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  WestConnex M4 -M5 Link - Construction of the St Peters Interchange Ramps and

  The WestConnex M4-M5 Link Tunnels D&C project includes 21km of mined tunnels that connect two underground motorways and an underground interchange in metropolitan Sydney, the M4 at Haberfield, and the M8 at the St Peters Interchange (SPI) and the Rozelle Interchange. The project started in 2018 and is due to be completed in 2023. The M4-M5 Link tunnel alignment crosscuts the Woolloomooloo Fault Zone (WFZ), one of the most significant geological structures in the Sydney metropolitan region. The SPI access ramps, and ventilation tunnels provided a unique opportunity to view this major fault zone over many different spans of tunnels and across the three dominant geological units in Sydney, namely the Ashfield Shale, the Mittagong Formation and the Hawkesbury Sandstone. This paper details the geological conditions encountered during the construction of the SPI tunnels within and adjacent to the WFZ across these three geological units, as well as the challenges faced to tunnel through the WFZ. The fault zone was overall more extensive than expected and was observed to have a much greater influence on the Ashfield Shale than the Hawkesbury Sandstone. Ground conditions encountered during construction were generally in line with the extent and characteristics anticipated at design stage in the deep ventilation tunnels. Ground conditions were poorer than expected in the shallower access ramps, which were entirely within the Ashfield Shale.

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