The Taiwan North-South High Speed Rail Project (THSRP) is regarded as a key element in the country’s economic future. It is also the first build operate and transfer infrastructure project in Taiwan and believed to be the largest in the world to date.
The line stretches 345km almost the length of the 394km long main island between terminal stations in the capital Taipei in the north and Kaohsiung in the south. Initially six intermediate stations are planned with more to be added later. Commencement of operations is planned for October 2005, the system is expected to become fully operational by 2006.
The civil works contracts include:
The project is being designed and built by joint venture teams comprising both national and international designers and contractors with typical contract periods ranging between 36 and 42 months.
The civil works contracts were awarded from March 2000 as multiple design-build self-certification contracts. Track work follows on from 30 to 36 months after the start of each contract.
Island geography
Taiwan is situated about 160km off the east coast of the Chinese mainland. The main island covers 35,980km². Forested mountains cover about 70% of the main island, predominantly on the eastern side, with the plain areas on the western side where some 95% of the country’s 22 million people are concentrated.
Project components
The central component of Taiwan High Speed Rail Corporation‘s (THRSC) task is 330km of guideway. In addition there will be:
The railway will have double tracks for the main line (northbound and southbound) plus sidetracks for stations. The main line will use 1,435mm standard track gauge at 4.5m track centre distance designed for operation at speeds of up to 300km/h (350km/h design speed).
The project will include some 47 separate double-track tunnels, totalling 50km, which will be generally shallow, with the geological conditions typically consisting of relatively homogeneous formations along the entire length.
The shallow tunnels allow construction of multiple shafts and adits for multi-faced drives, thus reducing the length of tunnel sections to be driven from any one face.
The high-speed rail service will operate 18 hours daily giving a journey time between Taipei and Kaohsiung by express train (with one stop in Taichung) of 90 minutes, and by regular train (with stops at intermediate stations) of 120 minutes.
When the THSRP first phase becomes fully operational a total of 38 train sets with minimum passenger capacity of 900 seats each will be put into service. The fleet will increase to 48 train sets by 2010.
Seismic activity
Taiwan shares similar earthquake characteristics with Japan, with an average return period (as determined by insurance information service Axco) for a seismic event in excess of Richter Scale 6 of 15 years.
The project design criteria follow National Earthquake Center regulations for high speed trains. For the THSRC project, a conservative approach to the seismic risk has been adopted and the division of seismic zones is based on the hazard analysis.
Although the maximum peak ground acceleration for each zone is different, they are equivalent to the same earthquake intensity. There are four zones and the maximum peak ground acceleration (PGA) ranges from 0.22g to 0.40g. A PGA of 0.40g would equate to a seismic event intensity of between 7 and 8 on the Richter Scale. These criteria are equal to those used in the design of nuclear power plants in Taiwan.
In contrast, the Japanese use a 130-year return period for their high-speed train designs.
Two different levels of earthquake are considered in design of the structures:
Tunnel construction methods
The route alignment generally follows the most populated area along the west coast of the island. The tunnels do not have to cross mountain ranges with high overburden, and the geological and hydrological conditions are generally well known. The bores are generally shallow and the geological conditions are visible along much of the alignment.
Geological conditions in the tunnel vary between soil, gravel and sedimentary rock, or a combination. Only in certain cases are there excessive hydrostatic conditions that may be problematic for construction.
The tunnels will typically be advanced using the sequential excavate and support construction method, which best suited the proposed construction programme and tunnel size and geometry. Temporary support is typically achieved with an array of rock bolts from the spring line to the crown, lattice girders spaced 1-1.5m apart, and shotcrete ranging from 175mm to 350mm thick.
An excavation diameter approaching 15m produces the excavation area between 110m² and 120m² required to house the double tracks.
The finished tunnel must have a minimum 90m² cross-section to meet the aerodynamic requirements for the 300km/h trains, in accordance with the Union Internationale des Chemins de Fer. This is governed by the aural health and safety requirements when the trains are operating and passing one another at high speeds in the confined space of the tunnel.
The finished cross-sectional area also allows for a minimum of a 1.2m safety walkway on either side of the tracks.
To avoid sonic boom, some portals will have a 20m long enlarged pressure relief structure with a cross-sectional area 150% larger than the tunnel to allow for the pressure wave dissipation.
The majority of the mined and cut and cover tunnels are planned with internal drainage systems and niches put in by the core system contractor. In a few specified areas where environmental constraints have been identified, tunnels are undrained. The final tunnel lining is typically a reinforced concrete shell designed to support the tunnel without the benefit of the temporary lining.
Emergency egress will be provided for all tunnels where the portal to portal distance is in excess of 3,000m.
Works planning
For the tunnels along the high speed rail corridor, THSRC carefully reviewed the options and benefits for possible construction methods in choosing the sequential excavation and support method. The schedule benefits from the use of the multiple construction adits, even though it is very labour intensive solution.
However, the flexibility to adjust to the changing geological conditions makes this method the most effective way of maintaining the construction schedule, minimising the time and cost consequences, and allowing contractors to take appropriate action in the case of failure.
The tunnel cross-section will be excavated in stages, typically consisting of heading, bench and invert, thus restricting the open surface of each face and reducing the potential of collapse. In addition, the contractor will be required to perform exploratory probes ahead of the excavation to detect potential problem zones in advance of the actual face excavation.
Tunnel construction
Portal construction and other low areas along the alignment will require ground treatment for strengthening. For safety and environmental considerations the portal cuts are reduced to a minimum. Portals will be generally excavated using modern techniques such as jet piling or steel pipe roof allowing portal excavation in poor ground conditions with a minimum of overburden. This method is preferred project-wide, rather than other forms of ground treatment or large open-cut approaches, to avoid tunnelling in low overburden areas.
Various tunnelling methods will be used depending on location and geotechnical conditions. These will have implications for the speed at which the tunnelling can proceed, which depends upon the amount of heading that will be self-supporting. Typical planned excavation rates are around 1-2m/day.
With the exception of Hueilung Tunnel, the geological conditions and strength of the formations of the excavation will require the tunnel to be advanced by mechanical equipment such as hydraulic excavators.
Most of these tunnels cut through younger conglomerates where the matrix material is poorly cemented yet strong enough to hold substantial vertical cuts, or through sandstone, siltstone and mudstone, which are most commonly horizontally bedded.
Hueilung Tunnel is located in a unique geological condition on the alignment, passing through sharply folded sedimentary formations of alternating sandstone and shale with occasional thin coal beds.
While the rocks are relatively weakly cemented, the intact rock is strong enough to withstand the stresses induced around the tunnels even at the highest overburden of over 160m.
However, there are significantly faulted areas where the rock is extremely fractured, and needs continuous support.
Additionally, the presence of methane gas has been identified. Special precautions are therefore required during both construction and operation phases.
Full-time monitoring and possible active ventilation may be required, depending upon the results of data collected from the monitoring system that is installed.
Related Files
Typical tunnel cross section design for the THSRP
Map of Taiwan
