Investigation of Complex Ground Conditions at Sydney Park for the Sydney Metro City & Southwest Running Tunnels

The Sydney Metro City & Southwest (SMCSW) project will provide additional underground rail services between Chatswood and Marrickville. The two running tunnels pass close to Sydney Park, which was quarried in the 19th and 20th centuries for the manufacture of pottery and bricks and resulted in the excavation of large pits of up to 60 m depth. Quarrying ceased around 1948 with the pits progressively backfilled with municipal waste over the next three decades. The running tunnels were designed to be constructed through a long rock pillar formed by deep backfilled pits on either side. The site history posed numerous geotechnical risks to the design and tunnelling works. These risks included intersecting the waste in the pits, ground instability due to the proximity of the pit walls, and uncontrolled and potentially contaminated groundwater inflows into the tunnel during construction. To address the design and construction uncertainties the proposed tunnel alignment was initially verified by a detailed desktop study. Information on the geometry of the excavated pits was mostly limited to aerial photos from 1934 onwards. Comprehensive geotechnical investigations were subsequently undertaken to confirm that the rock pillar provided sufficient separation between the tunnel alignment and adjacent waste-filled pits. During the desktop study a reference to an abandoned tunnel was discovered on a survey plan dating from 1937, with the old tunnel shown crossing the SMCSW tunnel alignment. A targeted site investigation was undertaken using conventional geotechnical boreholes as well as a range of geophysical techniques to successfully locate the position, depth and condition of the abandoned tunnel. This paper presents the results of the desktop study and site investigation work used to define the extent of the backfilled pits and the abandoned tunnel and confirm that the tunnel alignment would avoid these hazards. This work provided critical input to managing the design and construction risk associated with the complex ground conditions caused by over 100 years of human activity and enabled successful construction of the running tunnels.

Keywords: Sydney Metro, Sydney Park, Brick pits, TBM Running tunnels.