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As part of Contract 855 of the Circle Line in Singapore,two slurry shield TBMs had to be driven from One North to Farrer Road Station,passing through Buona Vista and Holland Village Stations.The tunnels passed directly beneath a number of buildings.The ground consisted of a constantly varying face of mixed soil and rock grades of weathered Jurong Formation(sedimentary rock)and Bukit Timah Granite.The granite over part of the drives had been metamorphosed into massive gneiss.The boundary between the rock and soil weathering grades in the Granite and Gneiss varied,but could be abrupt.The completely weathered(grade Ⅴ)granite or gneiss at the boundary was granular and permeable in nature,while the overlying grade Ⅵ had a low permeability and was cohesive.The tunnelling conditions therefore varied widely.The basis of the face pressure calculations is presented.With the high groundwater levels characteristic of Singapore,there was a very small margin between the minimum pressure required to maintain face stability and the maximum pressure required to control the risk of slurry escape to the surface.