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A huge cofferdam on the mass rail transport system for Taiwan's second city shaved months off the construction programme. Sati Bhogal reports.

Building an entire, mainly underground, mass rail transit system to a very tight programme is a huge task in any city and Kaohsiung in south-west Taiwan is no exception. Throw in diffi cult ground conditions and the task becomes even more challenging.

Kaohsiung is Taiwan's second largest city, at the heart of the island's heavy industry, and home to its largest commercial port.

Its new mass rapid transit system will have two new lines linking the city centre to the suburbs: Red, running north-south, and Orange, running east-west. Their combined length will total 42.7km, with 37 stations (28 underground) and three train depots.

The project was awarded as a single concession contract to Kaohsiung Rapid Transit Corporation in 2001. KRTC is responsible for design, construction, manufacture, installation, testing and commissioning as well as operation of both lines. Its 36-year concession period includes six for construction.

KRTC is using multiple design and build subcontracts for civil, mechanical and engineering works and has an in-house project management team.

Mott MacDonald and Aerospace Industrial Development Corporation (AIDC) were engaged by government agency Kaohsiung Mass Rapid Transit (KMRT) to administer the quality and safety verification of the project.

Because of the city's diffi cult ground conditions - deep alluvium layers of silty clay and fi ne to medium sands, which are prone to seismic liquefaction to depths of 10m - engineers designed the stations to be built bottom-up within perimeter diaphragm walls. Decking over the station excavations, supported by temporary king piles, maintains traffic fl ow during construction and the diaphragm walls are propped apart with steel I beams.

While tunnelling is still under way (see box), one of the biggest and most unusual elements of the scheme was the 140m diameter cofferdam for the city centre interchange station between the Red and Orange lines. The interchange will be on two levels, with a transfer track between the two lines. The station will have a split base slab, at 20m and 27m below ground level.

Cofferdam stability relies totally on generating hoop load from external soil and water pressures, with water pressure contributing about 80% of the loading. This means the excavation is unbraced, allowing relatively free access for construction.

By proposing this solution, KRTC aimed to save up to seven months on the construction programme.

About 60% of the cofferdam is decked over during construction.

This allows uninterrupted traffic fl ow at the junction of the two busiest roads in the city.

Diaphragm wall panels for the cofferdam were installed using reverse circulation rigs. The panels, 3m wide, 1.8m thick and 60m deep, were placed with a vertical accuracy of 1 in 1000.

Although a similar size cofferdam had been built in Japan, this was the first time it had been attempted in an urban environment. Combined with the diffi ult ground conditions, it presented a significant challenge to the project team.

The cofferdam site had a cover of soft superfi cial fi ll and a 5-8m thick clay layer 25m down, separating highly permeable alluvium above and below. Groundwater was high, 2-4m below the surface.

A comprehensive risk assessment was called for to cover aspects of design, construction, dewatering and excavation.

As part of its role as safety and quality consultant, Mott MacDonald worked with KRTC to develop the risk assessment and any mitigation measures to reduce all risks as much as reasonably practical. This led to the consultant insisting on the water level inside the cofferdam being lowered to 20m below ground level before excavation began, to demonstrate the generation of hoop load.

But the use of deep wells for water extraction resulted in unpredicted behaviour, developing large negative pore-pressures in the lower sandy formation below the clay layer. This meant the dewatering design had to be reviewed, with additional dewatering pumps located above the clay.

Safety was a constant concern throughout excavation and Mott MacDonald reviewed the monitoring data closely during work.

Critical stages in dewatering and excavation were checked against the risk register, which led to an increased confi dence in the stability of the works, and spurred progress as the rate of excavation increased.

Excavation finished last year and permanent works are now under way.

Overall, the mass transit scheme is set to cost an estimated $6bn (£3.4bn), breaking down into £0.85bn for planning and land acquisition, £2bn for civil works and £0.57bn for mechanical and electrical systems.

The Taiwanese government has provided 86% of the funding, which will cover civil works, while most of the remaining 14%, sourced privately by the concessionaire, is for systems. The railway will be opened in two phases, with the entire system expected to be in full operation by October 2007.

Sati Bhogal is Mott Macdonald regional director for Taiwan.

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