67 Engineering geology of Kankai Hydroelectric tunnel alignment Journal of Nepal Geological Society, 2005, Vol. 31, pp. 67–74 Engineering geology of Kankai Hydroelectric tunnel alignment in east Nepal *Sunil Kumar Dwivedi 1 and Prakash Chandra Adhikary 2 1 Department of Physics and Earth Sciences, University of the Ryukyus, Okinawa, 901-0213, Japan 2 Central Department of Geology, Tribhuvan University, Kathmandu, Nepal (*Email: k048313@eve.u-ryukyu.ac.jp) ABSTRACT This paper describes the engineering geological characteristics of rock mass in the headrace tunnel, powerhouse, and intake portal of the Kankai Hydroelectric Project. The project area lies in the Lower Siwaliks of east Nepal and consists of alternating sandstone and mudstone beds with frequent siltstone intercalations. The rock mass of the project area was classified according to rock mass rating (RMR) and rock mass quality index (Q) systems. It is of very poor, poor, to fair quality (categories V, IV, and III) in the headrace tunnel; of very poor quality (category V) in the powerhouse; and of fair quality (category III) in the intake portal. The stability analysis of irregularly jointed and fractured rocks of the area was carried out using SWEDGE and UNWEDGE. The analysis gave the safety factor of 0.45, 0.64, and 0.45, respectively for the powerhouse, intake portal, and headrace tunnel. The final safety factors obtained after the installation of support for powerhouse, intake portal, and headrace tunnel were 1.14, 3.33, and 4.53, respectively. INTRODUCTION Every hydroelectric project is unique in terms of its engineering problems and the Kankai Hydroelectric Project is no exception. The engineering geological studies of roads, irrigation canals , and bridges are common in Nepal (Dhital et al. 1991; Deoja 2000). However, there are a limited number of studies on hydroelectric projects (Kaphle 1996; Paudel et al. 1998; Dwivedi 2003), and the study of soft rock tunnelling is rather rare. In these circumstances, the study of engineering geological and tunnelling problems in soft rock can contribute to the understanding of hydropower development in the Himalayn conditions. In the proposed Kankai Hydroelectric Project (KHP), the rock mass was classified according to the rock mass rating (RMR) (Bieniawski 1989) and NGI (Norwegian Technical Institute) tunnelling quality index (Q) systems (Barton et al. 1974). The support systems were further selected according to each of these classification systems. In order to come up with a suitable geometry and to determine the stability of slopes, cut slope and tunnel sections of the project were analysed using SWEDGE and UNWEDGE, and required support systems were worked out. PROJECT AREA The proposed KHP is situated on the left bank of the Kankai River, on the Siwalik foothills of east Nepal (Fig. 1). It is bounded by latitude 26 o 41′ 00″ N and 26 o 42′ 00″ N, and longitude 87 o 52'30 ″ E and 87 o 53′ 00″ E. The poject area exhibits very steep, rugged, and dissected topography, characterised by a typical hogback structure with a southward escarpment. Interbedded sandstone and mudstone beds have given rise to alternating ridges and furrows in the area. The Kankai River, which is a rain-fed river, originates from the Mahabharat range and exhibits a dendritic drainage pattern. Strong denudation and linear erosion below the forest cover are frequent in the area. The proposed KHP is a 60 MW storage- type multipurpose scheme for hydropower generation and irrigation. The project includes a 70 m high dam, a 320 m long headrace tunnel, a toe-powerhouse, a 900 m long diversion tunnel, a spillway, and a reservoir (NEA 2002). GEOLOGICAL SETTING The Siwalik Group in the study area is bordered in the north by the Main Boundary Thrust (MBT) and in the south by the Himalayan Frontal Thrust (HFT). The Group is divided into the Lower, Middle, and Upper Siwaliks (Schelling and Arita 1991; Upreti 1999) and is represented by a thick pile of mudstone, sandstone, and conglomerate (Fig. 1). The major engineering structures of KHP (viz. intake portal, powerhouse, headrace tunnel, and dam), lie in the Lower Siwaliks represented by alternating beds of sandstone, mudstone, and siltstone. The sandstone is grey in colour, fine- to medium-grained, soft, argillaceous, poorly indurated, and highly jointed. The mudstone is bioturbated, variegated, and poorly indurated. In this area, laminated mudstone and calcareous siltstone beds are also frequent. Fining-upward cycles and ripple marks are observed in the sandstone beds. ENGINEERING GEOLOGY The engineering geological investigation included the study of rock mass, discontinuity survey, examination of rock cores, and collection of rock samples for the laboratory