Limitation of shotcrete and rock bolting as supporting system in structurally weak rock formation in underground works

Tunnelling and underground excavation are the major works in hydroelectric projects. Major hydroelectric projects are situated mostly in Himalayan region. North-eastern and upper Himalayan region consists of rocle formations which are very strong to highly weathered and very weak consisting of highly jointed, closely jointed, sheared, crushed and permeable rock strata. Very strong to strong and slightly weathered rocks do not pose any problem in terms of rock supports. Such kind of strong rocks have very high stand-up time, ranging from months to years. These kinds of good strata can be kept unsupported and can be supported with only spot rock bolts or only with wiremesh or a thin layer of shot-spot rock bolts or only with wiremesh or a thin layer of shotcreting. Other class of rock which is highly weathered, weak and with closely spaced joints and permeable tends to pose problems of caving in, roof collapses, cavity and chimney formations to a larger extent. Proximity of seepage water and high pour pressure creates flowing ground conditions which become very difficult to control.

The paper deals with the support system prevailing and methodology of excavation compatible with equipment available in India discusses how far it is successful to provide rock bolts and shotcrete as a support in such weak rock strata.