TBM tunnelling through unfavourable ground conditions : a case study, SSDS tunnel F, Hong Kong

Abstract

A study of the Strategic Sewage Disposal Scheme (SSDS) Stage 1 Tunnel F in Hong Kong was carried out as it is a great example of deep sub-sea hard rock TBM tunnelling through unfavourable, fault-affected ground conditions with heavy water inflows. The main objective of this study was to document events that took place during Tunnel F excavation and collate geological and geotechnical data related to the excavation, to aid future tunnel designers and contractors to assess the risk involved with such tunnelling projects. Data relevant to Tunnel F was collected from many personnel involved with the excavation, and from the Hong Kong Geological Survey (HKGS) of the Geotechnical Engineering Office. Geological maps, ground investigation data, tunnel mapping records, ground water inflow records and grouting records were reviewed and used to summarise the ground conditions and to analyse various potential relationships.

Knowledge of ground conditions is the most important requirement for any tunnelling project. This dissertation addresses the differences between actual and predicted ground conditions and the consequences of lack of ground investigation data. The major difference between predicted and observed ground conditions during Tunnel F excavation was the amount of water ingress into the tunnel. Hundreds of liters per minute of water at up to 13bar pressure was encountered from a few probe and grout holes. In the Tolo Channel Fault area, low Q values and very high ground water inflows were experienced, and heavy temporary support and large grout volumes were required to complete the tunnel.