The 13 civil contracts for the 12.5km route include various different tunnelling methods: hard rock, cut+cover and the use of a version of NATM for soft ground tunnelling. This is the first application of the last mentioned method in Hong Kong, and the difficulties are exacerbated by the presence nearby of a major sewer line. Other problems include excavation under roads with minimal cover and the requirement to break into a live line in the immersed tube tunnel under the harbour. A demanding part of the project is the large-scale excavation needed for five new stations, two of them double level.
All this work, totalling US$3.9bn, is for a new line running from Hong Kong Island’s Quarry Bay and its extension to North Point. The line runs across the Eastern Harbour Crossing into a new development zone at Tseung Kwan O. This is now reclaimed land and the site of a new town with 200 000 inhabitants, due to rise to 520 000 ‘or perhaps even 600 000’ says George Turnbull, MTRC‘s overall project manager for the scheme.
A large proportion of the population will be commuters and the TKE line is scheduled to be in profit from the opening date in December 2002, not 2001 as originally intended. The first of the civils contracts and of another 21 M&E contracts were let in November 1998 and about one third have been awarded so far.
The new line will take over the Eastern Harbour crossing’s twin tracks from the existing Kwun Tong Line, which will be re-directed into an extension running parallel to the TKE. This will proceed as far as the first two new stations at Yau Tong and Tiu Keng Leng.
A second phase of the TKE will create a new station at Tseung Kwan O South, the site of the large depot on 11ha. of reclaimed land, reached by a southern branch in the line. The line out of the depot to Tseung Kwan O will involve some of the most complex tunnelling. The Pak Shing Kok running tunnels pass mainly through rock, but this ranges from highly fractured volcanic to completely decomposed volcanic ( CDV) near the southern portal. This contract was let to the Kier International/ Hyundai JV in November.
Typically, the CDV forms a 10-20m thick layer. The contractor is using shotcrete and steel arches at 750mm centres to support it, usually with 200mm thick steel fibre reinforced shotcrete. The technique could be described as a soft ground variant of NATM, and is used for ‘the first time in Hong Kong’ says Dr Gareth Page, the MTRC’s construction manager for the two main tunnelling contracts: the hard rock Black Hill tunnels and the Pak Shing Kok tunnels.
Cut+cover tunnels and excavation at Tseung Kwan O will form the main station and the route north. Australia’s Leighton and China State Construction & Engineering Company have won this work. Further tunnel sections lead on to a deep excavation through soft ground and granite, which has four tunnels leading out from it on the west side. These are the Black Hill tunnels, which are longer but more straightforward than the depot tunnels. The portal here and the major part of the station were constructed in a preliminary excavation contract carving a 92m near vertical face in the hill and forming two platforms for housing and station development, Work on the $74m Black Hill tunnels started in April. Dumez and local firm Chun Wo were awarded the contract.
"The main restraint is vibration" says Page. "Because the portal at Yau Tong is in a built-up area, this has to be kept to 13ppv, which means lower production." Even at the other portal, the need to maintain the stability of the steep slopes has meant a restriction to 25ppv.
Beyond Yau Tong, mostly cut+cover excavation is required for the re-routed link to the old line, the diverging new link to the Eastern Harbour crossing and a station excavation. The most complicated part of this job will be the break into the harbour tunnel, which must remain live throughout. There are also complications with the approach tunnel, since it must pass under the cross harbour road with only 1-2m of cover. This will be overcome by using top down construction.
