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  Tunnelling in Mixed Ground Condition - A Challenge to Design and Construction

  In one of the civil contracts of Downtown Line 2 in Singapore, the construction of twin bored tunnels was conducted using Slurry TBMs. The TBM drives faced challenges involving high abrasivity and high permeability of the Bukit Timah Granite Formation. Due to its variability of weathering properties, such geological formation often leads to frequent cutter-head interventions and other associated problems created by the presence of localized sand lenses as well as mixed face of fractured rock and soil, which could cause localized face collapse especially during the restarting of TBM after intervention. Geo-tomography survey was carried out to map the mixed face locations which had facilitated the TBMs to be operated in optimal condition prior to entering or leaving the “mixed face” conditions. This paper describes the major challenges faced in construction stages, and outlines the successful controlled measures that were implemented to minimize ground surface.

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