ABSTRACT

1. INTRODUCTION

The tunneling job of the Loktak Hydro Project in India consists of a 6.857 km long pressure tunnel from the Loktak Lake in the East to the power house (105 MW) situated in western Leimatak Valley. Tunnel driving started in 1970 at several faces applying a conventional steel support method. The works were stopped in January 1975 because of a methane gas explosion which killed 14 people. In October 1978 the tunnel works were started again with the owner's own organization. High pressure exerting rock in face V and heavy fractured and waterbearing rock in face IV caused such difficulties with the adopted steel support method that the tunnel became unstable, supports buckled, and reexcavations became necessary on a considerable length of the tunnel.

Alternative solutions were studied and it was concluded that the application of the "New Austrian Tunneling Method" (NATM) would be suitable to cope with the extremely difficult rock conditions. The concept of the NATM is to take advantage of the self carrying capacity of the surrounding ground. To utilize this, certain principals have to be followed, such as choice of a proper tunnel shape, avoidance of detrimental loosening, careful excavation, right sequence of working stages, semirigid lining, allowance of controlled deformations for stress relief, ring closure time and ring closure distance, in-situ measurements, etc. As stabilizing and supporting means mainly shotcrete and rock bolts are used.

After a stage of redesign and the necessary reinstallation of equipment and machinery, work according to the NATM started in February 1979. The tunnel section dealt with in detail has a length of some 3800 m with a maximum overburden of 460 m and was driven from two faces, face IV from E to W and face V from W to E. At the time of preparation of this paper about 700 m of the tunnel were left to be completed.

2. SITE CONDITIONS

2.1 General Geological View

The geologic study for the project was performed by the Geologic Survey of India. Unfortunately no core drillings brother investigation data were available and the geological interpretation between the faces IV and V was done on the basis of surface mapping only (Fig. 1).

The eastern part of the project is in Lake deposits (clays, peatlayers, siltlayers and gravels) which document the process of alluviation of the Loktak Lake. Further to the West terrace deposits (gravels, clays, sands, and occasionally big builders) follow until the Tertiary fold range is reached. This area is the most important zone for the project because the tunnel section is situated in this intensively folded and fractured rock mass. In general, the fold range consists of the DISANGS series (lower Eocene, Tertiary) which is Sub-divided into 3 portions. The lower portion contains dark to black shales finely foliated with different kinds of fold structures, the medium portion consists of well bedded dark siltstones with frequent intercalations of shales and some sandstone layers and the upper portion consists mainly of well bedded and fractured sandstones. The DISANGS are overlaid by the BARAILS (upper Eocene, Tertiary), mainly composed of sandstones with thin shale intercalations.

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