Stability analysis of subway box structure supported by modified underpinning method

When excavation under existing structure is planned for a new construction project, the underpinning method is one of the most applicable construction methods. This study introduces a new modified underpinning method which is applied to construct a new subway #9 line in Seoul Metropolitan in South Korea. The new subway line #9 was designed to pass underneath the existing subway line #2. Subway line #2 carries about 2 million passengers daily, which is 33% of total passengers using subway in Seoul, and is the only circulation line in Seoul. Subway trains are passing 540 times through this section in a day. By applying a new underpinning method, the subway box structure of line #2 is exposed 54. m in the air supported by bearing piles. The proposed method was carefully monitored using heavy instrumentation system during construction. This study proposed and verified the application of the modified underpinning method, which can reduce construction period by 1.5 times and the construction cost by 1.2 times comparing with conventional method. The importance of considering construction sequence is investigated and verified by using measured load cell data. The unexpected heaving which can bring up a dangerous situation for train running stability were measured, so this study shows that the upward movement has to be analyzed in designing process. The vibration from train operation and blasting excavation was thoroughly monitored and verified that the proposed construction method can be used with safety. As the use of underground space increases, the proposed method can be a good example of underground development.