Slope stability analysis and mitigation measuresfor selected landslide sites along NH-205 in HimachalPradesh, India

AKHILESH KUMAR1,*, RAVI KUMAR SHARMA

1 and BIKRAM SINGH MEHTA2

1Department of Civil Engineering, National Institute of Technology, Hamirpur, Himachal Pradesh 177 001, India.2Ghumarwin, District Bilaspur, Himachal Pradesh 174 021, India.*Corresponding author. e-mail: akhileshsharma54@gmail.com

MS received 3 July 2019; revised 27 January 2020; accepted 5 March 2020

Landslide is the most frequent geo-environmental natural hazard which significantly aAect human life andenvironment. It is the natural hazard when occurs especially in hilly regions mainly along highwaycorridor, results in obstruction to trafBc Cow. The road network of a developing country plays a vital rolein its overall development. Therefore, it is important to ascertain landslide hazard assessment alongroads. In this regard, the study was carried out in middle Himalayan region of Himachal Pradesh, India.The paper describes the investigations carried out on two major landslides, namely Panjpiri and Nalayanoccurred at Kiratpur Sahib to Nauni road stretch along NH-205 in Himachal Pradesh, India. The slopefailure occurred at Panjpiri was identiBed as plane failure, whereas at Nalayan it was circular failure. Thefactor of safety was determined by using Bishop’s method of slices and circular failure charts. For factor ofsafety calculation of Panjpiri plane failure, six conditions were considered based on physical attributes.Subsequently, the evaluation of slope was designed by reinforcement with rock bolts. While for Nalayancircular failure, soil anchors were designed. Thus, evaluation of slope stability of these two failed slopeswere carried out for suggesting appropriate mitigation measures. The results of the study conclude that,with an accurate and well-planned mitigation measures, the severe landslide susceptible sites can bestabilized. Adopting eAective engineering mitigation strategies may help the decision makers to choosethe appropriate strategies to minimize the landslide hazard.

Keywords. Landslide hazard; Himalayan region; slope failure; factor of safety; Himachal Pradesh;mitigation measures.

1. Introduction

Natural hazards are the events that occur suddenlyand swiftly causing heavy damage to life andproperty. These hazards are mainly in the form ofCoods, cyclones, mass movements, volcanic erup-tions, earthquake, droughts, tsunamis, wild Breand locust infestation. Among all these hazards,landslides are the most frequent occurring geo-hazard, significantly aAect environment and

human life in one or the other way. Landslides areshort-lived phenomenon, which can cause extraor-dinary landscape changes and destruction of lifeand property. Landslides in the strict sense denotethe rapid movement of sliding earth material,separated from the underlying stationary part ofthe slope by a definite plane of separation due toslope failure, under the inCuence of gravity.The problem of deaths and injuries due to

landslides has been aggravated by increasing

J. Earth Syst. Sci. (2020) 129:135 � Indian Academy of Scienceshttps://doi.org/10.1007/s12040-020-01396-y (0123456789().,-volV)(0123456789().,-volV)

population in landslide-prone areas. Varnes (1981)estimated that during the period 1971–1974, nearly600 people per year were killed worldwide by slopefailures. In last three decades, many researchershave worked on landslide hazard and risk zoningusing a variety of approaches. Mazumdar (1980),Seshagiri et al. (1982), Valdiya (1987), Mehrotraand Bhandari (1988), Gupta and Joshi (1990),Anbalagan (1992), Mehrotra et al. (1992),Pachauri and Pant (1992), Thigale and Khandge(1996), Naithani et al. (2002), Alc�antara Ayala(2002), Choubey et al. (2005), Sujatha et al. (2012),Wang et al. (2015, 2016), Kumar et al.(2018, 2019), Rahman et al. (2019), and Gholamiet al. (2019) are among vast pool of researchers whohave made significant contributions in under-standing the problems and prospective of thelandslide hazards susceptibility and riskassessment.Slope instability processes are the product of

local geomorphic, hydrologic, and geologic condi-tions; the modiBcation of these conditions is bygeodynamic processes, vegetation, land use prac-tices, and human activities. Gedney and Weber(1978) concluded the basic approaches to potentialslope stabilization problems which was later mod-iBed by Holtz and Schuster (1996) techniques.Later, Holtz and Schuster modiBed the stability ofslope for the following reasons: (i) avoid the prob-lem, (ii) reduce the forces that cause the move-ment, and (iii) increase the forces resistingmovements by deBning procedure, application,limitations and remarks to each category. Toreduce the driving force, designing and construc-tion of surface and sub-surface drainages is themost vital aspect in slope stabilization. Gedney andWeber (1978) provided a brief review of the theoryfor designing trench drains. Forsyth and Bieber(1984) proposed geogrid for external stabilizationsystem which includes several bar and mesh rein-forcement system. Carrara (1989) recommendedgood surface drainage as a part of treatment of anylandslide or potential landslide. Schuster (1995)provided a number of additional illustrations oflandslide stability using drainage wells. Less com-mon technique such as electro-osmosis, vacuumdewatering and blasting of rock slopes forimproving drainage were suggested in their study.Sharma et al. (2019) studied slope stability analysisby Bishop analysis using MATLAB program basedupon particle swarm optimization (PSO) tech-nique. Singh and Thakur (2019) have done slopestability analysis of three vulnerable sites within

the lower Siwalik in north-western Himalaya. Thestudy applied rock mass rating (RMR) and slopemass rating (SMR) for stability classiBcation andevaluating the FOS. Kadakci Koca and Koca(2020) applied limit-equilibrium method (LEM) forcalculation of FOS for performing deterministicslope stability analysis.In slope instability investigation, it is desirable

to know the causes of instability. Slope stabilityanalysis used to evaluate and estimate slopemovement and propose safe and economic designsto limit slope’s equilibrium conditions. Stability ofslopes is usually analyzed by methods of equilib-rium. Slope stability in limit equilibrium tech-niques is analyzed by computing the FOS. Thevalue is determined for the surface that most likelyto fail by sliding. Iterative procedures are used eachinvolving the selection of a potential sliding mass,subdivision of this mass into a series of slices andconsideration of the equilibrium of each of theseslices by one of several possible computationalmethods. These methods have varying degree ofcomputational accuracy depending on the suit-ability of the underlying simplifying assumptionsfor the situation being analyzed. The slope stabilityanalysis is essential for demarcating the endan-gered locales; delineating potential failure mecha-nism; understanding the slope sensitivity to variedtriggering mechanism; designing of optimal slope inrespect to safety, reliability and economic andsuggesting viable mitigation measures. The selec-ted two slope failure in this study, expose variedslip planes, slope geometry and slope formingmaterials. These landslides with variable aerialextent were disposed across different geomorphicterrains and geological units. Due to wide varia-tions in inherent characteristics and recognition ofpotential mode of failure at different geologicalconditions, a careful consideration has to be givento different methods to choose a reliable and mostaccurate analysis technique for slope stability ofthese landslides.Methods for stabilization of rock slopes were

developed by Hoek and Bray (1981). Later on,Wyllie (1991), Norrish and Wyllie (1996), andWyllie and Mah (2004) further reBned and devel-oped eAective rock stabilization designs. Adoptionof heuristic, statistical and deterministic approa-ches were incorporated as reliable techniques forthe assessment, recognition and analysis of slopeinstability. It becomes necessary to mitigate thedisastrous impact with a detailed understanding ofthe physical process and sufBcient amount of

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historical information quantifying the hazard withboth temporal and spatial approaches. A lot ofresearchers have adopted various mitigationstrategies in recent times (Ðuri�c et al. 2017; Luoet al. 2017; Song et al. 2019; Huang et al. 2019;Chen et al. 2020).The Brst landslide event triggered on 2nd August

2007 at near village Panjpiri, took the life of oneperson and swept one vehicle to the bottom of theslope. The second landslide event also occurred on2nd August 2007 near Nalayan, which disrupted thetrafBc route continuously for Bve days. To minimizethe loss of property, safeguard the public interestsand keep the national highway operational, it oftenbecomes essential to develop slope stabilizationtechniques essential for both natural and excavatedslopes. For this persistence, study area has beencarefully chosen along road corridor of NH-205 inHimachal Pradesh, India. NH-205 being chiefhighway of Himachal Pradesh, accounts for mostsevere sections prone to landslides. The intensityand frequency of landslides on these stretches notonly aAect trafBc movement, but also disrupt theeconomic activities of the region, which leads tomonetary losses. Two major devastating slopefailures were selected which needed mitigationstrategies on very urgent basis. A minor fall or slipand substantial slope failures on transport routescan severely disrupt trafBc Cow resulting in bothdirect and indirect economic losses. The segmentsof the NH-205 between Garamaura and Nauni,often witnessed repeated failure of slopes duringmonsoon. The occurrences of maximum number ofpotentially unstable slopes in these segments usu-ally disrupt trafBc Cow for days together and usu-ally witness frequent numbers of minor to majorslides/slips every year. This route also acts as amain connecting link for far-Cung tribal regions ofHimachal Pradesh and Jammu and Kashmir withthe rest of the country. This route is used not onlyfor supply of essential commodities to militarybases/posts across Pakistan and China borders, butalso caters the need of the Kullu and Lahaul andSpiti districts of Himachal Pradesh for the supply ofessential commodities and shipment of medicinalplant, timber, vegetable and horticulture produces.This route also provides connectivity to the alreadyestablished and upcoming major hydro projects fortheir transmission and maintenance works. Thestudy of landslide along this section becomes morepertinent to analyse different inherent causes andevolve viable mitigation measures to arrest themenace of slope instability along this section.

Keeping national highway operational, mini-mizing the economic loss to the exchequer andsafeguarding the public interests, adoption of slopestabilization techniques rather becomes an essen-tial tool. A proper planning for demarcating land-slide prone areas/segments in the entire route,delineating the probable cause of slopes failure andadoption of speedy mitigation and mitigationmeasures along the route become paramount.No significant work on the impacts of landslides

upon transport routes of strategic importance hasever been attempted. Records of detailed study onlandslide investigation covering parts of north-western Himalayan terrain in general and Hima-chal Himalaya in particular are rare. The presentstudy is an attempt to evaluate the mitigationdesign parameters of two selected landslide siteswith distinct slope forming material and type oflandslide. The inferred database thus generatedhas been utilized to delineate the unstable andsusceptible segments along the road section andadoption of appropriate mitigation measures.

2. Study area

The selection criteria for the study areawas based onthe number of landslide incidences reported andfrequent close down of this strategic route duringmonsoon season. The selected route being the mainfeeder trunk to different parts of the HimachalPradesh and adjoining states for trans-shipment ofessential commodities also endures utmost touristCow the Himachal Pradesh entertains every season.Exposure of a wide range of slope forming material,and avariety of slope facets developed along the roadsection make this route ideal for evaluation forlandslide study and becomes more meaningful. TheNH-205 is one of the main strategic and importantroad corridors transecting the entire physiographiczones of Himachal Himalaya, i.e., Siwalik Foothills,Lesser Himalaya and Central Himalaya/GreatHimalaya right fromGaramaura inBilaspur passingthrough rolled and rugged terrain of Himachal Pra-desh. The NH-205, a lifeline to the tribal belt ofHimachal Pradesh serves as main feeder in con-necting different places. The study area locationmap is shown in Bgure 1. Besides, important touristplaces like Manali, Kullu, Rohtang Pass, Mandi,Manikaran, and Bilaspur, etc., are connectedthrough this route. This route also caters the need ofthe many functional and under construction hydroelectrical projects for the trans-shipment of raw

J. Earth Syst. Sci. (2020) 129:135 Page 3 of 14 135

materials, heavymachineries, allied equipments andconduit for laying of electricity transmission lines.The terrain encompassing the entire stretch of NH-205 exhibits undulatory and rugged topography.Though the lowest altitude of the highway is around501 m atGaramaura, the relief is around 600 mnearSiwalik terrain. The area exhibits moderate to highrelief. The Garamaura–Swarghat section of the NH-205 falls in the Siwalik Hills Tracts of Outer Hima-laya, where sub-tropical sub-humid climate pre-vails. Baring this section, the rest of the areaencompassing entire stretch of NH-205 falls in moisttemperate climate zone. The climatic conditions ofthe area under which the NH-205 falls, are mainlyinCuenced by altitudinal variation and disposition ofthe Dhauladhar and Pir Panjal ranges. The areawitnesses slightly prolonged winters and shortsummers. The maximum temperature of the areavaries from 40�C in Garamaura to 32.5�C at Manaliwith average maximum temperature of about17.3�C, whereas minimum temperatures of the areafall below freezing point duringwinters. The averagerainfall of the section is around 100 cm duringmonsoons and intensity of which decreases fromPunjab Plain in the southwest to Manali in thenortheast. Amongst the two selected sites, adjoiningthe study area receives themaximumannual rainfallwith average of about 150 cm followed by Bilaspur

sub-segmentwith average of 140 cm. Themaximumprecipitation in one month during monsoon exceeds650 mm with 40–120 mm rainfall during a day.Varied lithological assemblages of different geolog-ical formations and groups ranging in age fromTertiary to Proterozoic are exposed along andadjoining theNH-205 (Bgure 2).Most of the soil cumdebris exposed in the entire road section ofNH-205 ismade up of debris deposit and drifted soil developedmostly on moderate reposed slopes. These soils aremedium deep to medium soil 2–5 m thick mostlyderived from scree/talus and hill-wash material. Asper the geological map of India prepared by GSI andmacro level seismic zoning map of Bureau of IndianStandard, BIS 1993–2002 the entire stretch of NH-205 transecting the outer and lesserHimalaya zone isseismically active and falls in zone IV and Zone V.

3. Methodology

The main objective of the study was to keep thesusceptible zones of the NH-205 stable by provid-ing the mitigation design parameters and designingof slopes to provide safe access of vehicular move-ment throughout the year with acceptable mainte-nance and use of minimum economy. Accurateinterpretation of the surface feature of a landslide

Figure 1. Study area map.

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can be used to evaluate the mode of movement,judge the direction and rate of movement from thegeometry of the slip surface. To determine theFOS, slope stability analysis of selected landslideswith varied type, material and extent was studied.A detailed investigation of these two slope failureswas carried out in three stages involving Beldassessment, laboratory tests and slope stabilityanalysis. It was broadly comprising of variousphases which involves site selection, detailed geo-morphic survey on large scale with mapping of rockoutcrops, overburden material and existing cuts,recording of structural data and collection ofsamples.Firstly, the Beld assessment was carried out for

determining all inherent characteristics which alandslide possesses, viz., type of the failure and itscause, its aerial extent; type and nature of materialinvolved; surface of failed plane, impact to thesurface; type, inclination and orientation of theslope; terrain condition; characteristic type, incli-nation and orientation of the structural disconti-nuities; the physical properties/geotechnicalcharacteristics of the materials; sampling and itsimpact to the nature and human activity. Besidesthe selection of site, evaluation of regional geology,terrain and satellite imageries were considered asbasic inputs. Subsequently, physical parameters ofslope forming materials that is geotechnicalparameters, like water content, bulk density andshear parameters (c and u) of soil samples andcompressive strength of rocks were determined. To

determine the uniaxial compressive strength of theintact rock in the Beld, Schmidt hammer was used.For the collection of soil/earth sample from cir-

cular failure and representative rock sample fromrock outcrop, different procedures were adopted.Depending upon the availability and accessibility,4–6 soil samples each from different sections ofselected sites were collected keeping equal spacingand interval of the collected samples. Geotechnicaltests of the most of the soil and rock samples col-lected from the Beld sites were carried out in lab-oratory under controlled conditions. Direct sheartest was performed using shear test apparatus forthe determination of angle of internal friction (de-grees), a frictional resistance for translocation ofindividual soil particles at their contact points andcohesion and an adhesion between the surfaces ofthe soil particles. A Bxed number of samples weretested under increasing normal loads and therequired maximum shear force recorded andrespective cohesion (kN/m2). For the representa-tive rock samples, drilled core samples with diam-eter of 5 and 7.5 cm were put to shear box test fordetermination of friction angle of discontinuity.The two halves of the core sample were set in a pairof steel boxes (ISRM 1981) using plaster of Paris.Each sample was tested three to four times atprogressive higher normal loads to establish resid-ual shear stress for a normal load.Various types of laboratory tests were performed

on soil samples to measure a wide variety of soilproperties. Some soil properties were intrinsic to

Figure 2. Map of Panjpiri and Nalayan slope failure sites along NH-205, Himachal Pradesh depicting rock and clay mineralogysample locations (source: Geological map by Director General, Geological Survey of India (GSI), Kolkata, India).

J. Earth Syst. Sci. (2020) 129:135 Page 5 of 14 135

the composition of the soil and were not aAected bysample disturbance, while other properties dependon the structure of the soil as well as its composi-tion, and can only be eAectively tested on relativelyundisturbed samples. Following are few lab testswhich were conducted on samples for determiningthe input parameters for slope design: (i) In-situdensity, the bulk density of the soil; (ii) moisturecontent; (iii) grain size analysis; (iv) Atterberglimits, expansion index test; (v) direct shear test;and (vi) unconBned compression test. While, therock samples were collected mostly from ‘dis-turbed’ or ‘undisturbed’ parts of failed slope acrossthe main highway and were subjected to followinglaboratory test for determination of rock strength:(i) unconBned compression test and (ii) directshear test. Schmidt hammer test was also used inthe Beld for the estimation of compressive strengthof in-situ rocks at discontinuity surfaces.Out of the various in vogue methods for slope

stability, Bishop’s simpliBed methods of slices(Bishop 1955), Circular Failure Charts by Hoekand Bray (1981) and Software GEO5 were carriedout for stability analysis of circular failure. Thestability analysis of plane failure and wedge failurewere carried out using analytical formulae pro-posed by Hoek and Bray (1981). These formulaewere based on geometry of the slope and slidingplane, position of tension cracks and stereographicprojection of discontinuity planes deBning thewedges.On the basis of above input parameters, detailed

exercise on calculation of FOS of the failed slopeand FOS of slope on reinforcement with bolts andanchors was done. The results obtained from theslope stability analysis which was carried out usingthe methodology suggested by Wyllie and Mah(2004) for the plane and circular failure modes,adopted for suggesting mitigation strategies.Exercises for designing of rock bolts, pattern ofrock bolts and anchors was also designed andsuggested.

4. Results and discussion

Slopes are generally evaluated in terms of the FOSagainst sliding. The slope conditions in the entirestretch of NH-205 is highly variable with differencein topography and material properties. The shearstrength of the material changes with the change inmoisture content/conditions and physical–chemical

properties. The distribution of different types ofmaterial with inherent variability in physical prop-erties has made to focus more attention on the needfor accurate model for the mitigation of differenttype and dimension of the landslides exposed and/orlikely to take place in future. In the present study, itwas inferred that inherent physical properties of theslope forming material like shear strength capacity,slope geometry, and structural attributes whichcritically reduces the eAective shear strength are themain cause for instability in the parts of NH-205.Besides, modiBcation in slope geometry due tohuman interventions has also led to slope instabilityof the NH-205.Taking into geotechnical considerations of

selected landslides, mitigation design parameters ofslope failures involving rock slope at Panjpiri planefailure (Bgure 3) and Nalayan circular failure(Bgure 5) were carried out.

4.1 Panjpiri plane failure – Rock slope surface

The slope stability analysis of Panjpiri plane failure(Bgure 3) where 12 9 4.5 9 1.5 m block of ferrug-inous micaceous sandstone of middle Siwalik for-mation was failed, concluded FOS 1.22, 1.17 and1.03 for dry slope, wet slope (zw = 0.5 m) and wetslope (zw = 2.25 m) conditions, respectively. Basedon the determined FOS, the proposed mitigationmeasures were suggested: (i) rock bolting, (ii)reinforced concrete dowels, and (iii) removal ofoverhang rock by trim blasting.

4.1.1 Design parameters

The slope stability analysis was carried outassuming plane failure for the upper slope withinclination of plane of discontinuity at 40� and 50�,respectively. For design parameters, the conditionfor slope was assumed to be possessing 40� incli-nation of discontinuity plane. The differentparameters considered for the designing of rockbolts were enumerated in table 1.

4.1.1.1 Analysis: The slope stability analysis wascarried out using the approach given by Wyllie andMah (2014) for the plane failure mode. Usually thebolts are installed at an angle Catter than thenormal to the dip line to draw maximum beneBtfrom the bolt force. The slope of the bolts formaximum eDciency was designed equal to thedifference between the angle of friction and the dip

135 Page 6 of 14 J. Earth Syst. Sci. (2020) 129:135

angle (40�). However, the angle was kept at such anangle so that it can be practical to install the boltsand to keep the length of the bolts limited(Bgure 4).

4.1.1.2 Derivation for FOS: Calculation of FOSfor the following different conditions was carriedout on the determined values of different attributes(table 2), based on the physical parameters enu-merated in table 1. For evaluation of FOS of theslope, following conditions were considered: (i) dry– when the slope is completely dry, (ii) tensioncrack Blled with water – up to 50% of the critical

depth, (iii) tension crack completely Blled withwater (due to surface run-oA/freezing and thaw-ing), (iv) completely drained slope, and (v) com-pletely drained slope with cohesion reduced to zerodue to blasting/earthquake.

(1) When the backBll is dry:

FOS ¼ cAþW cosWP tanuÞ=W sinWPð Þ¼ 1:07 � 1:00;

the slope is nearly stable.(2) When the tension crack is half Blled with

water, i.e., zw = zc/2 = 1.36 m.

Figure 3. Proposed mitigation measures for Panjpiri plane failure (a) frontal view, (b and c) vertical section and broadview-showing different types of mitigation structures.

J. Earth Syst. Sci. (2020) 129:135 Page 7 of 14 135

V ¼ cwz2w

� �=2 ¼ 9:248 kN=m:

U ¼ cwzwð Þ=2 H þ b tanWS�zcð Þ cosecWP

¼ 101:12 kN=m:

FOS ¼ cAþ W cosWP�U�V sinWPð Þ tanu½ Þ =�

½W sinWP þV cosWP � ¼ 0:67\ 1:00:

Hence, the slope is unstable.(3) When the tension crack is completely Blled

with water, i.e., zw = zc = 2.72 m.

V ¼ cwz2w

� �=2 ¼ 36:99 kN=m:

U ¼ cwzwð Þ=2 H þ b tanWS�zcð ÞcosecWP

¼ 0:63 kN=m:

FOS ¼ cAþ W cosWP�U�V sinWPð Þ tanu½ Þ =�

½W sinWP þ V cosWP � ¼ 0:63\1:00:

Hence, the slope is unstable.(4) Slope is completely drained, i.e., U = V = 0.

FOS ¼ cAþ W cosWP�U�V sinWPð Þ tanu½ Þ =�

½W sinWP þ V cosWP � ¼ 0:68\1:00:

Hence, the slope is unstable.(5) When the slope is completely drained and the

cohesion is zero due to the vibrations caused bytrafBc/blasting:

FOS ¼ cAþW cosWP tanuð Þ=W sinWP

¼ 0:44\1:00:

Hence, slope is unstable and sensitive to cohesionon vibrations.

Figure 4. Schematic diagram of Panjpiri rock slope withreinforced rock bolts.

Table 1. Parameters for the analysis of rock slope, Panjpiri plane failure, NH-205.

Sl. no. Input data Calculated values

1 Type of slope failure Plane

2 Height of the slope, H 16 m

3 Angle of inclination of the slope, Wf N 215�/60�4 Angle of internal friction, u 28�5 Dip angle of joints, WP N 210�/50�6 Unit weight of rock, c 26.5 kN/m3

7 Unit weight of water, cw 10 kN/m3

8 Angle of inclination of the top slope, WS 40�9 Cohesion, c 20 kN/m2

10 UnconBned compressive strength (UCS) of rock 23 MPa

Table 2. Different parameters for the determination of FOS based on the physical attributes enumerated intable 1.

Sl. no. Input data Calculated value

1 Critical height of slope, zc H (1 � (tan WP cot Wf)1/2) = 2.72 m

2 Distance of critical tension crack, b H ((cot WP cot Wf)1/2 � cot Wf) = 1.9 m

3 Area of tension crack on upper slope, A (H + b tan WS � zc) cosec WP = 19.45 m2

4 Area of tension crack on main slope, Aa (H � zc) cosec WP = 17.33 m2

5 Weight, W c [(1 � tan WP cot Wf) (bH + (H2/2) cot Wf)

+(b2/2) (tan WS � tan WP)] = 811.27 kN/m

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(6) When the slope is completely drained and thecohesion is zero due to the vibrations caused byearthquake:

FOS ¼ cAþ W cosWP�kT sin WP þWkð Þ½ � tanuf g=

W sinWP þ kT cos WP þWkð Þf g;

where

kT ¼ kH 1þ kV=kHð Þ2n o1=2

¼ 0:055 for zone IVð Þ

and

Wk ¼ tan�1 kV=kHð Þ ¼ 26:56�;

FOS ¼ 0:43\ 1:00:

Hence, slope is unstable and sensitive to earth-quake tremors.

4.1.2 FOS of slope on reinforcement with bolts

Reinforcement of drained slope with zero cohesionwas done by installing tensioned rock boltsanchored into rock beneath the sliding plane. FOSfor the rock bolts installed at right angles to thesliding plane, i.e., WT = 90 � 50 = 40� keepingtotal load of 400 kN on anchors per linear meter ofslope was determined. Exercises were carried out toevaluate FOS of rock bolt installed at a Catterangle so that WT is reduced from 40� to 20� andthen even at Cattest angles. Following steps werecarried out to evaluate FOS of rock bolt installedat different angles.On Brst trial, laying out number of bolts per

vertical and horizontal rows with vertical spacingin between, the working load of 200 kN for eachbolt was suggested to achieve bolt load of 400 kN/m of the slope length.

(1) For drained slope having zero cohesion (c =U = V = 0) and a reinforcement force T of 400kN/m installed at dip angle Wt = 40�,

FOS ¼ W cosWP þ T sin Wt þWPð Þ½ � tanuÞ=

W sinWP�T cos Wt þWPð Þ½ � ¼ 0:78\1:00:

Hence, slope is unstable.(2) Bolts installed at a Catter angle, i.e.,Wt = 20�,

FOS ¼ W cosWP þ T sin Wt þWPð Þ½ � tanuÞ=

W sinWP�T cos Wt þWPð Þ½ � ¼ 0:98\1:00:

Hence, the slope is unstable.(3) Bolts installed at a Catter angle Wt = 3�,

FOS ¼ W cosWP þ T sin Wt þWPð Þ½ � tanuÞ=

W sinWP � T cos Wt þWPð Þ½ � ¼ 1:17[ 1:

Hence, FOS value greater than 1 makes slopeslightly stable.

After providing maximum Catter angle of 3� tothe installed rock bolts, the FOS at total workingload of 400 kN on anchors remained nearly equalto 1.00 making the slope nearly unstable. Thelower values of FOS may be due to the com-paratively steep inclination of both slope anddiscontinuity plane, where a total working load of400 kN on anchors proved inadequate to resistshear stress of the rock mass. To make reinforcedslope stable which can withstand the shear stressof bolted rock mass, the total bolt load onanchors has to be increased. Hence, an optimalbolt load of 600 kN/m was proposed forredesigning the rock bolts to achieve satisfactoryFOS.

(4) For drained slope having zero cohesion (c =U = V = 0) and a reinforcement force T of 600kN/m installed at dip angle Wt = 40�,

FOS ¼ W cosWP þ T sin Wt þWPð Þ½ � tanuÞ=

W sinWP�T cos Wt þWPð Þ½ � ¼ 0:96\1:00:

Thus, the slope is unstable.(5) Bolts installed at a Catter angle, i.e.,Wt = 20�,

FOS ¼ W cosWP þ T sin Wt þWPð Þ½ � tanuÞ=

W sinWP�T cos Wt þWPð Þ½ � ¼ 1:38[ 1:00:

The slope is considerably stable.(6) Bolts installed at more Cat angle Wt = 10�,

FOS ¼ W cosWP þ T sin Wt þWPð Þ½ � tanuÞ=

W sinWP�T cos Wt þWPð Þ½ � ¼ 1:72[ 1:00:

The slope is most stable.

FOS determined on assessing different slopeconditions that were considered for designing therock bolt anchors is given in table 3. Since the FOSis more than 1.3 when reinforcement force,T = 600 kN is at 10� and 20�, hence, the rock boltsmay be installed either at an angle, Wt = 10�(FOS = 1.73) or angle falling between 10� and 20�.

J. Earth Syst. Sci. (2020) 129:135 Page 9 of 14 135

4.1.3 Design of rock bolts

4.1.3.1 Pattern of rock bolts: The pattern of rockbolts should be such that the distribution of boltson the slope is as uniform as possible. If Bve boltsare installed in each row, the horizontal spacing ofvertical rows should be:

S ¼ TBN=T ¼ 2:5m:

Hence, the bolts were suggested to be installed ata spacing of 2 m in either direction.

4.1.3.2 Bond length: For cement and resin grou-ted anchored bolts,

lb ¼ T= pdhsað Þ;

where sa = UCS/30 = 0.766 MPa = 766 kN/m2,T = 600/2 = 300 kN (for FOS of 2) and diameterof hole, dh = 75 mm.The bond length, lb = 300/(p 9 0.075 9 766) =

1.7 & 2.00 m (say). However, proposing anchors oflength = 2 9 2 = 4.0 m.

4.1.3.3 Anchor design: Proposing 75 mm diame-ter cement grouted anchors having length of 4.0 mwith steel bars of diameter C 35 mm having ulti-mate strength of 600 kN; or 5 mm diameter wireto form 12 mm diameter strands with ultimatestrength of 260 kN to give a total ultimate strengthof 600 kN (i.e., 2 nos.). Resin anchors may not beused since they lose strength due to vibrations.Main reinforcement of 10 mm diameter may beused at 150 mm c/c in both the grout layers.

4.1.4 Additional mitigation measures

4.1.4.1 Removal of overhang rock: Carry outlocal trimming and removal of loose blocks as theoverhanging rocks are of less than 1 m3 volume.

4.1.4.2 Reinforced rock dowels: As the anglebetween the slope at the rock face and the potentialsliding surface is equal to 10, the dowel bars shallbe 32 mm in diameter, hot dip galvanised typesteel bars. The angle of dowels should be approxi-mately perpendicular to potential sliding surface ofthe rock block. The dowel length = 3 9 thicknessof potentially unstable rock block subject to aminimum length of 3 m and a maximum length of6 m should be considered.

4.1.4.3 Pneumatically applied shotcrete: In orderto avoid the water seepage and inBltration, the topslope should be lined with shotcrete having Bneaggregate in two layers of 100 mm each may beapplied with compressive strength at 3 days = 20MPa and 7 days = 30 MPa.

4.1.4.4 Rock trap/clean ditch: As the rock facegradient b is nearly vertical and height, H of rockslope is more than 10 m, the depth, D and width,W of the rock trap/clean ditch should be 1.2 and4 m, respectively.

4.2 Nalayan circular failure – Debris-soil slopesurface

Nalayan landslide located along Garamaura–Swarghat section of NH-205. It was identiBed ascircular failure, where debris material failed.2–10 m thick debris deposits comprising of clayeymatrix admixing boulder and pebbles of sandstoneconstitutes the slope forming material of the slidingmass. Mitigation measures suggested: (i) soilanchors, (ii) masonry wall (75 cm top and base1.85 m), (iii) removal of muck (2 m thick) andlevelling and stepping, (iv) grass seeding inbetween, and (v) diversion and interceptordrainage system (Bgure 5).

Table 3. FOS of the rock slope of Panjpiri plane failure for the consideration of rock bolts design.

FOS of slope with slope inclination of 608

Dry

Tension cracks Blled

with water

Fully drained

Fully drained cohesion

zero

Drained slope, zero cohesion. Reinforced with rock bolts

(WT) and total load on anchors (T)

T = 400 kN T = 600 kN

50% Full Blasting Earthquake 40� 20� 03� 40� 20� 10�

1.07 0.67 0.63 0.68 0.44 0.43 0.78 0.98 1.17 0.96 1.38 1.72

135 Page 10 of 14 J. Earth Syst. Sci. (2020) 129:135

Figure 5. Proposed mitigation measures for Nalayan Landslide (a) frontal view, (b and c) vertical section and broadview-showing different types of mitigation structures.

J. Earth Syst. Sci. (2020) 129:135 Page 11 of 14 135

4.2.1 Design of soil anchors

The different parameters considered for thedesigning soil anchors at Nalayan circular failurewere enumerated in table 4.

4.2.2 Design analysis

Based on the different parameters, a schematicdiagram of the soil slope of Nalayan slide (Bgure 6)was prepared for analyzing soil anchors design. Thefollowing steps were followed for design analysis:

Ka ¼ cosb cos b� cos2 b� cos2 u� �1=2h i�

cos bþ cos2 b� cos2 u� �1=2h i

¼ 0:289;

pa ¼ KacH ¼ 105:43 kN=m2;

PA ¼ 0:5KacH2 ¼ 1054: 3 kN=m;

Maximum load at the bottom anchor =1054.3 9 (18/20.0) 9 1 = 948.87 kN/m.For straight circular anchor, Qu = pdL0Sb,

where, using 100 mm diameter anchors and takingSb = 72 kN/m2;

948:87 ¼ p� 0:1� L0 � 200 or

L0 ¼ 948:87=p� 0:2� 72;

L0 ¼ 21:21 sayð Þ ¼ 21m;

where L0 is the Bxed anchor length.Hence, proposing 100 mm diameter anchors of

21.0 m Bxed length.

4.2.3 Design considerations of soil anchor

Reinforcement: Main reinforcement of 10 mmdiameter may be used at 150 mm c/c in two layers.Soil anchors: Use under-reamed multi-bell soilanchors of 150 mm shaft diameter (over freelength) and 300 mm bell diameter (over Bxedanchor length).

Tie backs: The tie backs should be HYSD 20 mmdiameter bars at 1.1 9 1.1 m c/c.Wire mesh: Wire mesh may be provided betweentwo 100 mm thick shotcrete layers. The weldedwire mesh fabricated from 3.5 mm diameter wire at100 mm centers with the steel pins complete withwashers and nuts should be grouted using150 9 150 9 12 mm plates. Steel pins should beanchored at 0.45 m centers to avoid loosening dueto load.Shotcrete: Pneumatically applied shotcrete withBne aggregate in two layers of 100 mm each may beapplied with the following characteristics: Com-pressive strength at 3 days = 20 MPa, and com-pressive strength at 7 days = 30 MPa.Dry mix: Cement content (Type I) = 400 kg/m3

(minimum) (18.3%), silica fume = 50 kg/m3

(2.3%), aggregate (2.5–10 mm size) = 500 kg/m3

(22.9%), sand = 1170 kg/m3 (53.7%), steelBbres = 60 kg/m3, high strength steel (HSS) Bbres,30–38 mm long, 0.5 mm diameter crimped) (2.8%),water\ 170 litres, total wet mass = 2350 kg.Drainage holes: The drainage holes inclined at anangle of 50 upwards with horizontal (intersectingthe discontinuities) having 100 mm diameter and5–7.5 m length may be provided at a spacing of2.2 m c/c. Holes should be lined with 100 mmdiameter perforated casing with perforations sizedto minimize the inBltration of Bnes that are washedfrom fracture in Bllings. The seepage water shouldbe collected in a drain (0.45 9 0.60 m deep) andshould be disposed of at safe distance from theslope.

5. Conclusion

In the present study, the mitigation measures weresuggested for the critical rock and soil slopes fortheir stabilization based on the geotechnicalinvestigations done for the micro level study of

Table 4. Different parameters for designing soil anchor forNalayan slide, NH-205.

Sl. no. Input data Calculated values

1 Unit weight of soil, c 18.3 kN/m3

2 Cohesion, c 12 kN/m2

3 Angle of friction, u 24�4 Bond strength of clay, Sb 72 kN/m2

5 Height of slope, H 20 m

6 Natural slope of soil, i 35�7 Surcharge angle, b 15�

Figure 6. Schematic diagram of soil slope reinforced withanchors for Nalayan circular failure.

135 Page 12 of 14 J. Earth Syst. Sci. (2020) 129:135

selected landslides. The selected landslide sitesalong NH-205 from Kiratpur Sahib to Nauni sec-tion where unconsolidated materials are in abun-dance, the reinforcement of slopes by externallystabilized techniques were suggested.The Panjpiri plane failure of 12 9 4.5 9 1.5 m

block of ferruginous sandstone took place on jointplane dipping N 210�/50� with 1.5 m deep tensioncrack dipping N 240�/76�. The slope analysis ofthis failed surface has given FOS = 1.22 on dryslope condition and 1.17 on wet slope condition.Where water level depth was zw = 0.5 m, the FOSvalue concluded was 1.03 on wet slope conditionassuming water depth zw = 2.25 m. Subsequently,six conditions were considered for evaluation ofFOS for Panjpiri plane failure involving rock slopesurface. Reinforcement of drained slope with zerocohesion value was proposed by installing ten-sioned rock bolts anchored into a sound rockbeneath the sliding plane. To make reinforced slopestable withstanding shear stress of bolted rockmass, a total bolt load of 600 kN/m was proposedon anchors with 4 m anchor length and installationof Bve bolts at a spacing of 2 m in either direction.Rock bolting, reinforced concrete dowels, trim-ming, cleaning of ditch and removal of overhangrock by trim blasting was suggested. Panjpiricritical rock site also was analyzed for differentconditions which includes trafBc/blasting andearthquake, according to which designing of rockbolts were suggested.The Nalayan landslide which was triggered on

August 2, 2007 was concavo-convex nearly pearshaped, southwesterly disposed, 30 m wide in thetoe region and 8–10 m in crown region. The generalslope angle of the slide area was between 30� and38�; however, the slope close to crown was found tobe nearly vertical. 2–10 m thick debris depositscomprising clayey matrix admixing boulder andpebbles of sandstone constitutes the slope formingmaterial of this sliding mass. Subsequently, theFOS of Nalayan landslide for drained slope derivedby Bishop’s simpliBed method of slices indicatesthat the slope was critically stable at FOS = 1.18.The average values of the FOS of entire slide areaobtained from circular failure charts, for dry con-dition, 25%, 50% and 100% ground water condi-tions were 1.36, 1.26 and 0.93, respectively,indicating moderately stable under dry conditionand unstable under fully drained condition. Thus,for Nalayan circular failure, 100 mm diameteranchor of 21.0 Bxed length, 10 mm diameter soilanchor at 150 mm c/c in two reinforcement,

150 mm shaft diameter under reamed soil anchorsalong with tie breaks, wire mesh and pneumaticallyapplied shotcrete with Bne aggregate in two layersof 100 mm were proposed. Two layer of masonrywall, removal of muck, grass seeding and inter-ceptor drainage system was also suggested. Theentire surface of each slope was suggested to becovered using a geosynthetic polymer wire meshwith the help of T-nails. In addition, soil anchors,shotcreting and provision of drainage holes weresuggested.In the present study, the selected landslide sites

were selected which comprises of rock and soilslopes, which were at severe falling probability.The mitigation strategies were needed for thesesites to minimize the impact for both Bnancial lossand damages. Thus, appropriate mitigation mea-sures were suggested, and few minor modiBcationswere also proposed. The design standards adoptedfor the landslide sites may help the decision makersto choose the most eAective engineering solutionsfor the planning mitigation strategies of landslidehazard.

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