Performance of primary tunnel support systems under seismic loads in weak rock masses

Abstract

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.