HILL SLOPE STABILITY STUDIES OF A PART OF TEHRI RESERVOIR, HIMALAYA, INDIA

Abstract

A rock-fill dam planned to a height of 260.5 m, is under construction across the Bhagirathi river - 1.5 km. downstream of its confluence with the river Bhilangna near Tehri township (Uttar Pradesh, India). On its completion the reservoir is estimated to be of35.5 * O -J 10 m storage capacity at target ofmaximum reservoir level of840 m. The reservoir when full, will extend upstream from the Dam to 44 km along the river Bhagirathi and 25 km along the river Bhilangna . The stability ofhill slopes flanking the reservoir, therefore, forms avery important aspect with regard to this dam. The present study is an endeavour to evaluate the stability ofthe reservoir bounding hill slopes of the Lesser Himalaya falling in the Survey of India Toposheet Number 53 J/7 and 53 J/11, with the following major objectives: (i) Identification ofhazard prone slopes. (ii) Impact oflandslide induced water waves in the reservoir on the proposed dam. (iii) Suggest remedial measures for improving stability ofhazard prone slopes. The rocks exposed in Tehri reservoir area are the Chandpur phyUites and quartzites of the Garhwal Group. In the Bhilangna valley, towards north and east of dam site, Chandpur phyUites come in contact with quartzite of Garhwal Group and are separated by the Srinagar thrust. The rocks are foUated with average dip of about 57° towards N2240. The rocks are invariably jointed. The joint orientations in decreasing order ofprominence are N247°/52°, N101°/47°, N103°/27° and N121°/26°. InitiaUy, a slope facet map of the area of study has been prepared which served as a base map to evaluate the hiU slope stability around reservoir. Of the total 156 facets studied, 93 belong to the Bhagirathi and 63 to the BhUangna river vaUey slopes. In Bhagirathi river vaUey, out of 93, 57 facets are on right bank and 36 facets on left bank. Similarly, out of 63 facets in BhUangna river valley, 32 facets are on right bank and 31 facets on left bank. Cross sections across all these 156 slope facets have been investigated for their stabUity aspects in Bhagirathi and BhUangna valley. The conventional approach, by using Markland test indicates that the slopes, in general, are stable except four potential faUure slopes at different locations. However, when Hoek and Bray's approach of calculating Factor of Safety (F.O.S.) was used, only one slope facet on left bank of Bhagirathi vaUey appears to be unstable in both dry and wet of dynamic conditions, considering earthquake acceleration 0.16. No slope on right bank of Bhagirthi vaUey appears to be unstable. Likewise, simUar slope analysis was done in the BhUangna river vaUey, one ofthe three potential faUure slopes appears to be unstable under 'dynamic and wet' condition. The other two slopes are just criticaUy stable under 'dynamic and wet' condition. No slope facet on left bank of Bhilangna vaUey is potentiaUy unstable. The conventional approach based on average slope ofthe facets does not take into account the variation ofthe slope within a facet. Hence realizing this limitation a more pragmatic approach has been adopted. In this approach, smallest possible unit between two closely spaced successive contours has been considered for slope stabUity investigation. It involves back analysis, which takes into account the foUowing steps : 1. Identification of locaUy steep slopes (potential faUure slopes), of the facet, based on Markland test. 2. Determination of Factor ofSafety (F.O.S.) for the potentiaUy unstable slopes for various conditions i.e. 'static and dry', 'static and wet', 'dynamic and dry' and 'dynamic and wet'. 3. Based on Factor of Safety the slopes are classified into three different categories (i) stable (F.O.S.>1.5) (ii) criticaUy stable (1<F.0.S.<1.5) and (hi) unstable (F.O.S. <1). Based on this new approach, atotal of 72 potential faUure slopes have been identified as against one by conventional approach in Bhagirathi vaUey. Out of these 72 slopes, 20 are potential wedge faUure and 52 are potential plane faUure slopes. Results obtained by calculation of Factor ofSafety ofall the potential failure slopes show that all potential wedge faUures are stable even under worst possible condition (dynamic and wet). Out of 52 potential plane faUures, 43 are unstable (i.e. 23 possible slides on right bank and 20 on left bank). Field observations confirm the results obtained by new approach. In Bhagirathi valley most of the slides (unstable slopes) are located near Bhaldgaon, Raolakot and Bhaldiyana vUlages. In Bhilangna vaUey there are 65 potential failure slopes as per the new approach as against three by conventional approach. Out of these 65 potential faUure slopes, 25 are wedge faUures and 40 are plane faUures. AU the 25 potential wedge faUures have FOS >1.5 and therefore are stable under four different conditions. Out of 40 potential plane faUures, 11 are unstable (FOS < 1), and located on right bank. There is no shde on left bank of Bhilangna vaUey. In BhUangna vaUey, most of the sUdes are close to Myunda and Dewal vUlages. As the geological and environmental conditions are same throughout the area of study, the stabUity measures have been suggested keeping in view of the results obtained by stability analysis. They include modification of slopes, installation of rock anchors , shotcreting on steel chain linked wire mesh and afforestation. For afforestation locaUy avaUable plant species can be used namely Chir pine (Pinus roxburghu), Buras (Rhododendron arboreum). Oak (Quercus incana\ Kilmore (Berberis sgp_.), Dhaula (Woodfordia fruiticosa). Hinselu (Rubus ellipticus). Deodar (Cedrus Deodara). Pipal (Fias religrose), Neem (Azardirachta indica). Barh (Ficus benghalensis) and other simUar plant species. Sudden failure of slopes around the reservoir would generate water waves in the reservoir. The distance, to which these waves would travel depends mainly upon kinetic energy involved in it. A sufficiently massive slide from the rim slopes could cause overtopping ofthe dam, either by wave action or simply by raising the water surface faster than the spiUway could discharge. In the present analysis, an attempt has been made to calculate the wave height generated by the possible slides in the reservoir area. In Bhagirathi vaUey, total 43 slides have been considered for this purpose (23 on right bank and 20 on left bank). Wave height due to possible slides for the reservoir rim slope, pertaining to left bank of Bhagirathi vaUey, varies from 0.101*10-6 mto 0.4493*10-5 m. For right bank, wave height varies from 0.165*10- mto 0.2*10"3 m. In BhUangna vaUey, variation is from 0.116*10-6 mto 0.1*10"5 m. As the dam has 5mfree board such waves are not likely to affect it adversely. The present study indicates that even in adverse condition of slope faUure the wave generated have amaximum amplitude to 0.2* 103m which is much less than 5m. Therefore, it can be concluded that due to possible slides in the reservoir area, stabUity ofthe dam'structure wUl not be endangered.