as close as 9.2m (30ft) to the alignment. Extended traffic disruptions and community opposition further increased overall project risk and uncertainty.


3.2 Microtunnel pipeline construction Microtunnelling is a highly controlled trenchless construction method that allows for precise installation of pipelines beneath surface obstacles with minimal disturbance. For the College Hill crossing, the construction method was evaluated to be a technically robust alternative to open-cut, capable of addressing both geotechnical and community constraints. Required construction elements: The microtunnelling


approach required construction of a Launch Shaft, 11m by 6.7m (36ft by 22ft), and a Reception Shaft, 4.6m by 4.6m (15ft by 15ft), each strategically located to minimise surface impacts. Major equipment would use a slurry- type MTBM, jacking frame, jacking pipe, slurry separation plant, and guidance and control systems. Steel casing was installed by jacking behind the MTBM, providing immediate ground support and a stable conduit for subsequent carrier pipe installation. Surface area & excavated materials: Surface disturbance


would be limited primarily to the locations of the Launch Shaft and Reception Shaft. Excavated materials would be from the shafts and tunnel boring, for which spoil would be removed hydraulically via slurry separation. Dewatering would be limited to shaft construction and localised groundwater management. Risks: Primary microtunnelling risks included over-


excavation, loss of line and grade, and the potential for a stuck MTBM. These risks were mitigated through detailed geotechnical investigation, appropriate cutterhead selection, real-time monitoring of jacking forces and slurry pressures, and experienced contractor execution.


Microtunneling Faster Yes Yes


Medium Better


Highest


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