Characterisation Of Complex Ground Conditions For The Rozelle Interchange Project

Beatriz Estrada, Tim Nash, Andrew de Ambrosis and Irene Chan

The Rozelle Interchange Project (RIC) in Sydney is an underground motorway interchange connecting multiple underground and surface arterial roads as well as the future Western Harbour Tunnel and Beaches Link. RIC completes the WestConnex program of works and is a complex array of approximately 22 km of multiple level tunnels, all constructed in an area 2.5 km long and 1.5 km wide.

RIC is located within complex ground conditions that include deep soils, regional faults, structural zones and igneous intrusions. Deep natural soils infilling a valley near Rozelle Bay are mostly recent Holocene alluvial, marginal marine and marine deposits. These soils are interlayered, discontinuous, normally to slightly over consolidated and capped by sand and coarse rockfill from 19th century reclamation.

There is a strong contrast in the level of detail between borehole and CPT data. Distilling this to provide a geological and geotechnical model for a project wide interpretive report for designers of multiple structures required a hybrid approach to model presentation. This included providing a simplified graphical model and including details from specific investigations and laboratory testing allowing designers flexibility to adopt appropriate parameters for their specific application.

Similarly, the rock structural model evolved from development of structural domains to identification and inclusion of regional geological structures overprinting the structural model. Regional scale thrust faults, corridors of structural complexity and igneous intrusions were identified and refined prior to and throughout the design process. These were considered in the design by modification of excavation sequencing and changes to tunnel support.

Tunnel excavations encountered these regional features at the locations predicted and with similar character as those described in the model allowing the safe construction of the tunnels.