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  Excavation of Dry Subsea Rock Tunnels in Hong Kong using Micro-Fine Cement and Colloidal Silica for Groundwater Control

  The 20 km of HATS2A sewage conveyance tunnels excavated by drill & blast are placed from 70 to 160 m below sea level and pass underneath the Victoria Harbour shoreline of Hong Kong Island. Extremely strict limits on residual groundwater ingress were imposed to avoid surface settlement and damage, requiring Pre-Excavation Grouting (PEG) to limits between 2.5 and 50 L/min/100 m of tunnel. 89% of the excavated 15 km shows less water ingress than the local limit value. This has been achieved by continuous probe drilling, grouting with micro fine cement only, supplemented with colloidal silica. The implemented overall PEG approach represents a unique combination of elements described in detail in this paper and in references. Of the two main rock types (granite and volcanic tuff), the tuff presented the more demanding case. In tuff, substantially more drilling and grouting materials were needed to reach the targeted tightness of the ground.

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