Behaviour of underground structures subjected to dynamic loading is different from the surface structures. Underground openings in rock mass generally do not suffer as much damage as the surface structures during major earthquakes (M < 7), particularly if they are located at a depth of more than 20m and there is no fault zone or weak zone in the neighborhood. Considering the ground acceleration of dynamic loading, a pseudo-static approach for estimating the dynamic support pressure in tunnels has been suggested in the paper. Rock mass quality in seismic areas (Seismic) proposed by Barton (1984) has also been correlated with the coefficient of peak acceleration. Higher the ground acceleration or blast loading more shall be the overburden for an underground opening to be safe.
Underground structures are an integral part of infra structural development programme. These are used for a wide range of applications, including subways and railways, material storage and sewage and water transport. Underground facilities built in areas subject to earthquake activity must withstand both seismic and static loading. Underground structures have features that make their seismic behaviour distinct from most surface structures, notably
their complete enclosures in soil or rock and
their significant length (i.e. tunnels) (Hashish et al, 2001).
In general, underground structures have a lower rate of damage than surface structures. Experience suggests that there is no cause for worry for tunnel stability because of earthquakes in rock masses below 20m from ground surface, except in the active fault zones or weak zones. Nevertheless, some underground structures have experienced significant damage in large earthquakes (M>7), including 1995 Kobe, 1999 Chi-Chi, 2004 Cheese, 2005 Kashmir and 2008 Henchman earthquakes (Hashish et al, 2001 and Aidan et al, 2010). Hence to make the underground infrastructures safe from seismic and other dynamic forces like heavy blast loading, it is necessary to carryout a dynamic analysis of an underground structure. This is all the more essential when it comes to accommodate human activities. For studying the effect of dynamic loading on underground structures, a pseudo-static approach is presented in the paper. Before this general observations on the seismic performance of underground structures as reported by Hashish et al. (2001) are given.
Following are the general observations pertaining to dynamic performance of underground structures (Hashish et al, 2001). Underground structures suffer appreciably less damage than surface structures. Reported damage decreases with increasing overburden depth. Deep tunnels seem to be safer and less vulnerable to earthquake shaking than are shallow tunnels. Underground facilities constructed in soils can be expected to suffer more damage compared to openings constructed in competent rock. Lined and grouted tunnels are safer than unlined tunnels in rock. Shaking damage can be reduced by stabilizing the ground around the tunnel and by improving the bond strength between the lining and the surrounding ground through grouting. Improving the tunnel lining by placing thicker and stiffer sections without stabilizing surrounding poor ground may result in excess seismic forces in the lining.