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, 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, . 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 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 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.

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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

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Garhwal-Kumaun Himalaya on 29th March 1999 with its epicenter located near Gopeshwar in 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. The soils have been classified as silty gravels (GM) as per IS 1498-1970 on the basis of sieve analysis and Liquid and Plastic limit tests. The relative density and strength parameters tests were also performed as per the procedure given in IS: 2720 (Part XIV) 1995. Shear parameters obtained from direct shear test have cohesion varying between 0.03 to 0.41 Kg/cm2 and coefficient of friction between 27.5˚ and 43.5˚. These strength parameters obtained from tests and slope geometry obtained from total station readings was then used to carry out slope stability analysis for computation of factor of safety.

5. System scheme:

The automated remote monitoring of landslide (ARMOL) was initiated under the project URUSWATI in the year 2010 by Defence Terrain Research Laboratory (DTRL), DRDO in collaboration with Central Building Research institute (CBRI), Roorkee at Tangni landslide site. Under the project, an integrated approach was used, consisting of (1) detailed geological, structural and geomorphological study of site and nearby areas (2) Surface monitoring of landslide area using total station and extensometer and (3) subsurface investigations (Borehole drillings and Inclinometer and Piezometer measurements).

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The engineering geological field works were conducted through field survey and drilling of boreholes to determine the bedding attitude of rock units, tectonic effect on topographical and local geo stratigraphic setup of region and for correlation of interpreted data taken from field with geotechnical parameters of studied area. The geotechnical borehole logging give single point information in lateral direction and hence geophysical methods were also employed to obtain two dimension distributed data. The ERT profile and GPR investigations give information not only about internal composition of sliding mass, slide geometry and depth of bedrock but also save time and cost in identification of locations for borehole drilling for installation of sensors. The resistivity data in Figure 3 at road level showed lower resistivity (due to debris) compared to crown level (due to fractured rock mass).

Figure 3 Geophysical and structural survey results

Structurally, the bedding joint has a slope of 32˚ in the dip direction of N5˚ and landslide site has a slope of 35˚ in the dip direction of N10˚. The orientation of slope face and bedding joint along with two prominent sets of continuous joints observed were plotted on stereo net. The projection diagram in Figure 3 showed that the bedding joint has a dip in the same direction as that of slope. Hence Tangni landslide has planar failure along the bedding joint.

The surface displacement measurements of 100 numbers of pedestals (blue dots in figure 4) installed over the entire landslide body were determined using total station acquired

Figure 4 Pedestal and borehole locations on study area

149 Journal of Engineering Geology Volume XL, No. 1, A bi-annual journal of ISEG July 2015 every three months in relation with known reference station (placed on opposite side of road) to obtain sloping, horizontal and vertical distances of the points measured. The peaks observed in Figure 5 shows substantial movement of pedestals all along the landslide

Figure 5 Surfacial displacements using total station

Figure 6 Group of wired extensometer

Figure 7 Displacement monitoring through extensometer

150 Journal of Engineering Geology Volume XL, No. 1, A bi-annual journal of ISEG July 2015 body. Alternately, eight nos. of wired extensometers seen in figure 6 were also installed during Aug-Sept 2013 through suspected zone of movement. The amount and rate of slope movement was measured due to weight pull along the graduated track. The displacement measured between Sept to Nov. 2013 ranged from 40mm to 100mm perpendicular to the sliding direction as seen in Figure 7.

Figure 8 Schematic of real time landslide monitoring system

These types of surface investigations define the distribution of unstable area and give an overview of deformation in different sectors of the landslide.

The development of new sensors, data storage facility, low cost methods of communication and data analysis software have made remote monitoring and analysis easy and affordable. Keeping in view the above fact, the establishment of EWS facility was initiated with installation of subsurface sensors done to the depth of 15m, required depth being obtained from geophysical survey and expected sliding surface depth. Five Boreholes were drilled at different locations of landslide, with each Boreholes housed five nos. of IPI biaxial sensors spaced at 3m interval to the depth of 15m for measurement of ground displacement. As most of the landslide is triggered by hydrologic conditions, additional four numbers of boreholes were also drilled to the depth of 15m near to IPI sensors for installation of VW piezometers so that correlation between movement and pore water pressure generation could be established. The frequency and intensity of rainfall was monitored through tipping bucket rain gauge installed near to enclosure at site. All these devices were calibrated for measurement depths and specific calibration values. Data obtained from all these different sensors were stored in the logger housed in an enclosure near to site and then transferred to control station at DTRL through Cellular GPRS network/FTP for onward processing and analysis. The whole system was powered by rechargeable batteries and solar panels for acquisition system and communication system. The data received at DTRL is analyzed and correlated with the rainfall pattern of the area to arrive at the critical or limiting rainfall threshold that triggers an event of landslide. Alarm alerts could then only be generated by SMS messages or emails to defined users. The schematic diagram of real time landslide monitoring system is shown in figure 8.

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6. Analysis and interpretation of data:

Automatic monitoring measurements started in the year 2012 and till date the measurement is in progress. The displacement data obtained are basically compared and correlated with the pore pressure readings and rainfall data to form the base for prediction of alarm stages on exceedance of threshold values. The inclinometer measurements carried out in Boreholes BH1, BH3, BH5, BH7 and BH9 (Red dots in Figure 4) from Dec 2012 to Dec 2013 showed variable displacements particularly in the upper and middle portion of slide. Starting with the toe portion, displacement pattern of sensors in IPI-01 were observed in both the directions. That means the sensors were free to move about their position. This became evident when during the field visit; ABS casing in figure 9 was seen popped out of ground. In the middle portion of slide, roughly 10m above road, the sensors of IPI-03 and IPI-05 were analysed for displacement.

Figure 9 Monitoring of IPI-01 inclinometer

The IPI-03 (B-axis) sensors showed cumulative displacement reaching values of 4cm at 3m depth and 6cm at 6m depth, with largest variation registered at 12m depth about both A and B axes. The cumulative displacement record of sensors in IPI-05 (A-axis) showed

Figure 10 Monitoring of IPI-05 inclinometer

152 Journal of Engineering Geology Volume XL, No. 1, A bi-annual journal of ISEG July 2015 development of sliding zone at depths of 3m and 12m respectively while along B-axis, the sliding zone developed at depths of 3m as seen in Fig. 10. Similarly, the sensors installed in IPI-07 at 72m height above road showed large displacement all along the depth with largest being observed at depth of 15m. That means the sliding surface depth

Figure 11 Monitoring of VW piezometer was much below the anticipated 15m depth. Finally, the sensors installed in IPI-09 at 95 m height above road showed development of sliding zone at 3m and 12m depth

600

400 200

mm 0

Cumulative rainfall in Months

Figure 12 Monthly rainfall data respectively about both A & B axis with largest displacement registered again at depth of 15m. Here also the sliding surface depth was observed to slip much below the anticipated depth of 15m. Consequently, the pore water pressure distributions developed within the slope for the period Dec 2012 to Dec 13 were observed through four nos. of VW based piezometers installed next to IPI’s. As seen in figure 11, the PZ-02 sensor shows no variation in pore water pressure, could be due to its position being displaced from its initial one after the landslide; the reason which was responsible for popping out of IPI-01 sensors. The PZ-08 sensor also showed no variation in pore water pressure because the sliding surface depth exceeded the anticipated depth of 15m. The PZ-04 sensor installed next to IPI-03 (BH3) showed increase in pore water pressure while PZ-06 sensor installed next to IPI-05 (BH5) showed comparatively opposite trend of pore water pressure. This could be because PZ-04 sensor was installed on stable ground while PZ-06 sensor was installed on stable but sloping ground. This led to draining away of water and a decline in pore water pressure development. The data obtained from these five IPI’s and four piezometers were transferred and stored in data logger through 40 core cable for onward

153 Journal of Engineering Geology Volume XL, No. 1, A bi-annual journal of ISEG July 2015 transmission to control station at DTRL. Adjacent to enclosure was also installed an automatic rain measuring gauge, programmed in such a way that sampling rate could be varied based on the intensity and frequency of rainfall. The increase in pore pressure is generally related to increase in rainfall and as seen from monthly rainfall record in fig. 12, the cumulative rainfall recorded every month from June to August 2013 exceeded 400mm which was sufficient enough to cause perceptible change in the pore water pressure. However changes in the flow of both surface and sub-surface water will have a significant effect on the pore water pressures.

7. Stability analysis:

After geotechnical field and laboratory investigation and generation of 2D section from elevation and horizontal data of total station, slope stability analysis was carried out for three 2D sections of Tangni landslide namely, section 51, section 44&53 and section 76 to investigate the safety and stability state of the landslide. The result of analysis for section 76 was only covered in this paper.

Figure 13 Slope profile and critical slip surface

Assuming the slope to be dry, the steady state analysis was performed using strength parameters of soil material sampled and tested for selected locations of slide. The analysis was carried out on GEO Slope, Slope/W software (GEO-Slope 2007). The critical slip surface was obtained corresponding to minimum factor of safety.

In any of the landslide site, rainfall generally contributes as the main triggering factor for landslide which not only alter the structure of soil, but also reduce or eliminate the frictional and cohesive parameters of soil; the similar condition of systematic reduction sequence of available shear strength parameters were simulated using the strength reduction technique of finite element analysis. The soil materials were modelled with Mohr-Coulomb failure criterion in the commercially available finite element code of PLAXIS (Brinkgreve 2002). FoS obtained under static condition showed that the slope was stable under dry condition (Factor of safety: 1.28) which dropped down to 1.11 under strength reduction technique. This type of analysis does not give an insight into the process of instability and hence cannot be used as basis for stability evaluation before failure.

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Figure 14 Factor of safety based on in-situ stresses

The dynamic nonlinear analysis was also performed using 2D finite element code PLAXIS. The soil material was modelled using a non-associated elastoplastic constitutive model with Mohr Coulomb failure criterion. The material damping was simulated

At Stress Point At Base

5.00 0.00 0.00 5.00 10.00

Acc. In In m/s^2 Acc. Time in sec.

Figure 15 Acceleration time history at Figure 16 Contours of total displacement base and surface with well-known mass and stiffness proportional Rayleigh damping. Horizontal fixity was introduced at the far end vertical boundaries while fixed boundary conditions was applied at the bottom of FE model. The lateral domain was provided with absorbing boundary condition to limit spurious reflections from the boundaries. The rigid bedrock was modeled by imposing an acceleration time history at the base of numerical model. The response of soil to seismic excitation was observed over the surface described in time domain by its amplification factor. The amplification was observed to be comparatively negligible, could be due to presence of stable rock material underneath.

8. Control measures:

From the field observations and stability analysis, it was clear that the Tangni slope is unstable and in the active state of instability. Taking into consideration the nature of instability, the remedial measures were proposed for the site.

i. The existing retaining wall at the edge of the road was damaged due to increased pressure of the moving mass down the slope creating cracks and pot holes on the surface. The wall needs to be constructed again with increased length and height and fill behind wall be replaced with properly tamped crushed stone material.

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ii. Provision of surface water drainage through trenches and open channels to collect and divert surface runoff away from unstable portion of slide area. They should be lined as well to minimise erosion and uncontrolled infiltration. iii. Cracks available at the crest of slide should be sealed with puddle clay or impervious fill so that the surface water does not accumulate/pond in the area. The minor openings/cracks can also be covered with impervious membranes as a temporary measure. iv. The loose / unstable boulders and debris from the unstable slope should be removed or alternately unstable slope can be modified to gentle slope to restrain falling of materials to road and causing hindrance to vehicles movement. v. The toe portion of slide can be dumped with local material or toe berms can be provided to increase the resisting forces and thereby improve the overall stability of slope. vi. Further construction of houses/new facilities should be discouraged as it increases the anthropogenic activities in the area making the slope vulnerable to landslides. vii. On an experiment basis, the rock fall nets can be provided on a small portion of slope anchored at the head to prevent falling of loose blocks/ boulders to road, which otherwise can disrupt the traffic for long hours.

9. Results and discussions:

The investigations, monitoring and implementation of near real time monitoring system at Tangni Landslide site on NH-58 was presented in this paper. All relevant data (geological, geo morphological, geophysical and geotechnical) for the site under study were collected and examined. The outcomes of these investigations combined with the results obtained from surface and subsurface monitoring provided not only the detailed understanding of Tangni site conditions but also the framework to interpret those conditions for issuing of alarm alerts. This integrated approach allowed us to work at both spatial and temporal scales. The inclinometers detected the sliding surface of landslide at depth between 6 to 12m with total displacement recorded up to 6cm per year. At toe and crest, the depth could not be ascertained due to reasons covered in the analysis section. It is imperative to understand the deformation process from crest to toe of slide but as seen from data results, it cannot actually be precisely reconstructed as inclinometer data was lacking along the longitudinal and transverse axis of the landslide both at crown and toe of slide. Stability analysis was performed to determine the degree of instability in terms of factor of safety. The steady state analysis showed that the slope was stable under strength reduction technique but under seismic condition the section 44&53 develops higher amplification at surface compared to section 51 and section 76. Taking into consideration the nature of slope instability, type of slope materials and slope geometry, few Control measures were also suggested to arrest the further movement and damage due to landslide.

Acknowledgement:

I have great pleasure and privilege to express my deep sense of gratitude and thankfulness towards my Director, DTRL and head Dr. P.N. Joglekar. I also acknowledge

156 Journal of Engineering Geology Volume XL, No. 1, A bi-annual journal of ISEG July 2015 the encouragement and support provided by Sh. Sunil Dhar, PD and Sh. SP Mishra, DPD in writing this paper. Their timely guidance & valuable suggestions, time to time have steered me in clearing out difficulties at every juncture. Thanks are also due to my project team mates Sh. Pratik Chaturvedi, Sh. Brajesh Jaiswal and Ms. Neetu Tyagi for their support and continuous encouragement during the preparation of this paper. Thanks are also due to Sh. Yadvendra Pandey, CBRI and his team and to M/s AIMIL Pvt. Ltd. for their valuable support and assistance during the study.

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