Synopsis:

Tunneling through Himalayan ranges is always a challenging job especially in the weak rock formation with high ingress of water creating flowing ground conditions. Occurrence of extremely poor strata in the form of shear zones or thrust zones or highly jointed rock mass while tunneling in Himalayas makes the job of construction engineers most difficult during execution and has a greater degree of uncertainty both in respect of time and cost. Different methodologies are required to be adopted while excavating very poor rock mass (class V) and exceptionally poor (beyond class V) rock mass. Most of the time proximity of seepage water creates flowing ground. Whenever such difficult situation is encountered, it is managed by many methods ranging from primitive manual methods to modern methods using state of the art technology and latest materials. Most of the hydro projects are encountered with such tunneling problems in a small or big way. A system was evolved while working in one of the difficult project where large lengths of extra ordinary geological conditions were encountered. The system has been termed as P 5 system of tunneling through extremely weak rock formations. The P 5 system is a systematic way of dealing with extremely weak rock formation with high amount of seepage water. This paper discusses P 5 System in detail and the methodology to carry out the same for tunneling through extremely weak rock mass with high ingress of water creating flowing ground. The System was successfully used in one of the projects in Himalayan geology.

Introduction

During excavation of a tunnel the rock strata encountered may include very hard rock with high stand up time termed as class-I / II or a rock with good stand up time termed as Class- III or low Stand up time termed as Class- IV or very low stand up time or no stand up time, termed as Class-V or even Class-VI. The last categories when mixed with water create "Flowing ground conditions". The strata may include; highly jointed rocks, shear zone material, thrust zone material, lake deposits, terrace deposits, bouldery strata or sandy, silty, clayey strata, overburden or glacial deposits etc.

The old Process

If the material encountered in the strata is flowing or collapsible in nature, then a plug of concrete of about 3 to 4 m thick is normally constructed at the face where flowing ground is encountered. In the concrete plug few 50 mm dia nipples are also embedded for drainage of face. In addition some nipples should be embedded on the side walls and periphery of the tunnel so that in the periphery at least 16 to 20 m long holes could be drilled in such a manner that the strata above the crown and the sides of the tunnel is well grouted to a height of 6 to 7m. After solidifying the reach by grouting, full face excavation be carried out to a point, where the grouted strata face of about 5–6 m thickness is left for further treatment.

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