Abstract-With the decrease in the over ground utilizable space and increase in growth of metro structures, cut and cover structures are becoming rather impossible to conceptualize and construct. Hence they are slowly replaced by underground openings made by NATM methods. These tunnels could be using rock bolts and shotcrete as support systems for poor-fair quality rock masses whereas ground improvement will be needed for extremely poor quality deposits. At this point it will be prudent enough to understand the seismic performance of these support systems if such a demand comes in short term. Since for extremely weak deposits grout curtain for ground improvement acts as a barrier, the focus will be on better quality rock masses where rock bolts and shotcrete are only used as primary supports. For the same a relatively low RMR value is adopted for the rock mass under consideration. A typical NATM tunnel geometry is adopted which has to be excavated using conventional methods. Suitable support systems for the same are designed. Then seismic performance of these support systems are deduced using two different approaches namely (1) Pseudo Static Method as per IS 1893 and (2) Quasi Static displacement approach as per EC-8. A sensitivity analyses for the support systems at different cover depths are explored. Comparative studies of the above approaches are made and their applicability for similar problems is discussed.
New Delhi is a city affected by near/far-field earthquakes, where underground metro construction activities have been undertaken since the early 2000s. Due to the seismic effects, these structures are designed by conventional force/displacement methods. The present study validates the efficacy of these methods, numerically. For this, varying geotechnical conditions in static and seismic cases were quantified with the expected ground motion. Bored tunnels at different cover depths were modeled in 2D and 3D, to ascertain the static response of these structures, followed by a true dynamic analysis. It was found that conventional design procedures using force methods gave conservative estimates for increments on the tunnel liner. Also, the importance of a full dynamic analysis could not be ignored because (i) higher surface spectral responses were found at the tunnel pillar location and (ii) the cumulative strains on the tunnel liner were found to exceed permissible limits.
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