Back Analysis along LANDSLIDE affected slopes of THE Bandarkot village in Uttarkashi district of Uttarakhand Sebabrata Das*, Prathvi Singh*, Bhupender Singh* and Bhupendra Singh** *Geological Survey of India, S.U: Uttarakhand, Dehradun ** Geological Survey of India, S.U: Madhya Pradesh, Bhopal Presented by Bhupendra Singh

INTRODUCTION The Bandarkot village is located 8 kilometer west of Uttarkashi township on the right bank of Bhagirathi river on NH-134. The area is dissected by many small tributaries of Bhagirathi River and shows moderately rugged topography. Quartzite, slates and metabasic of Berinag Formation of Garhwal Group of Mesoproterozoic Age belonging to Lesser Himalaya are exposed in the study area. The area is bounded by Main Central Thrust (MCT) in the North and North Almora Thrust (NAT) in the South.

LOCATION MAP OF THE AREA Bhagirathi River AOI

FEATURES OF THE SLIDE AFFECTED SLOPE It is predominantly a debris slide and is retrogressive in nature. The slide has a slope length of about 290 meter and maximum width of about 550 m having a height of 200m. About 150 meter stretch of the road was washed off severely affecting settlements along the roadside. Slope of the ground varies from 20°-65° towards South-Southeast direction and is undulatory in nature. At present the crown of the slide is located at an elevation of 1225 m whereas the toe occurs at the right bank of Bhagirathi River at an elevation of 1025 m. Main settlement is situated just above the road and behind the crown of the slide. Tensional cracks are observed in the village just above the crown of the slide. The debris and RBM consists of boulders, cobbles and pebbles embedded in fine matrix of clay- silt and sand.

Cultivated Land Settlements Crown Toe cutting by Bhagirathi River

Escarpment Crown of the slide Road

Bandarkot Village Road Bhagirathi River

Damaged Settlement

Steep Slope above road composed of RBM

CAUSATIVE FACTORS The main triggering factor for its reactivation was heavy rainfall which led to considerable discharge in the Bhagirathi River during June 2013. This led to toe cutting ultimately resulting in damage to NH, shops and settlement. During rain, due to the absence of lined drainage and presence of tension cracks, surface run off gets enough time to percolate down into the ground making the ground material saturated which reduces shearing strength of the material and causes failure. The slope above the road level from elevation 1110 m to 1145 m is cut at a steeper angle and at places negative slope has formed which makes the slope vulnerable to fail. The slope below the road is very steep and is covered with RBM with no break in slope. Absence of proper drainage system. Continuous river bank erosion by Bhagirathi River is also one of the cause for instability of the slope resulting into ground subsidence and landslide in the Bandarkot area.

BACK ANALYSIS ALONG THE SLOPE USING BISHOPS METHOD Unit Weight 18.5 kN/m3 Cohesion 0.2 kg/cm2 Angle of internal friction 37⁰ Analysis of the samples were carried out in Geotechnical Lab., GSI, NR

RECOMMENDATIONS Toe of the slide should be protected by RCC wall of height 8-10 m with provision of weep holes. The slope should be modified to an angle of 30-40° and benching should be done at least in 5 levels in the entire slope. Slope above the road should be modified and 2-3 m wide berms at least at three levels should be provided to check the direct damage to the NH and to ease out steepness of slope. The above berms should also be provided with contour lined drainages. Road can be constructed in the same elevation of 1110 m with proper slope protection below the road. Construction of suitably designed effective surface (including cross drainage) drainage network for release of water so that the pore water pressure can be reduced. All the cracks in upper, middle part and on road must be filled with impervious material.

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