Engineering geological and geotechnical characteristics of the Kankai hydro-power tunnel in soft rock, Nepal

Nepal, which is located in the Himalayan region of South Asia, is gifted with rich natural resources including huge rivers, and innumerable rivulets criss-crossing the country and, thus in recent years, has become a favourable site for the development of hydro-power. The construction of tunnels through soft rock is rare in Nepal and this paper gives details of the proposed Kankai hydro-power project, located in eastern Nepal. The scheme which will comprise a dam, intake structure, headrace tunnel and semi-underground powerhouse will be constructed in an area of soft sedimentary rocks in a sequence known as the Siwalik Group of Plio- Pleistocene age. The headrace tunnel, which will be located on the left bank of the Kankai River, will be driven through interbedded sandstones, mudstones and siltstones of the Lower Siwalik Group. The area is seismically active due to the presence of two major regional thrust faults, the Main Boundary Thrust (MBT) to the north and the Himalayan Frontal Thrust (HFT) to the south. Detailed engineering geological and geotechnical investigations were undertaken as part of the feasibility stage of the project the results of which were used to develop a rock mass classification scheme for the project. Boreholes were sunk to obtain rock samples for uniaxial compression tests, triaxial tests, deformability potential tests and for the purpose of undertaking in situ permeability tests. The results indicated the rock formation along the tunnel section to have low shear strength and high deformability potential thus having a rock mass classification of very poor to fair. X-ray diffraction tests were also carried out and confirmed the presence of swelling clay minerals. The results of slope stability analysis of typical slope sections along the tunnel route are also presented in the paper. Finally a guide to excavation methods/techniques and the use of appropriate support systems for the headrace tunnel are presented. Resume: Le Nepal, qui est situe dans la region de l'Himalaya de l'Asie Du sud, est doue avec les ressources naturelles riches comprenant les fleuves enormes, et les rivulets innombrables entrecroisant le pays et, en consequence, est devenu un emplacement bien connu pour le developpement d'hydro-electricite ces dernieres annees. L'excavation et la construction des tunnels par la roche molle est rare au Nepal. Le projet propose d'hydro-electricite de Kankai avec le potentiel de l'electricite de 60 MW est situe dans l'ordre de roche sedimentaire doux de Plio-Pleistocene, le groupe de Siwalik, du Nepal oriental. Le tunnel de headrace dont est situe a la banque gauche du fleuve de Kankai et sera conduit par les gres, les schistes et les siltstones intercales du Siwalik inferieur. Le secteur est seismicalement du actif a la presence de deux defauts regionaux principaux de poussee, la poussee principale de frontiere (MBT) dans le bandeau du nord et de l'Himalaya pousse (HFT) dans les sud. Machinant des observations geologiques, des investigations geotechniques et les arrangements de masse de classification de roche ont ete employes dans la presente etude. Les essais de compressibilite uniaxiaux, les essais a trois axes, les essais potentiels de deformabilite et les essais de permeabilite etaient insitu effectue aussi bien que des echantillons de roche de forage. L'exposition de resultats la formation de roche le long de la section de tunnel a la basse resistance au cisaillement, le hauts potentiel de deformabilite et tres pauvre, pauvre au type juste categorie de roche. Des essais de diffraction de rayon X ont ete effectues pour confirmer la presence du minerai d'argile de gonflement. En outre, l'analyse de stabilite de pente des sections typiques de pente le long de l'itineraire de tunnel est egalement presentee dans le papier. Enfin les suggestions pour empecher de futurs dommages dans le tunnel sont proposees.