Hong Kong’s Drainage Services Department has for years 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. Due to a dissatisfying experience on the prior Strategic Sewage Disposal Scheme (SSDS) Stage 1 where Tunnel Boring Machines were used, the chosen method for HATS II was Drill and Blast.
There were several challenges to this high profile project. Firstly, 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 and also be highly versatile, nimble and maneuverable to enable the tight tunnel excavation program to be achieved. There were extremely tight water ingress controls required by the client, so the quality, speed and efficiency of pre-injection grouting were going to play a major part in achieving the tunnel advancement rates.
Normet OY signed a contract for the supply of 11 Variomec 1090D’s, 6 Multimec 6600’s with 29 various cassettes, 3 Spraymec 1050 WPC’s, together with various grouting and hand spraying shotcrete equipment. An additional Spraymec 6050 and a Multimec 6600 were later supplied on a monthly rental basis.
A comprehensive LTC service package was also provided to repair, service and maintain the Normet Fleet on the project. The tunnel conditions were considered highly aggressive due to salt water encountered. This proved to be very harsh on all tunnelling equipment.
In addition to the extensive range of equipment, Normet also provided unique tunnelling chemical solutions to the whole project, which 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 accelerator and steel fibres for the project.
The logistical issues of supplying sprayed concrete from working shafts are problematic by themselves 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.
Although the project commencement was delayed by over 12 months due to other unforeseen circumstances in the pre-qualification and tender stage, the contractor safely completed excavation and temporary support within a positive timeframe, and the tight water ingress requirements were kept well within the allowable limits.
The customer reported that the combination of TamCrete MFC and TamCrete 400CS had performed well and had proved to be the correct choice. The Normet equipment had performed well under the aggressive conditions underground but showed the wear when finally returned to Normet.