In recent years, the most densely populated cities in the world have accelerated effort in developing underground space as a mode of transportation for eco-friendly and sustainable cities. However, it appears to become more difficult to use underground space in metropolitan cities due to existing underground structures. Consequently, it leads to development of underground space beneath existing underground structures and the underground structures become multi-layered vertically. Considering the fact, this case study aims to introduce the design and construction methodology of the largest tunnel station in Seoul, which has been constructed 15cm under the existing subway line 3 and the superannuated underground shopping complex within highly-congested area. To overcome the obstacles incurred by the site condition, for instance, neighboring construction, civil petition and severe traffic jam during the construction, the combined method of the TRcM(Tubular Roof construction Method) and the CAM(Cellular Arch Method) has been created and applied. Although this construction project was considered as the most difficult project ever in the history of the Korea's subway construction, the subway station has been constructed successfully without any damages on the adjacent structures and has launched from July 2009.
Seoul, the most populous city in Korea, has the population density of 16,582 people per square kilometer in 2009. Until the 1990s, Seoul had experienced rapid economic growth as well as population concentration, and decided to promote the development of underground space. The utilization had been focused on the shallow depth underground space by using the cut and cover method which was relatively easier and less intricate to apply. As a result, the most of shallow-depth underground space in urban areas has already been utilized for transportation, electric power, communication, water supply and sewer networks. Nowadays, these existing underground structures are considered as one of the major obstacles in development of underground space. Consequently, the cases of tunneling work under existing underground structures have been increasing rapidly.
The stage 1 of the new Subway line 9 has launched in July 2009 to ease the traffic flow between the west and the east part of the Seoul and to promote the use of underground space as a mode of transportation for the development of eco-friendly and sustainable cities by reducing air pollution and economic cost of traffic congestion. The stage 2 and stage 3 of subway line 9 will be completed in 2013 and in 2015, respectively. The stage 1 of subway line 9 starts from Gimpo Intl. Airport and ends in the Gangnam Area, the most urbanized area in the Seoul. The total length of the subway line 9 is 25.5 km and it has 25 stations. The 913 project of the stage 1 is composed of two subway stations (922 and 923 subway station) and the 1.78km of subway lines as shown in figure 1. The 923 subway station of the 913 project, which is named the Banpo(Express Bus Terminal) station, is located in the center of the life culture and transportation.
