Journal of Engineering Geology Volume XL, No. 1, A bi-annual journal of ISEG July 2015 Instrumentation and slope stability analysis of Tangni landslide Sharma, Sumit Dhar, Sunil Mishra, S.P. Chaturvedi, Pratik Jaiswal, Brajesh Defence Terrain Research Laboratory, DRDO, Delhi, India E-mail: [email protected], [email protected] Abstract The paper illustrates the results of an integrated study done on Tangni landslide located on NH-58 at Chamoli, Uttarakhand. The slide was selected on the basis of high landslide risk to human life, natural resources and infrastructure located in risk area and also on the evidences of road subsidence and development of cracks on retaining wall and on walls of few houses present at the crest of slide. Geological, geo-morphological and geotechnical investigations were carried out to understand the mechanism of landslide and to plan further investigation and monitoring. Surface monitoring using total station and extensometer coupled with subsurface measurements of inclinometer and piezometer data were carried out to determine nature, magnitude, rate and direction of movement. The steady state analyses along with site specific seismic response analysis were also attempted to investigate the dominant factors responsible for landslide in terms of factor of safety. The innovative and cost effective remedial measures on the basis of such studies were also suggested for the site to arrest the further movement and damage due to landslide. Keywords: Landslide, Slope Monitoring, Instrumentation, EWS, Stability Analyses, Control measures. 1. Introduction: The increased incidences of landslides and mass movements have become widespread events, posing great threat and challenges to development processes around the world. Over the past few decades, the catastrophic and disastrous effects of landslides have caused extensive damage to life, property and public utility services [1]. Monitoring and Early warning system has therefore become need of time toward reduction of disasters induced by landslides and slope instabilities. The real-time monitoring not only provide immediate warning of landslide activity but also provide understanding of dynamics of slope failures. The system has been in use throughout the world to forecast landslide hazard. In US, USGS, the nodal agency for all landslide studies, carried out the real time monitoring of an active landslide called Cleveland Corral Landslide at Highway 50, California [2]. Publication also available on Capturing landslide dynamics and hydrologic triggers using real time monitoring (Reid, M.E., Baum, R.L., Lahusen, R.G., Ellis, W.L.) [3]. AlpEWAS (Early Warning System for Alpine slopes) jointly developed by Germany, Switzerland and Austria used the combination of underground cables and surface based video camera and laser scanner to detect and measure movement [4]. UK, China, Peru, Malaysia, Egypt countries also faced lots of problems related to landslides and at many places they established early warning systems based on conventional as well as wireless sensors. Site-specific, real time systems have been applied in many countries to monitor 145 Journal of Engineering Geology Volume XL, No. 1, A bi-annual journal of ISEG July 2015 critical structures like dams, or hazardous landslides (Angeli et al. 1994, Berti et al. 2000, Husaini & Ratnasamy 2001, Froese & Moreno 2007) [3]. In India, the work related to instrument aided landslide monitoring for developing EWS is in embryonic stage. However, attempts have been made by Amrita University in Munnar area (Western Ghats); CSIO, Chandigarh in Mansa Devi area and DST sponsored projects with various Universities/Institutes have been initiated in Indian Context. Geological Survey of India also initiated real time monitoring of three landslides viz. Surbee landslide in Uttarakhand, 9th mile slide in Sikkim and Hospital landslide in Nilgiri hills under Indo- Canada collaboration project. Defence Terrain Research Laboratory (DRDO), New Delhi has also installed set of instruments at Tangni landslide in year 2011 for monitoring of ground movement and development of EWS. Tangni landslide is located near Pakhi village in between Pipalkoti and Joshimath on NH- 58. The slide is an active one, as was evident from road subsidence and manifestations of cracks on retaining wall near road and on the walls of few houses present at the crest of slide. The assessment of slide also becomes essential as highway connects important Hindu pilgrimage centre called Badrinath Dham to rest of the country and therefore any kind of slope failure along the route not only disrupts the traffic but also cause lots of damage to infrastructure and public utility services. An integrated study including geological, geo morphological and geotechnical investigations were therefore planned to determine the features and dynamics of landslide to plan further investigation and monitoring of the slide. The main structural discontinuities below ground surface were identified using resistivity tomography profiles and GPR investigations. The surface displacements of the landslide were determined using surface wired extensometers and total station whereas subsurface monitoring were planned using conventional sensors i.e. biaxial In Place Inclinometers and Vibrating Wire Piezometers for determination of landslide kinematics, change in pore water pressure and rate and direction of movement. The rain gauge was also installed near to data logger enclosure. The data of all these sensors were transferred through data logger installed at field to control station at DTRL via GPRS modem/FTP. Analysis of data allows recognizing of landslide processes and dynamics and once the threshold value of precipitation exceeded, alarm warnings will be generated accordingly. Till date, the data received is not sufficient enough to arrive at some logical conclusion and therefore continuous data monitoring for at least another three years is required to identify the risk conditions for sending the early warning SMS messages. The results obtained from the outputs of surface and subsurface monitoring were used as stepping stone and numerical model were developed for validation with field observations. The analysis showed that the slope was stable under strength reduction technique but under dynamic analysis, the same slope suffered large amplification which in future, could lead to landslide, if earthquake of higher magnitude occur in the area. 146 Journal of Engineering Geology Volume XL, No. 1, A bi-annual journal of ISEG July 2015 2. Location of study area: The study area is located on NH-58, roughly 65Km south of Badrinath Dham at an altitude of 1524m near Pakhi village. The precise geographic location as seen in fig. 1 is Lat 30° 27’ 54.3” N and Long 79° 27’ 26.3”. The average slope of the slide area is about 35˚ towards North. The slide is an active one and signs of movement were evident from road subsidence, cracks opening at crest and settlement cracks on retaining wall and on walls of houses located at crest of slide. In first glance, the movement could be attributed to water flow through nallah adjacent to slide leading to removal of old debris material from body and toe and also to continuous stretch of crack formation at crest leading to negative pore pressure generation and slope instability. The trees on right flank of slide were also observed to be tilted towards road. The exact cause for movement would be ascertained once the data analysis of sensors will be carried out. The zone area runs parallel to the river course of Alaknanda and its tributaries. Figure 1 Tangni landslide site 3. Geological setting of area: The geology of the area is complex, consisting of Precambrian lithological units of Garhwal region of the NW lesser Himalaya. The height of upper and lower slope is 107m and 57m respectively. The slope is continuous and shows an inclination of about 35˚ above road level and 42˚ belo w road level in the dip direction of N10˚. The main escarpment in the slide area comprised of rocks and debris. The left flank has hard jointed rocks present while right flank is mostly covered with loose debris and soil. The rocks are well jointed and mainly comprised of slate / phyllite / dolomite and limestone under the Tejam & Damtha group. The dolomites in the proximal area of this zone are generally fractured and pulverized. The area is known to be affected by fault zone which extends from village Pakhi to near Belakuchi, Patal Ganga. Due to this, multiple cracks are seen originated at crest of slide which extends through agricultural land to road, thereby making slope unstable. Besides these, the other factors responsible for landslide are surface and subsurface water flow, unfavourable discontinuity and tectonically active zone. From tectonic viewpoint, the slide lies in the vicinity of Main Central Thrust (MCT), result area is tectonically active and experiences moderate to high magnitude earthquake round the year. One major earthquake of magnitude Ms=6.6 rocked the 147 Journal of Engineering Geology Volume XL, No. 1, A bi-annual journal of ISEG July 2015 Garhwal-Kumaun Himalaya on 29th March 1999 with its epicenter located near Gopeshwar in Chamoli district of Garhwal region. The earthquake belongs to highest Seismic Zone V of Indian Seismic Code (IS: 1893-2002); this incident evoked the interest to study the influence of earthquake on the geology of area. Figure 2 Contour and DEM map of site and indications of slope instability on retaining wall and road 4. Geotechnical investigations: The laboratory tests were conducted on soil samples obtained from boreholes drilled at selected location of slide above road for determination of soil index properties, subsurface stratification, strength parameters etc. The boreholes were advanced to varying depth depending on relief features and geological details and the representative soil samples were collected from crown, middle and toe portion of slide. The soil samples were tested for grain size analysis (mechanical sieve analysis) as per IS: 2720 (Part V) 1995. The % age content of gravels varied from 31.3 to 60.4, sand from 15.5 to 24.4 and fine aggregate from 22.5 to 48.8.