Shani Wallis, Editor

Two drill+blast tunnelling contracts worth a total HK$6.3 billion (US$815 million) to construct a 20km long sewage conveyance system on Hong Kong Island and under Victoria Harbour to the central treatment works on Stonecutters Island are awarded officially to Gammon Construction Ltd and a joint venture of Leighton Asia and Leonhard Nilsen & Sonner AS (LNS) of Norway.

Fig 1. Alignment of the new tunnels for Stage 2 of Hong Kong's HATS sewage conveyance system

At an official contract signing ceremony last week, the largest contract for a total 12km of tunnelling beneath the dense urban highrise that is the northern fringe of Hong Kong Island from North Point in the east and including the under sea connection to the treatment works is awarded for HK$3.76 billion (US$485 million) to Gammon Construction.

The second, worth HK$2.54 billion (approximately $US330 million) and comprising 7.5km of the project from Aberdeen on the south side of the Island to Sai Ying Pun on the north side where it junctions with Gammon's undersea crossing, is awarded to the Leighton Asia/LNS JV.

Paul Y is awarded a third contract worth HK$236.5 million (US$30.5 million) to complete two 900mm diameter directional-drilling connections under the sea from Aberdeen to Ap Lei Chau Island south of Hong Kong Island.

Developed by the Hong Kong Drainage Services Department, the two tunnel contracts are the central components of the total HK$19 billion (US$2.5 billion) Stage 2A Harbour Area Treatment Scheme or HATS project designed to collect the remaining 25% of Hong Kong's wastewater, direct it to Stonecutters Island treatment works and improve substantially the quality of the oceans surrounding the island. Stage 1 of the project, with included substantial lengths of tunnelling on Hong Kong Kowloon side, was completed in 2001 and diverted about 75% of the sewage normally discharged into Victoria Harbour for treatment at Stonecutters Island.

Thomas Ho, Chief Executive of Gammon Construction (left) and K.K. Lau, Director of Drainage Services sign Contract 24, observed by Y. T. Loong, Chief Engineer of the DSD HATS Division

Where excavation of the 23.5km of tunneling for Stage 1 was all based on TBM excavation, all tunnelling for Stage 2 is specified by the client as drill+blast. This decision draws on hard lessons learned during the Stage 1 experience. An initial contract let to one contracting joint venture had eventually to be terminated and the works completed under continuing geotechnical difficulties by three new contractors under new contracts. One of these contractors was Gammon and Thomas Ho, the company's Chief Executive said at the signing of this Stage 2 contract: "Our expertise in tunneling work and our experience on HATS Stage 1 are key factors in the contract award."

Stage 1 was eventually completed at the end of 2001, some four years later than originally planned and at an increased HK$2 billion cost for the tunnels as compared to the original $1.3 billion contract. A paper written by senior engineers at the Drainage Service Department and presented at the ITA World Tunneling Congress 2009 in Budapest in May and reprinted below, explains the risk analysis work carried out by the DSD and its consultant, the Metcalf Eddy and Maunsell JV, that lead to the selection and specification of drill+blast for the new HATS 2 tunnels.

For Gammon, the project's Contract 23 is its largest ever single civil engineering contract. The 12km of tunnelling is divided into three drives of up to 165m deep and includes eight deep shafts. To enhance safety of the contract, Gammon has said it will adopt a specialized Tunnel Personnel Tracking System to monitor automatically the whereabouts and movement of workers underground and mobilize only electrically powered equipment, using the latest in computerized drilling jumbos and computerized grout injection systems to control predicted high volumes of water ingress. It says a total workforce of nearly 500 will be employed to complete the contract over the next five years to 2014. Jointly owned by Jardine Matheson and Balfour Beatty, Gammon has been headquartered in Hong Kong for over 50 years and has worked on many of its tunnelling and underground construction projects.

Director K.K. Lau (centre) confirms contract award with Ian Edwards, Executive General Manager, Leighton Asia, Hong Kong area.

The tunnels on the Leighton/LNS JV's Contract 24 run at depths of 70m to 120m below sea and the contract includes five deep level vertical shafts. Twin oval-shaped sewer pipes of 1.38m to 1.65m equivalent diameter will be installed to complete the in-situ concrete lined conveyance system.

Of the contract, Hamish Tyrwhitt, Managing Director of Leighton Asia said: "There are a number of challenges to address. Our partner LNS brings to the project leading drill+blast technologies to address complex geological conditions and material transport issues. We also propose to work with Macmahon to draw on their experience with construction of deep mining shafts in Australia." Leighton Asia, part of the Australia's Leighton Group, has been operating in Asia for more than 30 years and LNS (Leonhard Nilsen & Sonner) has extensive tunneling experience, having constructed numerous drill+blast tunnels throughout Scandinavia. Ian Edwards, Executive General Manager for Leighton's Hong Kong area operations signed the contract for the joint venture with Frode Nilsen, Project Director of LNS.

DSD Assistant Director Shiu Wing-yu (left) at the Paul Y HDD contract signing on 12 August

Speaking at the contract signing ceremony, Director of Drainage Services K.K. Lau said: "Complicated geological conditions are expected. The project is going to be a real engineering challenge for us."

When commissioned, HATS Stage 2 will serve about one million people on Hong Kong Island.

www.cleanharbour.gov.hk
www.maunsell.aecom.com
www.leightonasia.com
www.lns.no
www.macmahon.com.au
www.gammonconstruction.com