Accepted Manuscript Slope stability analysis of Balia Nala landslide, Kumaun Lesser Himalaya, Nainital, Uttarakhand, India Mohit Kumar, Shruti Rana, Pitamber Dutt Pant, Ramesh Chandra Patel PII: S1674-7755(16)30218-9 DOI: 10.1016/j.jrmge.2016.05.009 Reference: JRMGE 291 To appear in: Journal of Rock Mechanics and Geotechnical Engineering Received Date: 16 January 2016 Revised Date: 26 April 2016 Accepted Date: 17 May 2016 Please cite this article as: Kumar M, Rana S, Pant PD, Chandra Patel R, Slope stability analysis of Balia Nala landslide, Kumaun Lesser Himalaya, Nainital, Uttarakhand, India, Journal of Rock Mechanics and Geotechnical Engineering (2016), doi: 10.1016/j.jrmge.2016.05.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT Slope stability analysis of Balia Nala landslide, Kumaun Lesser Himalaya, Nainital, Uttarakhand, India Mohit Kumar a,*, Shruti Rana a, Pitamber Dutt Pant a, Ramesh Chandra Patel b a Department of Geology, Kumaun University, Nainital, India b Department of Geophysics, Kurukshetra University, Kurukshetra, India Received 16 January 2016; received in revised form 26 April 2016; accepted 17 May 2016 Abstract: Balia Nala is the outlet of the Nainital lake, flowing towards southeast direction. Presence of Nainital habitation at its right bank has high socio-economic importance. This study presents the stability analysis of a ravine/valley along Balia Nala. Variegated slates (lower Krol and upper Blaini formations) are the main rock types, wherever the outcrop does exist and rest of the area is covered by slope wash and river borne materials. Three sets of joints are presented in the area, but 4 sets of joints also exist at some locations. Nainital lake fault intersected by Manora fault from southwest direction passes through eastern side of the study area, and some small faults, which are sub-branches of Nainital lake fault, are observed (with 10 m offset) and promote the landslide in the area. This study shows that different kinds of discontinuities (joints, faults and shear zones) and rapid down cutting by the stream due to neotectonic activity affect the stability of the slope. The fragile lithology and deep V-shaped valley further accelerate the mass movement in the study area. In addition, rock mass rating (RMR), factor of safety (FOS) and graphical analysis of the joints indicate the study area as landslide-prone zone. This study will be helpful in not only reducing the risk on life of people, but also in assisting the ongoing civil work in the study area. Keywords: rock mass rating (RMR); factor of safety (FOS); Balia Nala landslide; slope stability analysis composed of pyriteous slates of Kailakhan member (Infra Krol), slates of 1. Introduction Manora member (calcareous slates, greyish to greenish in color), purple slates of Hanumangarhi member (ferruginous slates), and dolomite blocks Slope failure may lead to loss of lives, property and environmental of Pashandevi member (Valdiya, 1988). In Balia ravine near Tallital of degradation. In the Himalayan region, slope instability (Panikkar and Nainital hills, the major lithologies are crumpled Kailakhan slate Subramanyam, 1997), tectonic activity and mass movement frequently (pyritiferous slates of Infra Krol) and lower Krol slate. occur due to steep slopes (Paul and Mahajan, 1999), and highly sheared, The study area is bound by the MBT in the south along which the LHS crushed and deformed rocks. In the southern margin of Kumaun sub- has thrust over the sub-Himalaya (Fig. 1). The MBT is characterized by Himalaya, the frequency of landslides is high due to structural and imbricating thrusts and faults (Valdiya, 1984). It influences the active neotectonic activities along the main boundary thrust (MBT) zone tectonic movements in this area. Other major faults in this area are (Valdiya and Bartariya, 1989; Valdiya, 2001, 2003). In the past two Nainital lake fault (Middlemiss, 1890) and Manora fault (Valdiya, 1988) decades, Malpa rockfall in 1998 (Pant and Luirei, 1999), Okhimath (Fig.MANUSCRIPT 1). The Nainital lake fault passes through the Nainital lake and landslide along Mandakini valley in 1998 (Sah and Bist, 1998), Amiya rotational movement along this NW-SE trending lake fault is described as landslide of southern Kumaun (Pant and Luirei, 2005), Phata Byung the mechanism of development of the Nainital lake. The landslide of Rudraparyag district in 2001 (Naithani et al., 2002; thermoluminescence dating of neotectonic events as recorded in fault Chaudhary et al., 2010), Budha Kedar landslide in Balganga valley (Sah gages and buried soils formed on landslide debris related to the lake fault et al., 2003), Varunawat landslide in 2003 (Gupta and Bist, 2004), indicates that the Nainital lake was formed at calibrated annum 40 −50 ka Agastyamuni landslide in 2005 (Rautela and Pande, 2005), natural (Singhvi et al., 1994). The eastern ridge, named as Sher-Ka-Dada, is the hazards in Alaknanda valley (Joshi and Kumar, 2006), landslide in up thrown block exposing the lower Krol sediments, whereas the western Pitthoragarh district in 2009 (Sarkar and Kanungo, 2010), and landslide in block (Ayarpatta ridge) is down thrown block exposing the upper Krol Asi Ganga in 2012 (Gupta et al., 2013; Martha and Kumar, 2013) have succession. This fault has dextrally offset the MBT near Beluakhan west devastatingly affected Uttarakhand, India. of Jeolikot (Valdiya, 1984) and Manora fault near Alukhet (Fig. 1). Nainital is a popular hill station in the Indian state of Uttarakhand and Development of Balia Nala is related to movements along the Nainital headquarter of Nainital district in the Kumaun foothills of the outer lake fault oriented obliquely or transversely to the regional trend of Himalaya. The slopes of the nearby mountains are most populated, with orographic arc. The drainage is trellis type considerably sharpened and an elevation ranging from 1940 m to 2100 m. To prevent and/or reduce suitably modified by neotectonic tear faults (Valdiya, 1988). Drainage the landslide hazards, stability analysis has been carried out in this area density is high due to softness of the rocks (Valdiya, 1988). (Fig. 1). ACCEPTED3. Study area 2. Geology and tectonic setup The geological setting of the Kumaun Himalaya has been studied in details by many scholars (e.g. Auden, 1934; Heim and Gansser, 1939; Fuchs and Sinha, 1974; Hukku et al., 1974; Pal and Merh, 1974; Pande, 1974; Valdiya, 1980). Nainital hills, the area of study, represent the outer sequence of the lesser Himalayan sequence (LHS). It is the southeastern part of a strip of en echelon basins of the Krol belt. The study area is *Corresponding author. E-mail address: [email protected] ACCEPTED MANUSCRIPT Fig. 1. Geological and location map of the study area (Valdiya, 1988). The study area stretches 2.1 km along the Balia Nala and is delineated by 29 °22 ′E−29 °21 ′E latitude and 79 °28 ′N−79 °28 ′23.2 ″ longitude 4. Methodology (towards southwest of Nainital, Fig. 1). The elevation ranges from 1450 m to 1920 m. The right bank of this stream has two populated localities, i.e. Geological and engineering geological mapping has been carried out in (a) locality nearby Government Inter College (GIC) and (b) Saraswati theMANUSCRIPT study area on 1:1000 scale (Fig. 3a and b). Geological cross-sections Vihar, both affected by landslides. have been plotted in Fig. 4 along four section lines (1-1′, 2-2′, 3-3′ and 4- 4′) marked in Fig. 3. This work has focused on the rock mass characterization by rock mass rating (RMR), factor of safety (FOS) and kinematics analysis (stereographic projection) of the discontinuities. RMR has wide application in tunnels, slopes, foundations and mines (Bieniawski, 1989). In rock mass classification system, methodology is proposed to identify the quantitative condition of road slope. It is broadly used in underground rock tunnels and road cut slopes. RMR also plays a key role in calculation of the slope mass rating. RMR includes the collection of field data, i.e. orientations of different discontinuities, uniaxial compressive strength (UCS) (measured using Schmidt hammer according to ISRM (1978, 1981, 2007), Bieniawski (1989), and Brencich et al. (2013)), spacing, slope direction and dip, conditions of discontinuities and groundwater conditions. Rock quality designation (RQD) has been calculated according to Singh and Goel (1999) in the field. RQD is calculated using number of joints per unit volume Jv and equal to 115 −3.3 Jv. FOS has been calculated for every outcrop according ACCEPTEDto Hoek and Bray (1981). FOS of a rock slope is the ratio of resisting forces to driving forces. If FOS is less than or equal to 1, the slope will fail. If FOS is much larger than 1, the slope will be quite stable. However, Fig. 2. Average monthly and annual rainfall during 2007 −2013 (Source: Aryabhata if the FOS is slightly greater than 1, small disturbance may cause the Research Institute of Observational Science, Nainital). slope to fail (Hoek and Bray, 1981). To analyze various modes of rock slope failures (plane, wedge, and toppling failures), Markland’s test has been performed as described by Hoek and Bray (1981). Various modes of Annual rainfall recorded in this area is 2468 mm during 1995 −2009, failures have occurred due to presence of unfavorable oriented while it is 4190 mm during 2007 −2013 (Fig. 2). The recorded rainfall discontinuities (Hoek, 2007). Kinematics refers to the motion of bodies shows 70% increase (Gupta et al., 2015).