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

PPT Presentation

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The Melbourne Arts Centre redevelopment required a 10–15 m deep basement beneath a heritage structure, in very soft Coode Island Silt with high groundwater and limited working space. Existing basement elements, raked piles and redundant ground anchors constrained the excavation footprint and the layout of the support system.<br /><br />The design integrated a 900 mm CFA soldier-pile wall, jet-grouted infill and base block, CFA piles and bored tension piles socketed into siltstone. The system had to control groundwater, heave and uplift while avoiding legacy obstructions. Serviceability crack control governed reinforcement in many tension piles, and analyses showed significant shear near the tops of the rock sockets. This required full-length reinforcement cages to depths of about 30 m, making cage handling, concrete workability and installation sequencing major construction risks.<br /><br />Eight of 490 piles struck redundant anchors. Each clash was investigated, used to refine the inferred anchor paths, and addressed through replacement piles or design changes. Jet-grout column positions were monitored downhole and adjusted progressively to limit shadowing and avoid unnecessary overlap.<br /><br />Verification was built into construction. PDA/CAPWAP testing confirmed socket shaft friction above the design value, and pull-out tests confirmed pile-to-grout bond strength above the design value. Jet-grout strength and permeability were also tested, while inclinometer arrays tracked wall movement. Maximum measured deflection was approximately 15 mm, below trigger levels. Back-analysis indicated that the wall was stiffer than predicted, partly because of jet-grout performance and its connection to the existing basement.<br /><br />The project demonstrated that deep basements in very soft clay can be delivered using an integrated retention and uplift system. Key lessons were to treat historical anchor records as guidance rather than precise set-out data, make jet-grout layouts adaptable, plan verification early, and address constructability and design flexibility alongside structural calculations.

Keywords

Melbourne Arts Centre

deep basement excavation

heritage structure

Coode Island Silt

groundwater control

CFA soldier-pile wall

jet grouting

bored tension piles

socketed into siltstone

construction verification