Extreme underwater tunnelling conditions tackled step by step in Hong Kong
For years, Hong Kong’s Drainage Services Department has been undertaking work to improve the waters in and around Hong Kong Harbour. The Harbour Area Treatment Scheme Stage 2 (HATS II) consisted of two tunnelling contracts C-23 and C-24 and a total of 21 km of tunnels.
C-24 consisted of 7.5 km of tunnels from Aberdeen to Sai Yin Pun. Alignment followed closely along the coast passing Wah Fu, Cyberport, Sandy Bay, and completed at Sai Ying Pun connection facilities on the western side of Hong Kong Island. Five working shafts were located at each point.
The chosen method for HATS II was Drill and Blast after disappointing results on the prior Strategic Sewage Disposal Scheme (SSDS) Stage 1 where Tunnel Boring Machines were operated.
This high-profile project faced several challenges. The relatively small excavation diameter combined with the requirement of providing adequate tunnel ventilation meant all tunnelling equipment had to be small and compact enough to pass underneath the ventilation ducting.
Extremely tight water ingress controls were required by the client. The quality, speed and efficiency of pre-injection grouting were going to play a major part in achieving the tunnel advancement rates.
The equipment had to be highly versatile, nimble, and manoeuvrable to achieve the tight tunnel excavation program specifications.
Normet OY signed a contract for the supply of 11 Variomec 1090Ds, 6 Multimec 6600s with 29 various cassettes, 3 Spraymec 1050 WPC’s and various grouting and hand-spraying shotcrete equipment. Later in the project, an additional Spraymec 6050 and a Multimec 6600 were supplied on a monthly rental basis.
The tunnel conditions were considered highly aggressive due to salt water encountered, being harsh on all tunnelling equipment. A comprehensive LTC service package was provided to repair, service and maintain the Normet Fleet on the project.
Normet also provided unique tunnelling chemical solutions to the whole project in addition to the extensive range of equipment. The chemical solutions included 5,000 tons of TamCrete MFC and associated admixtures, 400 tons of TamCrete 400CS (Colloidal Silica Technology) and associated GMA grout packer systems for pre-injection grouting.
Sprayed concrete performance was also considered critical to the excavation cycle. Normet Hong Kong Ltd. assisted the contractor with mix design optimization, approval trials, full on-site training, and all sprayed concrete admixtures, alkali-free accelerators, and steel fibres for the project.
The logistical issues of supplying sprayed concrete from working shafts are difficult even without the added issues that come from using external companies to supply the sprayed concrete mixes on day shift only, especially when working 24 hours per day and 7 days per week.
The contractor made ultimate use of Normet’s TamCrete HCA for onsite storage of sprayed concrete, which enabled them to continue excavation throughout the night with the ability to spray temporary rock support when needed.
TamSeal 800 spray-applied membrane was trialed and selected to provide a watertight underground workshop/maintenance area.