Static and Seismic Analysis of Twin Metro Underground Tunnels

In any metropolitan city, underground structures are key elements of a mass rapid transit system. In situations where metro underground construction is to be undertaken in poor soil strata, it is not possible to excavate a single large diameter tunnel for accommodating the two-way traffic of trains. It becomes therefore essential to have two parallel tunnels aligned either horizontally or vertically. In this paper, an attempt has been made to predict the influence of construction of a second tunnel, aligned either horizontally or vertically, after the construction of the existing first tunnel. The analysis has been carried out for both static and seismic conditions by varying the pillar width between the tunnels. Elasto-plastic finite element analysis has been carried out through Plaxis 2D software for the 1999 Chamoli earthquake of lower Himalaya. The analysis brought forth the fact that vertical stresses at critical points and the forces in RC liners of both horizontally and vertically aligned twin tunnels increase during the earthquake for a pillar width equal to half the diameter of tunnels. These vertical stresses and forces in RC liners have been found to reduce with increase in pillar width in case of vertically aligned twin tunnels, whereas a reverse situation arises in case of horizontally aligned twin tunnels.