Abstract: Hydroelectric projects, when constructed in landslide prone area, a well-considered program of preventive measures is required for safety of the project components. The inclinometer monitoring study is a very useful tool for understanding the behavior of slide prone area. This paper presents gainful utilization of inclinometer monitoring at bypass tunnel area of Loktak Power Station (105MW) in Manipur, India, vulnerable to landslide activity. On the basis of the long-term inclinometer monitoring study, it has been possible to detect micro-level deflections and identifying possible slip surfaces towards understanding the mechanism of displacement vectors, thereby helping in evaluating the performance of the protection structures and effectiveness of the remedial measures undertaken.
1. INTRODUCTION In hydroelectric projects, located in landslide prone area, long-term periodical inclinometer monitoring is very useful in understanding behavior of the landslide activity and assessing the effectiveness of the slope protection measures implemented in restricting ground movement. Moreover, at times, it also acts as a forewarning aid in case of any possible major slide activity that may likely take place in the monitoring area. This paper presents a case history, describing the utilization of inclinometer monitoring study at Loktak Power Station (105MW) in Manipur towards evaluating the performance of the protection structures and the remedial measures undertaken in the bypass tunnel area of HRT, vulnerable to landslide. The project has witnessed a massive landslide on 1983, which resulted to a collapse of about 33m length of HRT: the cause of landslide in the area being due to heavy rainfall which had lubricated and forced subsurface material to slide down along the slip surfaces. Subsequently, the project was recommissioned in 1984 by constructing a steel lined bypass tunnel of 157m length in the affected area and various slope protection measures were undertaken as per recommendations of Technical Advisory Committee. The drainages in the hill slope above the bypass tunnel were extensively improved with a network of rock trenches, planned contour drains and soil cementing with a view to minimizing infiltration of water into the subsurface. Additionally, number of reinforced drainage cum anchorage shafts were also sunk on the upslope side of bypass tunnel to collect and drain groundwater from upslope reaches through siphoning action. The stream flowing downhill across the tunnel alignment had as well been trained by check dams and concrete crate works. Although, as a consequence, the bypass tunnel area had appreciably stabilized by constructing various slope protection structures, a program of periodical monitoring had also been instituted for understanding the time-to-time behavior of landslides in bypass tunnel area and evaluating the efficacy of the slope protection structures. Initially, in the period between 1986 and 1991, inclinometer study was outsourced and being conducted by Central Soil & Material Research Station (CSMRS), New Delhi . Subsequently, from 1991 onwards, the inclinometer monitoring study is being routinely carried out by NHPC through in-house expertise. The paper discusses the observations of the inclinometer monitoring study since 1991 and its gainful utility in evaluating the performance of the various slope protection measures.