Effect of Rock Weathering on the Seismic Stability of Different Shapes of the Tunnel

Tunnels in rock are integral part of contemporary old and modern smart cities in hilly terrain to provide services and transportation. Tunnels have been the lifeline of the metropolitan cities since decades. Tunnelling have reduced the time of travel, saved money, reduced number of accidents and also reduced the human impact on the ecology of the hilly regions. Hilly regions are seismically active zones and have several minor earthquakes at a shorter span of time. Himalayan mountains are young folded mountains and have been affected by several earthquakes of large magnitude in last century. Hence, stability study of tunnels in hills is very crucial. This present study focuses on the effects of rock mass weathering, due to geological and other changes, on the stability of tunnels subjected to earthquakes. The 2D plane strain elastoplastic model has been adopted for the numerical analysis using finite element software. A model of dimensions 42 m x 42 m has been developed having an overburden of 500 m. The different shapes of the tunnel are also varied to understand their stability. Plasticity theory of failure given by Mohr–Coulomb has been incorporated to simulate the constitutive properties of rock mass. The physical properties of basalt have been adopted for this study. The different weathering classes of basalt used in this study are fresh basalt, slightly weathered basalt, medium weathered basalt and highly weathered basalt. The earthquake data of acceleration time history has been taken from the records of the Koyna earthquake, which was a 6.4 M magnitude earthquake. This study will help to understand the stability of the smart city tunnels subjected to the earthquake in hilly regions. This paper concluded that weathering has detrimental effects on the seismic stability of smart city tunnels. The deformations reduce by 34% with the increase in the depth of overburden in case of arch-shaped tunnels, in case of circular-shaped tunnels the deformations reduce by 42% and in case of horseshoe-shaped tunnels deformations reduces by 28% of deformations at shallow depth of overburden.