Breakthrough at Lai Chi Kok main tunnel

5 January 2012

Breakthrough was reached last month at the main tunnel of the Lai Chi Kok Drainage Tunnel project in Hong Kong. Located in an urban Area, sensitive structures that included railways, viaducts and culverts were of primary concern to client Hong Kong Special Administrative Region Drainage Services Department.

Contractor Leighton-John Holland JV employed a Herrenknecht slurry TBM to bore through bedrock 40m deep under high pressure to avoid surface disruption. Supervising consultancy was provided by AECOM who worked for nearly two years on planning and design work with the client.

The 4.9m internal diameter, combined 3.7km main and branch tunnel, along with associated shafts and adits, will intercept stormwater in West Kowloon and transfer it into Victoria Harbour.

Geology comprised granite rock with limited sections of shallow cover and mixed ground in the high level branch tunnel. Faults and intrusions of dyke rock were also present. In the main tunnel there were distinct sections of granite rock at launch and completely decomposed granite at recovery, with a section of mixed ground in between in an almost 1:1:1 split.

Ray Chan, deputy project manager for the contractor’s detailed designer, Atkins, said, “The final breakthrough marks the successful completion of a highly complex and challenging tunnelling project. The scheme presented a huge engineering challenge: constructing tunnels beneath densely populated areas and a dry shaft right next to the harbour, and contending with extreme earth and water pressure.”

Drainage Services Department director Chan Chi-chiu, who officiated the breakthrough ceremony on 13 December, added “We have overcome a number of challenges during the construction of the main tunnel including driving the TBM across four overlying operational railway lines and through the foundations of numerous existing infrastructures.

“Moreover, the work has involved 90 construction operations under hyperbaric pressure up to 4.2 bar, which is unprecedented in Hong Kong. We have taken this opportunity to bring to Hong Kong new hyperbaric technology for tunnel construction.”

Aecom executive director Bob Frew added, “The feasibility study proposed drill and blast with pre-injection grouting 80 to 100m below ground for the main tunnel. We believed a closed face TBM was more appropriate as it eliminated cost uncertainties and major construction risk. I think this was why we were the client’s choice, because we did not offer the Hong Kong standard solution of drill and blast and grouting.

“The only unique aspect was the decision to locate the tunnel at a depth that required hyperbaric intervention at pressures above 50 psi (3.45 bar). Up to that point it was a commonly held view that intervention above this pressure was inherently more hazardous. Therefore special effort had to be made to demonstrate to the regulatory authority (Labour Department) that the health outcome would be as good, if not better, than that under the decompression tables prescribed for work up to 50 psi (3.45 bar). This placed additional burden on the contractor as the party responsible for obtaining approval but with the support of Aecom and the client, this was achieved.

“The project was also one of three Drainage Services Department projects where the use of a geotechnical baseline report (GBR) was considered. The others were the Hong Kong West Drainage Tunnel and the Tsuen Wan Drainage Tunnel. The former didn’t use one though both Lai Chi Kok and the latter did, and a major effort was put into the Lai Chi Kok GBR.”

Flood control has historically been an important concern for West Kowloon. The new system will catch surface runoff at six locations to protect low-lying urban areas. The system will be able to deal with a one in 50 year flood event.

The Lai Chi Kok branch tunnel was completed in February 2011. It took 11 months of construction. After this, construction on the main tunnel began upon arrival of a VMT navigation system for the hard rock machines.