West Gate Tunnel Project, Melbourne, Australia: Northern Portal Ground Engineering Works - Secant Piling and Cutter Soil Mix Works-Paper

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The West Gate Tunnel Project’s Northern Portal in Melbourne required extensive ground engineering for a 330 m cut-and-cover tunnel and TBM launch area. WPBA JV, a partnership between Wagstaff Piling and Bauer Foundations Australia, delivered a range of foundation works in variable ground conditions, including backfill, alluvial soils and basalt bedrock.<br /><br />The works included hard-hard and hard-soft secant pile walls, bored and CFA piles, plunge columns, sheet piles, driven piles and cutter soil mixing (CSM). Early contractor involvement enabled design changes to improve constructability and cost. In the deepest excavations, the reference hard-soft secant wall was replaced with a hard-hard arrangement, reinforcing both primary and secondary piles. This redistributed reinforcement more effectively, reduced congestion and improved cage installation and concrete flow.<br /><br />A CSM block was built behind the launch headwall to stabilize alluvial soils and control groundwater. Its rectangular panels provided continuous treatment down to competent basalt. Trial mixes and ongoing strength testing informed adjustments to the cement mix, with the block designed for a target unconfined compressive strength of 600 kPa.<br /><br />Construction was complicated by a restricted, congested site requiring coordination of numerous piling rigs, cranes, deliveries and follow-on contractors. Weekly and daily sequencing, updated site layouts, traffic controls and exclusion zones helped manage interfaces safely. To meet the stringent 1:200 verticality tolerance for deep secant piles, the team used accurately positioned guide walls, stiff double-wall casing and regular quality checks.<br /><br />Long reinforcement cages were spliced horizontally on the ground and installed in single lifts, improving efficiency, safety and coupler accuracy. Overall, tailored design and construction solutions enabled the complex ground engineering works to be completed to the required standards, budget and schedule.

Keywords

West Gate Tunnel Project

Northern Portal

ground engineering

secant pile walls

cutter soil mixing

basalt bedrock

TBM launch area

alluvial soils

foundation works

constructability