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Journal of Rock Mechanics and Geotechnical Engineering 9 (2017) 150e158

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Full Length Article

Slope stability analysis of Balia Nala landslide, Kumaun LesserHimalaya, Nainital, Uttarakhand, India

Mohit Kumar a,*, Shruti Rana a, Pitamber Dutt Pant a, Ramesh Chandra Patel b

aDepartment of Geology, Kumaun University, Nainital, IndiabDepartment of Geophysics, Kurukshetra University, Kurukshetra, India

a r t i c l e i n f o

Article history:Received 16 January 2016Received in revised form26 April 2016Accepted 17 May 2016Available online 19 November 2016

Keywords:Rock mass rating (RMR)Factor of safety (FOS)Balia Nala landslideSlope stability analysis

* Corresponding author.E-mail address: puniyamohit@gmail.com (M. KumPeer review under responsibility of Institute o

Chinese Academy of Sciences.

http://dx.doi.org/10.1016/j.jrmge.2016.05.0091674-7755 � 2017 Institute of Rock and Soil MechanicNC-ND license (http://creativecommons.org/licenses/

a b s t r a c t

Balia Nala is the outlet of the Nainital lake, flowing towards southeast direction. Presence of Nainitalhabitation at its right bank has high socio-economic importance. This study presents the stabilityanalysis of a ravine/valley along Balia Nala. Variegated slates (lower Krol and upper Blaini formations) arethe main rock types, wherever the outcrop does exist and rest of the area is covered by slope wash andriver borne materials. Three sets of joints are presented in the area, but 4 sets of joints also exist at somelocations. Nainital lake fault intersected by Manora fault from southwest direction passes througheastern side of the study area, and some small faults, which are sub-branches of Nainital lake fault, areobserved (with 10 m offset) and promote the landslide in the area. This study shows that different kindsof discontinuities (joints, faults and shear zones) and rapid down cutting by the stream due to neo-tectonic activity affect the stability of the slope. The fragile lithology and deep V-shaped valley furtheraccelerate 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 willbe helpful in not only reducing the risk on life of people, but also in assisting the ongoing civil work in thestudy area.� 2017 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/

licenses/by-nc-nd/4.0/).

1. Introduction

Slope failure may lead to loss of lives, property and environ-mental degradation. In the Himalayan region, slope instability(Panikkar and Subramanyam, 1997), tectonic activity and massmovement frequently occur due to steep slopes (Paul and Mahajan,1999), and highly sheared, crushed and deformed rocks. In thesouthern margin of Kumaun sub-Himalaya, the frequency of land-slides is high due to structural and neotectonic activities along themain boundary thrust (MBT) zone (Valdiya and Bartariya, 1989;Valdiya, 2001, 2003). In the past two decades, Malpa rockfall in1998 (Pant and Luirei, 1999), Okhimath landslide along Mandakinivalley in 1998 (Sah and Bist, 1998), Amiya landslide of southernKumaun (Pant and Luirei, 2005), Phata Byung landslide of Rudra-paryag district in 2001 (Naithani et al., 2002; Chaudhary et al.,2010), Budha Kedar landslide in Balganga valley (Sah et al., 2003),

ar).f Rock and Soil Mechanics,

s, Chinese Academy of Sciences. Prby-nc-nd/4.0/).

Varunawat landslide in 2003 (Gupta and Bist, 2004), Agastyamunilandslide in 2005 (Rautela and Pande, 2005), natural hazards inAlaknanda valley (Joshi and Kumar, 2006), landslide in Pitthoragarhdistrict in 2009 (Sarkar and Kanungo, 2010), and landslide in AsiGanga in 2012 (Gupta et al., 2013; Martha and Kumar, 2013) havedevastatingly affected Uttarakhand, India.

Nainital is a popular hill station in the Indian state of Uttarak-hand and headquarter of Nainital district in the Kumaun foothills ofthe outer Himalaya. The slopes of the nearby mountains are mostpopulated, with an elevation ranging from 1940 m to 2100 m. Toprevent and/or reduce the landslide hazards, stability analysis hasbeen carried out in this area (Fig. 1).

2. Geology and tectonic setup

The geological setting of the Kumaun Himalaya has beenstudied in details by many scholars (e.g. Auden, 1934; Heim andGansser, 1939; Fuchs and Sinha, 1974; Hukku et al., 1974; Paland Merh, 1974; Pande, 1974; Valdiya, 1980). Nainital hills, thearea of study, represent the outer sequence of the lesser Hima-layan sequence (LHS). It is the southeastern part of a strip of en

oduction and hosting by Elsevier B.V. This is an open access article under the CC BY-

Fig. 1. Geological and location map of the study area (Valdiya, 1988).

M. Kumar et al. / Journal of Rock Mechanics and Geotechnical Engineering 9 (2017) 150e158 151

echelon basins of the Krol belt. The study area is composed ofpyriteous slates of Kailakhan member (Infra Krol), slates ofManora member (calcareous slates, grayish to greenish in color),purple slates of Hanumangarhi member (ferruginous slates), anddolomite blocks of Pashandevi member (Valdiya, 1988). In Baliaravine near Tallital of Nainital hills, the major lithologies arecrumpled Kailakhan slate (pyritiferous slates of Infra Krol) andlower Krol slate.

The study area is bound by theMBT in the south alongwhich theLHS has thrust over the sub-Himalaya (Fig. 1). The MBT is charac-terized by imbricating thrusts and faults (Valdiya, 1984). It in-fluences the active tectonic movements in this area. Other majorfaults in this area are Nainital lake fault (Middlemiss, 1890) andManora fault (Valdiya, 1988) (Fig. 1). The Nainital lake fault passesthrough the Nainital lake and rotational movement along this NWe

SE trending lake fault is described as the mechanism of develop-ment of the Nainital lake. The thermoluminescence dating ofneotectonic events as recorded in fault gages and buried soilsformed on landslide debris related to the lake fault indicates thatthe Nainital lake was formed at calibrated annum 40e50 ka(Singhvi et al., 1994). The eastern ridge, named as Sher-Ka-Dada, isthe up thrown block exposing the lower Krol sediments, whereasthe western block (Ayarpatta ridge) is down thrown block exposingthe upper Krol succession. This fault has dextrally offset the MBTnear Beluakhan west of Jeolikot (Valdiya, 1984) and Manora faultnear Alukhet (Fig. 1).

Development of Balia Nala is related to movements along theNainital lake fault oriented obliquely or transversely to the regionaltrend of orographic arc. The drainage is trellis type considerablysharpened and suitably modified by neotectonic tear faults(Valdiya,1988). Drainage density is high due to softness of the rocks(Valdiya, 1988).

3. Study area

The study area stretches 2.1 km along the Balia Nala and isdelineated by 29�220E�29�210E latitude and 79�280N�79�28023.200

longitude (towards southwest of Nainital, Fig. 1). The elevationranges from 1450 m to 1920 m. The right bank of this stream hastwo populated localities, i.e. (a) locality nearby Government InterCollege (GIC) and (b) Saraswati Vihar, both affected by landslides.

Annual rainfall recorded in this area is 2468 mm during 1995e2009, while it is 4190 mm during 2007e2013 (Fig. 2). The recordedrainfall shows 70% increase (Gupta et al., 2015). Each monsoontriggers slope instability, as more than 80% of slope along Balia Nalais composed of argillaceous rocks (slates) (Fig. 3a).

4. Methodology

Geological and engineering geological mapping has been car-ried out in the study area on 1:1000 scale (Fig. 3a and b). Geologicalcross-sections have been plotted in Fig. 4 along four section lines(1-10, 2-20, 3-30 and 4-40) marked in Fig. 3. This work has focused onthe rock mass characterization by rock mass rating (RMR), factor ofsafety (FOS) and kinematics analysis (stereographic projection) ofthe discontinuities. RMR has wide application in tunnels, slopes,foundations and mines (Bieniawski, 1989). In rock mass classifica-tion system, methodology is proposed to identify the quantitativecondition of road slope. It is broadly used in underground rocktunnels and road cut slopes. RMR also plays a key role in calculationof the slope mass rating. RMR includes the collection of field data,i.e. orientations of different discontinuities, uniaxial compressivestrength (UCS) (measured using Schmidt hammer according toISRM (1978, 1981, 2007), Bieniawski (1989), and Brencich et al.(2013)), spacing, slope direction and dip, conditions of

Fig. 2. Average monthly and annual rainfall during 2007e2013 (Source: Aryabhata Research Institute of Observational Science, Nainital).

Fig. 3. Geological map (a) and engineering geological map (b) of the study area with four cross-sections (1-10, 2-20 , 3-30 and 4-40).

M. Kumar et al. / Journal of Rock Mechanics and Geotechnical Engineering 9 (2017) 150e158152

Fig. 4. Geological cross-sections (1-10, 2-20 , 3-30 and 4-40) along the marked section lines in Fig. 3.

M. Kumar et al. / Journal of Rock Mechanics and Geotechnical Engineering 9 (2017) 150e158 153

discontinuities and groundwater conditions. Rock quality desig-nation (RQD) has been calculated according to Singh and Goel(1999) in the field. RQD is calculated using number of joints perunit volume Jv and equal to 115�3.3Jv. FOS has been calculated forevery outcrop according to Hoek and Bray (1981). FOS of a rockslope is the ratio of resisting forces to driving forces. If FOS is lessthan or equal to 1, the slopewill fail. If FOS is much larger than 1, theslope will be quite stable. However, if the FOS is slightly greaterthan 1, small disturbance may cause the slope to fail (Hoek andBray, 1981). To analyze various modes of rock slope failures(plane, wedge, and toppling failures), Markland’s test has beenperformed as described by Hoek and Bray (1981). Various modes offailures have occurred due to presence of unfavorable orienteddiscontinuities (Hoek, 2007). Kinematics refers to the motion ofbodies without reference to the forces that causes them to move(Goodman, 1989). It is one of the most useful techniques in therecent years to investigate possible failure modes of rock masseswhich contain discontinuities (Hussain et al., 2015).

FOS for wedge failure of slope can be calculated by

F ¼ sin b tan f1

sinðx=2Þtan 4(1)

where b is the angle between intersection line of discontinuity andthe bisector, f is the friction angle, x is the wedge angle, and 4 is theplunge of intersection line of two discontinuities.

FOS for planar failure of slope can be calculated by

F ¼ ½cAþ ðW cos j� U � V sin jÞtan f�=ðW sin jþ V cos jÞ(2)

where c is the cohesion, A is the area of the block,W is the weight ofsliding block, U is the uplift force due to water pressure on slidingsurface, V is the force due towater pressure in tensile crack, and j isthe dip of failure plane.

5. Results and discussion

During mapping, many structural features have been docu-mented. Predominant rock type presented in the area is slate ofManora member (greenish color), Kailakhan member (gray to darkgray color) and Hanumangarhi member (reddish color), as shownin Fig. 3a. This rock has weak to very weak strength. Other rocktypes in the area include dolomite of Pashandevi member which iscompact with high strength, and marl. Engineering geological map(Fig. 3b) shows that most of the area is covered by slope wash andriver born materials, and at few localities, slates, dolomite and Talformation rocks are exposed as pinching and swelling shape on thesurface. Most of the rock mass is highly deformed and dissected by3e4 sets of joints (Fig. 5a).

Nainital lake fault has been described to develop during 40e50 ka (Singhvi et al., 1994) with a movement rate of 6 cm per year

Fig. 5. (a) Four sets of joints in the carbonaceous slate. (b) A dextral strike slip fault along the streamwith 10 m offset. (c) Shear sense indicator showing dextral movement. (d) 0.5e0.7 m high waterfall along the stream. (e) GIC ground showing 0.5 m subsidence. (f) Toppling failure at the right bank of the stream. (g) Cracks in the houses at the right bank. (h)Tensile cracks presented nearby GIC ground.

M. Kumar et al. / Journal of Rock Mechanics and Geotechnical Engineering 9 (2017) 150e158154

(Kotliya et al., 2009). This study indicates that the Nainital lake faultis a result of neotectonic activities in this area. Most prominentevidence is the rapid down cutting along the course of Balia Nala. Atlocations 7 and 10 (Fig. 3a), two subsidiary faults of the Nainital lake

Table 1Final results of this study.

Location Rock type Slope angle (�) RQD (%) RMR

L-1 Slate 60 65.5 54L-2 Slate 60 65.5 52e57L-6 Slate 60 45.7 59e60L-7 (left bank slope) Slate 60 72.2 55e56L-7 (right bank slope) Slate 60 72.2 55e56L-8 Slate 60 78.8 52e57L-9 Slate 60 72.2 50e54L-10 Slate 60 58.9 59e60L-11 Slate 60 72.2 56e61L-12 Slate 60 72.2 52L-13 Slate 60 49 36e37L-14 Slate 60 52.3 40L-16, 17 Slate 60 49 50e52L-27 Slate 60 45 51e53L-29 Slate 60 42.4 46e47

fault are identified along which w10 m lithological offset (Fig. 5b)of dextral sense has occurred (Fig. 5c). Waterfalls of 0.5e0.7 m inheight (Fig. 5d) are also characteristic features presented alongthese faults.

RMR class Failure type Responsible discontinuity FOS

Fair Wedge failure J1, J2 �0.29Fair Wedge failure J1, J2 0.012Fair Wedge failure J0, J1 �0.16Fair Wedge failure J1, J3 0.02Fair Planar failure J2 0.06Fair Planar failure J1 0.4Fair StableFairFairFairPoor Planar failure J1 0.12Poor StableFairFairPoor Wedge failure J1, J2 0.03

Fig. 6. Stereographic projection for different locations (L-13, L-14, L-16, L-17, L-27 and L-29) with respect to presented discontinuities, slope and river trend.

M. Kumar et al. / Journal of Rock Mechanics and Geotechnical Engineering 9 (2017) 150e158 155

All the geological cross-sections drawn in this area show therelationship between slope and discontinuities. The L-section of thearea (Section 1-10 in Fig. 4) shows the gradient of the stream andthe orientation and distribution of the discontinuities in the area. Inthis area, slope is steep (>60�) (Table 1). It has resulted in 5e10 mthick slope washmaterial (Sections 2-20, 3-30 and 4-40). Section 2-20

in Fig. 4 shows that the dip of joint J1 (64�) is steeper than that ofthe slope. Argillaceous slate is exposed along the course of thestream and dolomite is exposed at the top of it. The slates along thecourse of stream are eroded rapidly. Due to these and neotectonicactivities in the area (Kotliya et al., 2009), the rocks at the right bankslide downward due towhich toppling failure and subsidence (0.5e1.5 m) of ground are common at GIC ground (Fig. 5e and f). It is

supported by various evidences such as tilting of poles and ground(Fig. 5e), cracks in the houses (Fig. 5g) and tensile cracks on thesurface with 0.1e0.3 m wide and 2e3.3 m deep (Fig. 5h). Our sur-face data show that the GIC ground has 5e10 m thick overburdenbelow which there are tensile cracks (Section 2-20 in Fig. 4). How-ever, ground penetrating radar (GPR) data (Gupta et al., 2015) alsoconfirmed our observation on GIC. At the left bank, Kelakhan slateunderlies the Manora slate. Joint J2 presented in these rocks dips58�e78� towards valley, which accelerates the slope failure (L-27 inFig. 6).

Section 3-30 shows that, at the right bank, there is no planar andwedge formations (L-16 and L-17 in Fig. 6). At this location, Sar-aswati Vihar colony is situated. At the left bank, location L-12

Fig. 7. Stereographic projection for different locations (L-1, L-2, L-6, L-7, L-8, L-10, L-11 and L-12) with respect to presented discontinuities, slope and river trend.

M. Kumar et al. / Journal of Rock Mechanics and Geotechnical Engineering 9 (2017) 150e158156

M. Kumar et al. / Journal of Rock Mechanics and Geotechnical Engineering 9 (2017) 150e158 157

(Fig. 7) is stable, but away from the river bed, discontinuities J1 andJ2 form the wedge which has plunge of 266�/49� towards valley (L-29 in Fig. 6). Section 4-40 shows that the thickness of slope washmaterial varies from 10m to 15m. At this section, discontinuities J0,J1 and J2 formwedge (L-7 in Fig. 7) and planar failures (L-7 and L-8in Fig. 7).

RMR was calculated on the basis of various parameters. RMRvalue ranges from 36 to 55 for the locations L-7, L-8 and L-13,whereas it ranges from 46 to 60 at locations L-1, L-2, L-6, L-7 and L-29. Based on the RMR classification, rocks of this area fall into poorto fair class (Table 1).

Stereographic projection analysis has been carried out to un-derstand the kinematics based on the Markland’s test and usinginternal friction angle and relative slope direction/dip of disconti-nuities. Kinematics analyses of slope at locations L-1, L-2, L-6, L-7, L-8, L-13 and L-29 (Figs. 3, 6 and 7) show that these locations arestructurally controlled and not safe for the construction. Totally 15locations are analyzed for wedge and planar failures (Table 1, Figs. 6and 7). Out of 15 investigated sites, 8 sites (53.33%) are unstable.Internal friction angle of 25� and cohesion of 40 kPa are taken forslate according to Hoek and Bray (1981). Both the dry and wetconditions have been considered during the FOS calculation. FOS issmaller than 1 for the landslide-prone sites (Table 1). Stereographicprojection of discontinuities confirms the wedge and planar fail-ures in the study area (Figs. 6 and 7). At the left bank, wedge failureshave 285�/50� (L-7 in Fig. 7) and 266�/49� (L-29 in Fig. 6) trend andplunge with FOS of 0.02 and 0.03, respectively. Rocks at these lo-cations belong to class III (Table 1). Similarly, at the right bank,wedge failure has 85�/30� (L-2 in Fig. 7), 34�/43� (L-6 in Fig. 7) and99�/39� (L-1 in Fig. 7) trend and plunge with FOS of 0.012, 0.16 and0.29, respectively. Planar failure also shows FOS<1 (Table 1) at lo-cations L-7, L-8 and L-13 (Figs. 6 and 7). On average, the study areareceives 2000 mm rainfall per year during 2007e2013 (Fig. 2).Rainfall is a paramount factor which influences chemical weath-ering. It controls the supply of moisture for chemical reactions andfor the removal of soluble constituents of the minerals. In the studyarea, the average overburden thickness is 10e15 m (Fig. 5). So rainpercolates into the surface and increases the pore water pressurewhich promotes the failure along the discontinuity planes (Hoekand Bray, 1981). The rain acts as lubricant along discontinuity sur-faces, and facilitates the process of rock sliding. In soil (slope wash)slopes after raining, the weight of soil mass increases and thusthreatens the stability of the soil mass. Besides, groundwater helpsto develop the pore water pressure within the soil mass whichfurther aggravates the instability. Therefore, the role of rainfallcannot be negligible in this area as it acts as lubricant for argilla-ceous sediments.

6. Conclusions

Along Balia Nala, the area has 3e4 joint sets. The RMR valueobtained from the study area ranges from 36 to 61, representingpoor to fair rock class. The highest RMR value is obtained fromlocation L-11 and the lowest from L-13. Kinematics analysis alsoreveals that most joint planes intersect with each other and formdifferent potential failures. Out of 15 locations, 5 locations aresusceptible to the wedge failure and 3 locations to plane failure.Finally, FOS calculated for the failure-prone locations ranges from0.06 to 0.4 for plane failure and �0.29 to 0.03 for wedge failure.

Conflict of interest

Wewish to confirm that there are no known conflicts of interestassociated with this publication and there has been no significant

financial support for this work that could have influenced itsoutcome.

Acknowledgments

The authors are thankful to Department of Geology, KumaunUniversity at Nainital for necessary laboratory facilities.

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Mohit Kumar obtained a M.Sc. degree in applied geologyfrom Kurukshetra University, Kurukshetra and is perusingPhD from Kumaun University, Nainital. He has two years’experience in engineering geology and three years in ac-ademic as assistant professor in Kurukshetra Universityand guest faculty in Kumaun University, Nainital. He hasbeen involved in geotechnical consulting for hydropowerproject and structural mapping project in Kedar valley. Heis interested in engineering geology and structuralgeology.

Shruti Rana obtained a M.Sc. degree in geology fromKumaun University, Nainital and is perusing PhD fromKumaun University, Nainital. She is interested ingeochemistry and tectonics.

Pitamber Dutt Pant obtained M.Sc. and PhD in geologyfrom Kumaun University, Nainital and is professor inDepartment of Geology, Kumaun University, Nainital. Hehas published more than 30 research papers in nationaland international journals and is interested in landslideinvestigation, mitigation and structural geology.

Ramesh Chandra Patel obtained M.Sc. and PhD in geologyfrom Indian Institute of Technology Roorkee and is pro-fessor in Department of Geophysics, Kurukshetra Univer-sity, Kurukshetra. He has published more than 30 researchpapers in national and international journals (tectonics,techno physics, GSI) and is interested in low temperaturethermochronology (fission track dating) and structuralgeology.