Construction has little impact on traffic flow

The bridges are part of the Ngong Shuen Chau Viaduct under construction. The viaduct - a series of spans used to carry the road over the highway - is  2.2 kilometres long. It has more than 2,600 precast segments weighing from 70 to 120 tonnes each. It will connect the West Kowloon Highway and Route 8's Kowloon Section near Lai Wan Interchange to the eastern end of Stonecutters Bridge.

The department chose a balanced cantilever precast segmental construction method - one that allows most of the operations to be done in the airspace above the existing highway and without any scaffolding. 

Essentially, the segments are placed in a symmetrical fashion about a pier, using a pair of 116-metre-long, 400-tonne launching girders to set them in place. Cantilever tendons are the main reinforcement for the structural system. They are set in the deck slab and are anchored at the segment's ends.

"Segment erection over the West Kowloon Highway, a strategic route with railways underneath, is a main feature of the project that requires special attention of the project team as well as departments concerning traffic management," Mr Fok said.

Extensive planning required

"The planning and preparation for the temporary traffic arrangements, contingency plans and emergency procedures behind the scene are as extensive and complicated as the bridge construction that can be seen."

West Kowloon Highway southbound carries an average daily traffic of 3,600 vehicles an hour during peak periods. At night, about 900 vehicles an hour travel it southbound while 500 an hour travel northbound.

To minimise disruption to the public during construction, the project team started preparing six months in advance with its partners, including the Police, Transport Department, bus companies and container terminal operators.

A dozen temporary traffic-management schemes were worked out while 10 contingency plans were established to cater for scenarios relating to the delay of the highway's re-opening.

Goal: Zero incidents

"Zero" incidents was our target for the works over the West Kowloon Highway, as any incident would have serious consequences on the traffic and facilities below. Any blockage of the highway's southbound lanes would build up a 3km tailback within minutes, together with major impacts on the traffic in Kowloon and the container port area," Mr Fok said.

"Safety is of paramount importance because of the heavy lifting and long clear span length involved. In particular, one of the spans has a tricky geometry - a curve of 250 metres, a clear span of 78 metres, a weight of 120 tonnes - the planning and erection of this span was a big challenge to us."

Apart from safety and traffic flow, Mr Fok said the team has had to face another issue: restricted working hours.

Three checkpoints for on-time completion

"It was necessary to follow the ground rule of 'no live traffic below segment being lifted for transportation or erection'. Therefore construction works could only be carried out at night when traffic is less busy. Excluding the time for setting up and removal of temporary traffic arrangements, we only had 4.5 hours to erect a pair of segments, and install" cantilever tendons.

"To work within such a short span of time, all operations must be well planned and tightly controlled. Three 'checkpoints' were introduced in the night work cycle to ensure proper completion of each critical stage before permission was given to proceed to the next one to reduce risks."

The three checkpoints were set at 9pm, midnight and 4am for starting work on the first segment, permission to start work on the second segment and deciding the time for re-opening the West Kowloon Highway.

No emergencies, few complaints

Mr Fok said segment erection for the three bridges was completed in June, with no incident requiring the activation of emergency procedures. The number of complaints and enquiries received during the 10-month construction period was less than 10 cases.

"For such a long duration involving so many different temporary traffic-arrangement schemes, this figure is indeed a lot less than our original estimate thanks to the great efforts and co-operation of all parties involved, and of course to the road users who tolerated the detours with patience," he added.

Works on the Ngong Shuen Chau Viaduct, started in April 2002, are expected to end in the first half of 2007 at a cost of $1.5 billion. For project details, click here.