Construction of a Retaining Structure Within Complex Site Conditions-Paper

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The project involved constructing a diaphragm-wall retention structure for a tunnel access shaft on a major Melbourne rail project. The site was a former sand quarry backfilled with highly variable, uncontrolled fill to depths of 13–14 m, over natural sands with a high water table. All fill was contaminated with non-friable asbestos. The wall comprised 57 reinforced-concrete panels, 1 m thick and 24–31 m deep; 38 panels extended through the contaminated fill.<br /><br />Uncertain ground conditions created risks from obstructions, excavation instability, concrete overbreak, and changes to bentonite properties. Strict asbestos controls, mandatory breaks and decontamination also constrained working time. In addition, airport height limits restricted lifting equipment and reinforcement-cage lengths.<br /><br />The team used hydraulic diaphragm-wall grabs with bentonite support fluid and began with 19 panels in uncontaminated ground. Measures to maintain productivity included shortening stop-ends, prefabricating reinforcement cages up to 24.5 m, and coordinating staggered shifts. Silt and organic material in the fill affected bentonite density and viscosity, requiring additional desilting, polymer treatment, and fluid replacement. Concrete quality was managed through slump-flow testing of every delivery.<br /><br />Despite obstructions, difficult stop-end removal and variable concrete performance, the diaphragm wall was completed on schedule and within budget. Subsequent excavation revealed a noticeably less regular wall surface in the uncontrolled fill than in the natural ground, consistent with voids or minor excavation collapses. The case highlights the importance of detailed planning, adaptable construction methods and close client collaboration on contaminated, variable-ground sites.

Keywords

diaphragm wall

tunnel access shaft

Melbourne rail project

contaminated fill

asbestos

bentonite support fluid

hydraulic diaphragm-wall grabs

ground conditions

reinforced-concrete panels

construction planning