Seismic load consideration in design of tunnels and tunnel portals in the Himalayan region

Tunnels in rock are less susceptible to damage due to seismic event than the surface structure due to-proportionate reduction in ground motion with depth and subsequent reduction in seismic coefficient, increased-modulus with depth, small excavation dimension with respect to much larger seismic wavelength. The main-parameters which can influence the damage of tunnels owing to seismicity are earthquake magnitude, distance-from epicenter, peak ground acceleration, geology of tunnel media, rock mass quality, tunnel-depth from surface, location and orientation of any Faults, magnitude and direction of in-situ stresses, types of support system and purpose of tunnels. In Himalayan rocks many problems during tunneling experienced are due to high level of seismicity, encounter of many geological features as Fault zone, thrust zone, shear zone, high in-situ stresses, low rock cover, high ingress of water, buried channel, geothermal gradient, ingress of gases etc. Therefore, the impact of earthquake on tunnels is required to be studied in details so that adequates support measures are provided in design of the support system and lining of tunnel. This paper discusses a case study of design of tunnels and tunnel portals for seismic condition in Himalayan region and check performance and behavior of tunnel during any seismic event.