Design Concepts, Optimisation and Challenges of an Underground Metro Interchange Station in Mixed Ground Conditions

Chennai is a city with multidimensional socio-economic growth in population and infrastructures. Chennai Metro Railway provides an efficient, safer and high-capacity transport system of urban transportation network in Chennai. The Phase-II work of Chennai Metro is divided into three lines, i.e. Corridor 3, Corridor 4 and Corridor 5. Thirumayilai is an interchange station between Corridor 3 (C3) and Corridor 4 (C4) lines. It comprises an island station (Corridor C4) and a stacked station (Corridor C3) connected with various ventilation shafts and multiple entrances, and it is also connected with existing MRTS to promote the multimodal integration. There are three intermediate levels to connect the island station and the stacked station. The subsurface strata consist of cohesionless soil layers followed by weathered/fresh rock. In this complex hard rock strata, diaphragm walls are considered as external walls of the station box with slabs forming a part of internal framing. Due to variation in starting level of weathered rock layer throughout the station, two different sets of construction methodologies have been considered. Along with conventional top-down sequential method, bottom-up method has also been adopted with the help of rock-bolts, shotcrete and diaphragm walls with shear pins embedded into the rock strata. This paper describes the concept design, optimisation and challenges of this underground metro station having interchange in mixed ground conditions. The present paper also explains the construction sequence of four levels of slabs to connect the island station with stacked station. Soil structure interaction analysis has been performed in finite element-based software to get overall stability of the structure, surface settlements, bending moments, shear forces, etc.