ENGINEERING GEOLOGICAL APPRAISAL OF KISHAU DAM PROJECT, GARHWAL HIMALAYA

Abstract

With the increasing demand for power, the activities on river valley projects have been accelerated in the recent times. The Himalaya, which holds an enormous hydro-electric power potential provides many favourable sites for the construction of river valley projects. Number of schemes have already been implemented to harness the vast power and irrigation potential in this region and many more are under planning and construction. Kishau dam project, one of the major river valley projects, envisages the construction of a 236m high concrete gravity dam, across the river Tons with a surface power house of 600 MW of peak power. The dam site is located near village Samberkhera (30° 39' 39" : 77° 46' 55") in Dehradun District of Uttar Pradesh, India. The present research work is aimed to evaluate the engineering geological problems which may arise during or after the construction of Kishau dam. The study has been carried out under three broad heads. a) Engineering geological evaluation of suitability of the dam foundation b) Engineering geological evaluation of suitability of the power house site. c) Assessment of stability of hill slopes around the dam reservoir. On the basis of the surface and sub-surface investigations carried out at the dam site, an evaluation of the dam foundation condition has been presented. The dam foundation area has been mapped on 1:2000 scale. In the river section of the foundation of the dam, quartzitic slates with interbands of purple quartzites are present. Purple quartzite and white quartzite are exposed on the dam abutments. The rocks at the dam site are folded into an assymetrical upright anticlinal fold. The dam foundation falls on the hinge zone of the assymetrical anticlinal iv fold. The foundation rocks are highly fractured due to folding. The Tons thrust, a major structural feature of the area cuts across the right bank at the dam site between El. 845 mto 850 mand enters into the adjoining Meera river valley. It dips at 15° to 25° in Southwesterly direction and is associated with large scale shearing. The thrust will be submerged under the maximum reservoir level of 860 m. The interpretation of the drill hole data has revealed that the fluvial overburden in the river bed section ranges from 3.65 mto 11.8 m. On the basis of limited water pressure test data, it is estimated that the foundation rocks are semipervious to pervious in nature. The stability analysis of abutments has revealed that the left abutment slope is kinematically unstable for plane mode of failure. The plane failure analysis for right abutment slope has been carried out by modifying the analytical technique of Hoek &Bray, where the effects of inclined upper slope surface and tension crack have been incorporated. In order to improve the overall foundation condition, suitable remedial measures have been evolved. The Kishau dam project envisages the construction of a surface power house with an installed capacity of 600 MW of peak power, on the left bank of Tons river. For housing the power units, a terrace (22 x 70 m) has to be created by excavating the adjoining hill slopes. The stability analysis of hill slopes adjoining the power house site has revealed that the slopes are stable. Based on the stability analysis, a cut slope design has been suggested in addition to suitable stability measures. In view of the problems arising due to Jamar stream flowing in the centre of the site, an alternate site has been identified about 75 mdownstream of the present one. The reservoir area has been mapped on 1:15,000 scale. The rocks belonging to Dharagad Group (Limestone, Quartzite and Slate), Deoban Group (Limestone, Slates and Quartzite), Simla Group (Quartzite and Quartzitic slates) and Jaunsar Group (Slates) are exposed in the reservoir area. For the stability analysis, the data pertaining to structural discontinuities, mainly joints, has been collected from 100 localities. In the entire reservoir area 70 potentially unstable slopes have beep identified for detailed stability analysis. Acomputer program ROSS has been developed to identify the possible plane and wedge mode of failures. Besides, four slopes, having rotational mode of failure, have also been studied. The detailed analysis has been carried out for both natural and water impoundment conditions. An attempt has been made to study the possible impacts of these unstable slopes on the stability of dam in the event of failures.