Mined tunnel ground behaviour with faults and dykes in Sydney sandstone and Shale

This paper presents and discusses typical ground behaviour from a recent mined tunnel in Sydney in sandstone and shale where typical Sydney basin geological structures were encountered. Geological structures include high and low angle normal and reverse faults in both sandstone and shale and contacts between dykes and the country rock. Major geological features including faults and dykes in Sydney sandstone were rarely encountered. where they were intersected in the tunnel, they caused minor discontinuity-controlled gravity induced falling and sliding of blocks leading to overbreak < 1 m. Faults and dykes in shale were more common on this project. Faulting often combined with bedding planes and joints causing significant discontinuity-controlled gravity induced falling and sliding of blocks combined with local shear failure on discontinuities leading to overbreak up to several metres. Dykes reduced the quality of the shale rock mass up to several metres either side of the structure leading to spalling of the tunnel face. Spiles were effective at preventing ravelling of fresh dyke and shale country rock beyond the tunnel profile