ABSTRACT:

In the Nathpa-Jhakri Hydroelectric Project - under construction since 1993 - unusual stress conditions show a severe influence on the drilling and blasting conditions. It is believed, that the in-situ stress field has rotated from a vertical alignment to one that is parallel to the surface of a creeping landslide. For this reason, the tunnel shows a typical deformation pattern with convergence along the downhill spring line and along the uphill wall. Over all, very poor drilling conditions have been recorded along the headrace tunnel, especially in those regions with high rock cover and foliation striking parallel to the tunnel and dipping downhill towards the river. The main sliding and creeping surfaces coincide with this foliation which is encountered in a series of quartz-mica-schists, schistose quartzites and amphibolites. Estimating the true overburden load to account for the foliated nature of the creeping rock mass, the derived stress state explained why drilling velocities have been significantly influenced. Drilling rates were reduced in those areas corresponding to the rotated stress conditions at the tunnel face. In other words, the face was under increased confining stress, requiring more drilling effort to fragment the rock.

PROJECT OVERVIEW

Since 1993, the Nathpa-Jhakri Hydroelectric Project (NJHP) has been under construction and includes a 60.5 m high concrete gravity dam, an underground desilting arrangement, a 27.3 km long headrace tunnel, a 300 m deep surge shaft and an underground power house to generate 1500 MW of power. The project is situated in the higher Himalya in the middle reaches of the river Satluj in the northwestern part of India (Himashal Pradesh) and has been one of the largest civil works in India. Figure 1 provides an overview of the geological and tectonic situation (Schwan, 1980).

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