The Republic of Korea’s new high-speed railway project, which is based on French TGV technology, links the capital Seoul, in the north-west, to Busan (Pusan), in the south-east. The project is a key component of government plans to improve the speed and accessibility of transport facilities throughout the country. Wherever possible the route incorporates existing rail lines, with upgrades and electrification as required, and approximately follows the Gyeongbu Line where possible (Figure 1). For the second phase of the project, to further speed the trains and serve additional towns, a substantial portion consists of new routes with major tunnelling works required.
The entire route will be electrified and most of the line will be double-track, apart from the suburbs of Seoul to Cheonan where it will be four-track. All tunnels will be twin-track.
Project phases
Opened in April this year, the first phase of the Gyeong-bu high-speed rail line utilises 174.7km of existing line; 42.5% of the 409.8km between Seoul and Busan. Valued at approximately US$840M, the main features of this phase include the rebuild between Seoul and Taegu, and electrification of the existing route between Taegu and Busan, and in the cities of Taejon and Taegu.
Now under construction, the second phase will reduce the Seoul-Busan travelling time of 2hrs 40m for the first phase to only 1hr 56m (compared with over four hours for conventional trains). It will be 412km long, via Gyeongju, and is due to open in 2010. A traffic flow of 500,000 passengers a day is expected by 2010.
The second phase is a deviation from the original project plan, which included substantial tunnelling through old mining areas. Severe geotechnical problems lead to a diversion, raising the cost of the whole project from the original US$9.27bn to an estimated US$12.9bn.
Tunnels
Some idea of the importance of tunnels to the Korean rail network through the country’s uplands is that, of a total length of 3125km of track, 241km is in tunnels or subways (in 492 locations). This is mainly for existing conventional railways whereas high-speed railways, with a maximum grade of 2.5%, will use tunnels with greater frequency. In fact, 46% of the new track for this project is in 75 tunnels up to 18.4km long.
As can be seen from Figure 1, the need to achieve safe, high speeds means that track alignments need to be smooth in both horizontal as well as vertical axes. The minimum curve radius is 7000m, as it is for German high-speed trains.
The tunnels that are necessitated by these alignment criteria are double-track bores which, with 5m between track centres, result in a finished sectional area of around 107m². The designed excavated section is a horseshoe shape, 13.4m wide at the base and 11.2m high at the crown from the drain channels (Figure 2). The width at the spring-line is 14.2m.
Tae-A Construction won the tunnelling work on Section 11.4 on a competitive bidding process, and is a specialist tunnelling sub-contractor to Doosan Construction as main contractor. Headquartered in Busan, Tae-A Construction is a leading contractor in railway construction, both underground and on surface routes. The main contract is worth US$11.9M. The site is located near Gyengju (Kyeng-ju), which can be identified on maps as the main bend in the ‘dog-leg’ of the new Phase 2 route.
Drill and blast
The 2km long Songseun Tunnel horizon is entirely in dark, friable mudstone, which requires particular care in drill and blast operations. The early excavation method was to employ a Sandvik Tamrock Data Super 316-150 jumbo, supplied and supported by Sandvik Suh Jun of Seoul, on two 12hr shifts.
The Tamrock Data Super 316-150 bores a central wedge-cut drill pattern of 135-137 holes of 45mm diameter, with fanned peripheral holes finishing to create ‘smooth blasting’. The ‘wedge’ is located in the upper centre of the face to provide an early second ‘free’ face by detonator delays. With a maximum hole length of 2.75m in the friable ground, the round has a planned pull of around 2m. Removal of the upper face leaves a bench for later excavation.
Recently Tae-A has also purchased a new, fully computerised Tamrock Axera T11 Data-315. It was planned to use this machine from the other end of the tunnel once a portal had been established at the valley side. At the time of the site visit the jumbo was being commissioned with two fitters from Sandvik Soh Jong attaching hoses and testing the hydraulics, together with the Tae-A operator.
Drills compared
The Axera T11 Data-315 is equipped with new HLX series rock drills, representing a new concept in rock drill design, which allow drilling speeds substantially above those of conventional jumbo drills. The percussion power in excess of 22kW guarantees fast and efficient drilling in all rock conditions.
Lower drill steel consumption has been achieved by optimising the percussion dynamics and better matching the piston and current heavy-duty drill steels together. The two-stage hydraulic stabiliser in the new high-frequency HFX5T drill keeps the bit in ideal rock contact, thus prolonging the rod life. The stabiliser also helps to control the secondary vibrations in the string, which is especially important in extension drilling. The primary recoil absorption is done with the piston, which directs the energy into the high-pressure accumulator for the next stroke.
Tae-A decided to compare two HLX5T rock drills against one high frequency model HFX5T, which raises the drilling speed to higher levels. With a 40% increase in frequency it preserves efficiency as well as drilling stability. The energy transfer from rock drill to rock is more efficient than ever. The new way of breaking the rock, using more but lighter impacts, also helps maintain hole straightness and decreases the risk of jamming the rods in fractured rock.
At the Tae-A site the HFX5T rock drill has proved to give a 25% higher penetration rate compared to the HLX5Ts whilst using the same percussion pressure level of 210 bar. This results in remarkably lower stress levels in drill steels when using the HF-drifter. All this is done with standard 55-kW powerpacks. There has been also a significant improvement in the service life of drilling tools. Operators have also noticed another clear advantage; the noise level of HF-drifter has been reduced, with a more acceptable sound.
Following commissioning of the second jumbo, the plan is to advance with more caution in the increasingly friable ground. The normal procedure is to excavate a half-height top heading followed by a bench excavation about 400m behind the top face. This allows the round length to be increased to 3.5m-4m instead of 2.75m for a full face. Using this method the average daily advance achieved is around 6.7m.
The jumbo’s booms have a large arch sectional coverage, 358-deg rotation and automatic parallelism for faster positioning. With Tamrock’s fully computerised Data Control package the drills are positioned automatically according to designed drill pattern input, and penetration rates automatically controlled according to rock conditions. All but the bottom holes are being drilled in the fully automatic mode. With improvements in post-blast floor cleaning, Tae-A plans to also drill bottom holes using the Tamrock Data Control system.
The motivation for selecting fully computerised control is the relatively high wages payable for skilled operators in the Republic of Korea, caused by a general skills shortage. Use of computerised control is not thought to require such a high level of skill. As ‘smooth blasting’ is carried out to prevent excessive overbreak, a high level of accuracy is required that is easier to achieve with computerised control.
Tae-A Construction uses electrically detonated ANFO slurry explosives for the main charge with Special Finex 1st grade cartridge explosive in the peripheral holes.
The main mucking out is by a 166kW, 21.5t Caterpillar 966F wheel loader, filling 15t highway dump trucks of South Korean manufacture. A Daewoo Solar wheeled hydraulic excavator is used to scale the roof and face, to clean up remaining spoil, and to clear the drain channels.
A 150kW Shinhan axial-flow forcing fan provides ventilation through 1500mm diameter flexible ducting.
Support
The tunnel construction philosophy is based on NATM, with five levels of support and commensurate drill patterns. Doosan Construction carries out the surveying and checking of geological conditions to determine the correct level of support. The normal Type 2 support, for a 2m advance, includes substantial reinforcement in the sprayed concrete lining in view of the friability of the ground.
Firstly the crown is secured, if required, with a standard pattern of 10 x 5m long rockbolts using the Tamrock jumbo. This takes from an hour to an hour and a half. Then a primary 100mm layer of sprayed concrete is applied to the exposed ground with a Normet Spraymec 9150 rig to provide a working area safe from spalling.
Using a Normet Himec twin-basket lift, the miners erect lattice girder arches at 1m spacing to hold linked welded mesh sheets. Another 400mm layer of sprayed concrete is then applied to the depth of the lattice arches, up to the last arch installed, which must be within 5m of the face.
On the Spraymec rig the nozzleman can work from a seated position at a constant, safe distance from the sprayed concrete stream. The nozzle design and correct dosing of accelerator achieves low rebound and loss of materials. The hydraulically controlled Spraypump 300 has an output of approximately 33m³/h and can handle mixes of low water-cement ratios. Normal progress has been one ‘ring’ of the arch per shift, or an advance of 2m. This is a case where accuracy, and care to prevent overbreak in possibly fragile ground, assumes more importance than advance rate in order to achieve a good overall structure.
The excavated ground surrounding the portal is supported by reinforced sprayed concrete with soil nails and mesh. Tunnelling screens at the portals prevent drill and blast noise from travelling on night shift and keep cold air out during winter.
Economic context
Although the Republic of Korea (South Korea) has wide coastal plains in the West and South, ranges of hills and mountains (up to 1950m) dominate 70% of the country with generally steep grades on valley sides. In the past this has acted as a considerable obstacle to cross-country transport. Nevertheless, the national train and highway authorities are determined to create rapid transport links through these areas.
For its part, Korail is upgrading many parts of the rail system in addition to the Seoul-Busan high-speed line. Linked to this project, for example, is an electrification of the branch from Daejon (on the HSR) to Mokpo in the South-East.
The renewal of the transport infrastructure is symbolic, or perhaps one of the main causes of, the recovery of the whole economy. Well-known as one of the Asian ‘tiger economies’, the Republic of Korea suffered more than most from the economic crisis of 1997, but since then recovery has been good with an annual growth in Gross Domestic Product of 5.7%. In this climate, careful investment in infrastructure for rapid transport continues to be a major priority.
Conclusion
Although just one link in the trans-national chain of high-speed rail infrastructure, the tunnelling on contract section 11.4 is as important as any other on what is forecast to be the world’s busiest high-speed rail route, with a capacity of 520,000 passengers a day. For example, it is expected to carry six times more people than the Paris-Lyon TGV route, which it emulates, and so will be, arguably, the most important transport facility in the Republic of Korea.
Depending on political situations, Korail has outline plans to extend the high-speed network ‘over the horizon’ northwards. There is already a reopened conventional railway link with North Korea.
Related Files
Figure 2 – Design cross-section through the tunnel showing positions of drains, cableways, walkways and the double track
Figure 1 – Korail’s main Seoul-Busan route with First Phase modifications. Note journey reduction times and the ‘smoothing’ of the route through central uplands (inset)
