Three dimensional analyses to assess induced stresses around closely spaced tunnels in the WestConnex M4 East tunnels, Sydney, Australia

WestConnex will widen M4 between Parramatta and Homebush and extend it underground in multilane tunnels between Homebush and Haberfield. The underground portion of the WestConnex M4 East project also includes ramps at the interchanges and a ventilation system in the northwest of Sydney. The Parramatta Road Ventilation Facility (PRVF) is located at the east end of the project and consists of a three-lobed shaft with seven ventilation tunnels radiating out from it to the surrounding, nearby ramps and mainline tunnels. The ventilation tunnels are located approximately 30 to 40 metres below ground surface in fresh Sydney (Hawkesbury) Sandstone. Several of the tunnels are spaced as close as approximately 2.4 metres. The proximity of the tunnels to one another required a thorough assessment of the anticipated geotechnical conditions, and detailed three-dimensional analyses to assess induced stresses, stability of the narrow rock pillars formed between adjacent tunnels intersecting with the shaft, and associated rock support requirements. The primary focus of the assessment was the potential behaviour of the openings in areas where multiple tunnels and riser shafts intersect with each other, or different tunnels underpass or overpass in close proximity. The analyses considered the magnitude and direction of the in situ stress field including the effects of excavation sequence of the numerous nearby excavations. As well as reviewing the results of the three-dimensional analyses, this paper compares predicted to observed ground where possible at the time of publishing.