Jet Grout Retention and Deep Tension Piles in Very Soft, Normally Consolidated Clay: Challenges from the Melbourne Arts Centre Redevelopment-Paper

Paper

Pdf Summary

The Melbourne Arts Centre redevelopment required a 10–15 m basement excavation in very soft, normally consolidated Coode Island Silt, with a high groundwater table and significant heritage, access and construction constraints. Redundant ground anchors and piles made conventional secant or diaphragm walls impractical, while a heritage spire restricted equipment headroom across part of the site.<br /><br />The adopted system combined CFA soldier piles with jet-grouted infill columns and a jet-grout base block. CFA and polymer-supported bored tension piles, extending to depths of up to 32 m and socketed into siltstone, resisted uplift and restrained the excavation. The base block also helped control seepage, base heave and wall movement. Design and construction were complicated by variable geology, pressurised groundwater, legacy obstructions, low-headroom work and the need to install full-length reinforcement cages in deep piles.<br /><br />Verification included pile dynamic testing, jet-grout sampling and testing, a pull-out trial, and continuous inclinometer monitoring. The jet-grout pull-out tests exceeded the design bond value, while measured wall deflection reached about 15 mm—below predictions of up to 34 mm. Back-analysis attributed the difference mainly to conservative assumptions about grout stiffness and connectivity with the existing basement.<br /><br />Key lessons included the importance of early coordination and detailed sequencing; treating legacy as-built information cautiously; careful concrete and cage-placement planning for deep CFA piles; and using suitable low-headroom equipment. Monitoring column alignment enabled the team to reduce jet-grout column quantities while maintaining coverage. Overall, the project demonstrates that deep basements in very soft clay can be delivered using an integrated retention and foundation system, provided conservative design, comprehensive verification and close collaboration are maintained.

Keywords

Melbourne Arts Centre

basement excavation

Coode Island Silt

jet grouting

CFA soldier piles

groundwater control

heritage constraints

tension piles

wall deflection monitoring

retention system