© Author(s) 2025. CC Atribution 4.0 License Evaluation of cut slope stability in the Lesser Himalaya of Nepal Ocena stabilnosti vkopnih brežin v Nizki Himalaji v Nepalu Krishna Kumar SHRESTHA1, Kabi Raj PAUDYAL1, Dinesh PATHAK1, Alessandro FRANCI2 & Prem Bahadur THAPA3* 1Central Department of Geology, Tribhuvan University, Kirtipur, Kathmandu, Nepal 2International Center for Numerical Methods in Engineering (CIMNE), Universitat Politècnica de Catalunya (UPC), Carrer Gran Capitán, UPC Campus Nord, Barcelona, Spain 3Department of Geology, Tri-Chandra Multiple Campus, Tribhuvan University, *Corresponding author’s e-mail: prem.thapa@trc.tu.edu.np Prejeto / Received 9. 5. 2025; Sprejeto / Accepted 6. 6. 2025; Objavljeno na spletu / Published online 30. 7.2025 Key words: cut slope, slope stability, numerical modelling, evaluation and validation, Lesser Himalaya, Nepal Ključne besede: vkopna brežina, stabilnost pobočja, numerično modeliranje, vrednotenje in potrjevanje, Nizka Himalaja, Nepal Abstract A spatial inventory of cut slopes in the central and western Lesser Himalaya of Nepal was prepared and characterised to evaluate their stability. The stability of these cut slopes is governed by the geotechnical properties of rock/soil together with slope geometry, groundwater conditions and human interventions. Numerous cut slope failures were observed in areas where slope geometry is modified for engineering developments such as roads, dams, powerhouses, industrial development, etc. Two modelling sites were evaluated using the Limit Equilibrium Method (LEM), Finite Element Method (FEM), and Particle Finite Element Method (PFEM). Pre-failure analyses using LEM and FEM under dry and saturated conditions revealed that the stability of the Lesser Himalayan hillslopes with considerable soil thickness is predominantly controlled by the depth of groundwater level (GWL). Slopes remain stable with a factor of safety (FoS)>1.3 when the GWL lies below 7 m from the surface and gradually become unstable as it approaches the surface. This trend for both slopes confirms that elevated groundwater during the rainy season is the major cause of frequent cut slope failures in the Himalayan regions. The comparison of FoS from LEM and Strength Reduction Factor (SRF) from FEM showed a strong cross-correlation (90–99 %), revealing minimal variation which affirmed the validity of the adopted modelling techniques used in this study. Post-failure simulations of these sites were further analysed using an innovative approach, the robust PFEM modelling technique, to compute the dynamic failure mechanism. Sensitivity analysis of both modelled sites showed that friction angle and cohesion are the most significant parameters for slope stability evaluation. Moreover, forward and back analyses indicated that computed results are in good agreement, thus depicting reliability and performances along with the model validation. Izvleček Za oceno stabilnosti vkopnih brežin v osrednjem in zahodnem delu Nizke Himalaje v Nepalu je bil pripravljen in karakteriziran prostorski popis. Stabilnost teh brežin je odvisna od geotehničnih lastnosti kamnin in tal, geometrije pobočja, hidrogeoloških razmer ter človekovih posegov v prostor. Na območjih, kjer je bila zaradi inženirskih posegov, kot so gradnja cest, jezov in elektrarn, spremenjena geometrija pobočja, so bila opažena številna porušenja vkopnih brežin. Z uporabo metode mejnega ravnotežja (LEM), metodo končnih elementov (FEM) in metodo delnih končnih elementov (PFEM) sta bili izbrani dve lokaciji modeliranja. Analize, izvedene z metodama LEM in FEM v suhih in nasičenih pogojih so pokazale, da je stabilnost pobočij v Nizki Himalaji, prekritih z večjo debelino tal, pretežno odvisna od globine nivoja podzemne vode (GWL). Pobočja ostajajo stabilna s faktorjem varnosti (FoS) >1,3 kadar gladina podzemne vode (GWL) leži več kot 7 m pod površjem in postopoma postajajo vedno bolj nestabilna, ko se nivo vode približuje površju. Ta trend je opazen pri obeh izbranih pobočjih in potrjuje, da je povišana gladine podzemne vode v obdobju deževne dobe glavni vzrok pogostih porušitev pobočij v himalajski regiji. Primerjava faktorjev varnosti izračunanih z metodo LEM ter faktorja zmanjšane trdnosti (SRF) pridobljenega z metodo FEM je razkrila močno medsebojno korelacijo (90–99 %), kar kaže na minimalne razlike in potrjuje zanesljivost uporabljenih modelirnih tehnik v tej študiji. Simulacije po porušitvi na teh območjih so bile dodatno analizirane z uporabo inovativnega robustnega pristopa z PFEM modeliranjem za izračun dinamičnega mehanizma porušitve. Analiza občutljivosti obeh modeliranih območij je pokazala, da sta trenje in kohezija ključna parametra za oceno stabilnosti pobočij. Poleg tega so druge izvedene analize pokazale dobro ujemanje pridobljenih rezultatov, kar potrjuje zanesljivost modela, njegovo učinkovitost ter veljavnost. GEOLOGIJA 68/2, 123-145, Ljubljana 2025 https://doi.org/10.5474/geologija.2025.006 Article 124 Krishna Kumar SHRESTHA, Kabi Raj PAUDYAL, Dinesh PATHAK, Alessandro FRANCI & Prem Bahadur THAPA Introduction Mass movements such as landslides, debris f low and cut slope failures are significant hazards and risks in the Lesser Himalaya of Nepal, par- ticularly during the rainy season. These events are exacerbated by weak and fractured lithologies, intense seasonal rainfall, active tectonics and in- creasing human interventions. Average altitudes of this region vary from 300 to 3500 m (Groppo et al., 2023), and it is the most populated region in the Himalaya that comprises sedimentary and less metamorphosed rocks (Upreti, 1999). Im- pacts of active tectonics, extensive human inter- vention and rapidly growing infrastructure devel- opment in this region are extremely exacerbating the landslides and cut slope failures. Most of the infrastructures concentrated near the road, which declines as a function of distance from the road network, justifies the significance of the road in terms of socio-economic development (Rawat & Sharma, 1997). The occurrence of cut slope failures to large- scale landslides is frequent in the Lesser Himala- yan Terrain of Nepal (e.g., Hasegawa et al., 2009; Phuyal et al., 2022; Thapa et al., 2023; Phuyal et al., 2025). Major highways in Nepal often traverse through river valleys and steep mountainous re- gions where numerous cut slopes of varying geom- etries are highly susceptible to failure during rain- fall and earthquakes. About 46 % of the Prithvi Highway (NH04) runs through hillsides with mul- tiple cut slopes. The Krishnabhir landslide (83 km west of Kathmandu) in 2000 blocked the Prithvi Highway for over two weeks, causing a shortage of daily commodities (Maskey, 2016) in the capi- tal city. Similarly, the Jogimara landslide (90 km west of Kathmandu) also obstructed the highway for 10 days (Upreti & Dhital, 1996). These loca- tions were recurrently affected by landslides for almost a decade, and the annual blockages during the monsoon season caused repeated disruptions to public and local transportation networks. The Jure landslide in 2014 along the Araniko Highway killed 156 people and damaged 2 km of road (Pan- thi, 2021). Over 4,000 landslides and cut slope failures occurred between 1971 and 2020, causing 5,000+ deaths, averaging 111 annually (Adhika- ri & Gautam, 2022). In 2023, 45 deaths were re- corded, while in 2024, 343 deaths and 48 missing cases occurred by the end of the monsoon. Unusu- al intense rainfall from 26 to 28 September 2024 triggered more than 500 landslides and cut slope failures along the Prithvi Highway alone, caus- ing severe disruptions and 35 fatalities in a single incident. These data indicate the vulnerability of highways to slope failures in mountainous regions underscoring the urgent need for comprehensive slope stability assessment to reduce the socio-eco- nomic impacts. Geological structures, active seismic zones, steep topography, seasonal rainfall (hydro-mete- orological), and increasing anthropogenic activ- ities are the major causes of mass movement in weak areas like in the Lesser Himalaya (Varnes, 1958; Gerrard, 1994; Upreti, 1999; Shrestha et al., 2004; Dahal et al., 2006; Dahal & Hasegawa, 2008; Singh, 2009; Haigh & Rawat, 2011; Devkota et al., 2013; Regmi et al., 2013; Rahman et al., 2014; Da- hal, 2014; Pathak, 2016; Marc et al., 2019; Dikshit et al., 2020; Shrestha et al., 2023). These appear in the form of earth f low, debris, bulging, rock fall, avalanches and so on (Varnes, 1978; Cruden & Varnes, 1996; Hungr et al., 2014). Infrastructure development in this region requires natural slopes subject to cutting to create space for roads, hydro- power facilities, industries, railways, airports and canals, resulting in cut slopes of varying scales (DoR, 2007; Sutejo & Gofar, 2015). Geo-environ- mental factors like slope geometry, lithology, soil depth, weak band, groundwater condition, drain- age density and proximity to faults are the key fac- tors to control the stability of excavated cut slopes (Wyllie & Mah, 2004; Singh et al., 2020). Defective engineering techniques in changing the slope geometry, torrential rainfall (Dahal et al., 2006), long exposure to the atmosphere, the presence of weak bands, rapid weathering, shal- low groundwater, lack of surface drainage and dynamic loading frequently cause roadside fail- ures. Various incidents of landslides from small to large scales along the major highways of Nepal were reported (Schuster & Hubl, 1995; Upreti & Dhital, 1996; Martin, 2001; Bhattarai et al., 2004; Hearn, 2011; Dahal, 2014; Thapa, 2015; Regmi et al., 2016; Hearn & Shakya, 2017; Vuillez et al., 2018; Pant and Acharya, 2021; Pradhan et al., 2022; Robson et al., 2022; Pudasaini et al., 2024; Shrestha et al., 2023; Pokhrel et al., 2024; Robson et al., 2024; Sapkota & Timilsina, 2024; Phuyal et al., 2025, etc.). Within the different road sections in the Lesser Himalaya terrain of Nepal, various methods of analytical, conventional and numerical modelling techniques have been used to analyse the cut slope stability on soil and rock by different researchers (Ray & Smedt, 2009; Pathak, 2014; Dhakal & Acharya, 2019; Shrestha et al., 2023; Acharya & Dhital, 2023; Poudyal et al., 2024). Cut slope stability evaluation utilises various conventional and numerical modelling approach- es through the Limit Equilibrium Method (LEM) 125Evaluation of cut slope stability in the Lesser Himalaya of Nepal and the Finite Element Method (FEM). LEM often uses the “method of slices” to calculate the Fac- tor of Safety (FoS) based on driving and resist- ing forces (Morgenstern & Sangrey, 1978; Nian et al., 2012; Burman et al., 2015; Deng et al., 2017). FEM, on the other hand, offers a more complex ap- proach which incorporates stress-strain behaviour and material properties to analyse slope stabili- ty (Griffiths & Lane, 1999; Burman et al., 2015). The majority of cut slope stability assessments in the Nepal Himalaya have relied on field investi- gations, rock mass classification and calculation of FoS by conventional techniques. However, few studies have attempted to evaluate cut slope sta- bility in Nepal using numerical modelling tech- niques for comprehensive analyses (e.g., Kharel & Acharya, 2017; Khatri & Acharya, 2019; Shrestha et al., 2023). Thus, the present study has integrat- ed computational techniques of numerical model- ling in evaluating the cut slope stability within the Himalayan terrain. Based on the spatial inventory of cut slopes in the Lesser Himalaya of central and western Nepal, two cut slopes have been chosen for detailed study, as they represent typical slides in their respective regions and have seriously im- pacted the socio-economic conditions of the com- munity that depends entirely on the roads pass- ing through them. The modelling approaches of LEM, FEM and the particle finite element method (PFEM) have been implemented to compute the pre-failure and post-failure mechanisms of the respective slopes. The results obtained through these techniques were validated based on the field evidence and simulated results of the computed models. Study area The study area is situated in the western and central Nepal Himalaya, bounded by lati- tudes 27°48'52" N to 27°49'12" N and longitudes 83°17'33" E to 86°13'11" E (Fig. 1). The areal cov- erage of the study area is about 17,500 km2 and lies in the adjoining regions of the major cities Pokhara and Kathmandu, with elevations ranging from 230 m to 3,780 m. However, the actual inves- tigation is focused primarily along the road corri- dors, as the majority of cut slopes are located ad- jacent to road alignments. The total length of road sections assessed in this study is approximately 2000 km, encompassing both primary and sec- ondary highway networks. The geomorphology of this region is largely shaped by metamorphic and meta-sedimentary rocks of the Lesser Himalaya. The area has experienced significant uplift and erosion, which has shaped a complex geo-environ- ment, forming various geomorphic landforms. The major physiographic divisions, namely the Ma- habharat Range and Midland Zone, are the major landform units in this region. The Mahabharat Range is a distinct high mountain belt north of the Indo-Gangetic Plain, while the Midland Zone fea- tures a relatively subdued landscape typically cov- ered with residual, colluvial and alluvial deposits. The characteristics of landform conditions com- bined with extensive anthropogenic activities pre- dispose the region to frequent landslides and slope failures during the summer monsoon season. Geological setting and spatial distribution Geologically, the study area lies within the Lesser Himalayan Zone of Nepal and is bounded by two major thrust sheets of the Himalaya: the Main Boundary Thrust (MBT) to the south and the Main Central Thrust (MCT) to the north (Fig. 1). This region consists of two distinct metamor- phic zones that include the low-grade Lesser Him- alayan metasediments and the high-grade Lesser Himalayan Crystallines (Gansser, 1974). This zone features abundant faulting and folding as its pri- mary geological structures (Upreti, 1999; Dhital, 2015). The main rock types in this area comprise slate, phyllite, schist, quartzite, dolomite and lime- stone, with some intrusions of granites and me- ta-basic rocks. The differential strengths of rock strata with highly folded and faulted geo-environ- ments make the region particularly susceptible to landslides and cut slope failures. The prediction of slope failures and landslides together with as- sessment of mitigative measures becomes essen- tial because of the complex slope failure process and limited understanding of underlying mecha- nisms which are typical problems in mountainous regions, especially along the hill-cut slope of the Lesser Himalaya (Singh et al., 2008; Hasegawa et al., 2009). A cut slope inventory database has been devel- oped from various road sections of major high- ways within the study area of central and western Nepal. Two specific sites were selected for further detailed investigations, as they are the recurring slope failures annually and have considerable so- cio-economic impacts: the Kokhe Slide in Gorkha District (Site-1: 28°01'35"N, 84°40'13"E) along the Gorkha-Arughat rural road and the Udipur Slide in Lamjung District (Site-2: 28°10'51"N, 84°25'43"E) along the Dumre-Besishahar-Chame Highway (NH25) of Nepal (Fig. 1). Both slides are recurrently triggered during the monsoon season, inf luenced by shallow groundwater tables, weak and weathered lithology typical of slopes in the 126 Krishna Kumar SHRESTHA, Kabi Raj PAUDYAL, Dinesh PATHAK, Alessandro FRANCI & Prem Bahadur THAPA Lesser Himalaya, inadequate drainage, and vari- ous anthropogenic disturbances. The slide at Site- 1 initiated before 2009 and is still posing threats to public transport during the monsoon of every year as progressive sliding occurs annually. The major failure at Site-2 occurred in July 2022 and swept away the 90 m road section downhill and is reactivating during the monsoon of every year. Detailed investigations are based on typical slope geometries, rapidly rising groundwater tables and recurrent failures with a documented history that have a direct impact on public transportation. Findings from these slopes can be applied to many other slides with similar features in terms of ge- ometry, lithology, groundwater conditions, precip- itation patterns and socio-economic impacts. Cut slopes and site characteristics The characteristics of cut slopes in the Lesser Himalaya are inf luenced by slope geometry, lithol- ogy, weathering and hydrogeological conditions (Upreti & Dhital, 1996; Wyllie & Mah, 2004; Da- hal et al., 2008; Phuyal & Thapa, 2023; Shrestha et al., 2023). Cut slope failures in the study area are often observed on higher and steeper slope gradi- ents experiencing more gravitational forces that have increased the shear stress exceeding the soil strength (Fig. 2a). Improper and non-engineered methods of excavation during construction with- out due consideration of local geology are signifi- cant contributors to slope failures. Many steep and high cut slopes formed in heav- ily weathered rocks are in a marginal stable state even under dry conditions. During rainfall, these materials become saturated and fail due to exces- sive pore pressure development (Fig. 2b). Failures often occur in areas with highly jointed, fractured, folded or faulted rock masses within the Lesser Himalaya (Fig. 2c, d). Every year, high and prolonged precipitation during the monsoon saturates the soil and increas- es pore water pressure therein. This reduces the effective stress and shear strength of the soil and leads to instability (Terzaghi, 1943; Craig, 2004; Duncan & Wright, 2005). Tensional cracks devel- oped at the crown of the slope create a path for water to percolate easily into the slope and ag- gravate the failure mechanism (Fig. 2e). Residual soil found along the road section with unprotected higher slope height and steeper gradient is suscep- tible to failure during the wet season (Fig. 2f ). Fig. 1. Geological setting (modified after Dhital, 2015) and spatial distribution of cut slopes in the study area. 127Evaluation of cut slope stability in the Lesser Himalaya of Nepal The cut slopes failure at Site-1 consists of the residual soil formed from the phyllite rock of the Kuncha Formation. The lithology in this area is psammitic phyllite, which is characterised by al- ternating layers of crenulated micaceous material comprising microfolds and a quartz-rich layer em- bedded within the fine-grained matrix of well-fo- liated mica minerals. This type of phyllite consists of quartz, K-feldspar, and muscovite as prima- ry minerals, whereas tourmaline, biotite, opaque minerals, oxides and clay minerals as accessory minerals (Silwal et al., 2024). Sericitisation and Fig. 2. Cut slope characteristics in the Lesser Himalayan Zone of western and central Nepal: (a) weak rock exposure and steep cut slope failed during heavy rainfall along the Prithvi Highway, Tanahun, (b) completely weathered high slope along the Kanti Lokpath, Lalitpur, (c) rock blocks over loose materials failed along the Pushpalal Highway, Kaski, (d) folded rock showing numerous discontinuities prone to failure at Dolakha, (e) a translational slide along the Kathmandu-Melamchi Highway, Sindhupalchowk, and (f) a failed residual soil slope due to slope modification for highway expansion along the Prithvi Highway, Dhading. (a) (b) (c) (d) (e) (f) 128 Krishna Kumar SHRESTHA, Kabi Raj PAUDYAL, Dinesh PATHAK, Alessandro FRANCI & Prem Bahadur THAPA alteration effects are very common in this type of rock and are well observed in the slope mate- rials (Fig. 3a). Mineral composition and textural features (e.g., foliation, cracks, alterations) aid in classifying rock strength and weathering grades, which in turn help estimate geotechnical parame- ters (e.g., unit weight, Young’s modulus, Poisson’s ratio). Furthermore, the presence of micaceous minerals (e.g., sericite, clay) indicates weak rock mass that are associated with observed failure mechanisms. The cut slope failure at Site-2 lies under the Fagfog Quartzite of the Lower Nawakot Group, which is highly fractured and thinly foliated with poor rock mass quality (Panthi, 2006). The section above the road is characterized by highly fractured and weathered quartzite, and the lower section contains thick colluvium. A minor fault is expected in this area, as seen by the high fracture frequency observed in the quartzite rock (Fig. 3b). Data and methodology Representative soil and rock samples from the slope failure scarp and adjacent exposures of the selected sites 1 and 2 were collected for laboratory testing to determine geotechnical properties. Unit weight (g), cohesion (c), friction angle (f), Young’s modulus (E), Poisson’s ratio (n) and density (d) are the required input parameters for slope stability evaluation (Table 1). Fig. 3. Characteristic features of the modelling sites: (a) Site-1, the Kokhe Slide, inset shows the road conditions within the slide-affected section, (b) Site-2, the Udipur Slide, inset shows the slope section above the road. Table 1.Geotechnical parameters of Site-1 and Site-2. Parameters Site-1 Site-2 Soil Rock mass Soil Rock mass DC* SC** DC SC DC SC DC SC Unit weight (γ), kN/m3 17.2 18.4 26 22.81 17.0 19.6 26 27.5 Cohesion (c), kPa 16 4 42 35.8 40 20 200 185 Friction Angle (φ), ° 22 14 38 32 28 14 35 31 Young’s Modulus (E), kPa 40,000 40,000 52,000×103 52,000×103 400,000 400,000 36,000×103 36,000×103 Poisson’s Ratio (ν) 0.4 0.4 0.23 0.23 0.30 0.30 0.25 0.25 Density (d) (kg/m3) 1753 2331 - - 1733 2180 - - Note: *DC = Dry condition, **SC = Saturated condition 129Evaluation of cut slope stability in the Lesser Himalaya of Nepal Field investigation of each specific site was carried out in detail using a Total Station (TS) survey for generating the hill slope profiles and a geophysical survey using Electrical Resistivity Tomography (ERT) for delineating sub-surface ge- ological layers and identifying groundwater posi- tions, which are important controlling factors in model development and slope stability analysis (Loke, 2004; Loke et al., 2013; Cardarelli & Fis- changer, 2006; Panda et al., 2023). The interpreta- tion of ERT correlates the variations in resistivity with different subsurface materials and condi- tions, such as lithology, moisture content and the groundwater level (Singh et al., 2014). Interpre- tative models derived from the processed tomo- grams of the slopes at sites 1 and 2 were integrat- ed into slope-section profiles to evaluate cut slope stability (Fig. 4). Modelling methods Numerous advanced numerical techniques, encompassing continuum, discontinuum, and hybrid methods, are available for soil and rock slope stability analyses (Griffiths & Lane, 1999; Cheng et al., 2007; Zheng et al., 2014; Sharma et al., 2017). Numerical modelling of two cut slope failures (Kokhe and Udipur) was evaluated using Slide v.6.0 and Phase2 v.8.0, followed by a com- parative analysis of Factor of Safety (FoS) under varying input conditions. Moreover, both sites were simulated utilising the multi-physics simula- tion framework “GiD” interface equipped with the Kratos platform. Input parameters for these simu- lations were also determined in the laboratory as per ASTM standards. The numerical modelling processes involved a series of analytical computation workf lows: defin- ing problems based on site conditions, selecting the method, creating numerical models, comput- ing outcomes and interpreting & validating results (Fig. 5). LEM and FEM modelling have evalu- ated the pre-failure state of cut slopes, and their post-failure mechanism has been analysed by Par- ticle Finite Element Method (PFEM). Model setup and computation Numerical modelling of cut slopes involves slope behaviour simulation under various condi- tions to assess stability and failure modes (Singh et al., 2008). The model setup was performed by defining the geometry of the slope, assigning mate- rial properties, and applying boundary conditions (Fig. 6). The model geometry has encompassed the Fig. 4. Geophysical survey for subsurface investigation by electrical resistivity tomography: (a) Site-1 and (b) Site-2. Fig. 5. Methodological framework for cut slope stability evaluation. 130 Krishna Kumar SHRESTHA, Kabi Raj PAUDYAL, Dinesh PATHAK, Alessandro FRANCI & Prem Bahadur THAPA actual slope, including its height and angle. Tri- angulated meshes were generated throughout the slope model with finer meshes around high-stress gradients and failure slip surfaces. The GLE/Morgenstern-Price method (Slide v.6.0) was used in the LEM model to determine the safety factors in both dry and saturated conditions for evaluating the slopes in two different geo-en- vironments. In addition, the same slope was com- puted by the FEM technique using Phase2 v.8.0. The FEM model was discretised using six-noded triangular meshes of 3,000 elements under gravi- tational loading for dry and saturated conditions. Boundary conditions significantly inf luence slope stability analyses in numerical modelling (Chugh, 2003). Therefore, appropriate selection of bound- ary conditions is essential for reliable numeri- cal modelling, which has been considered in this study. After model setup with defined boundary con- ditions, assigned material properties and hydrau- lic parameters, simulations were conducted for varying groundwater levels. The variation of the FoS was evaluated in terms of f luctuating ground- water levels to identify the critical failure surface due to pore water pressure during rainy seasons. A plane strain analysis was further applied by se- lecting metric units and using the Gaussian elim- ination solver for accuracy purposes. The stress analysis was performed with a maximum of 500 iterations and a tolerance level of 0.001 to ensure convergence. The gravity loading was applied to the created model, and meshes were generated using a six-noded graded triangle as recommend- ed by Komadja et al. (2020). Before running the simulation, the base and the right boundary of the model were restrained in both horizontal (X) and vertical (Y) directions to prevent movement. The model slope face was kept unrestrained to allow free deformation during numerical modelling. The Limit Equilibrium Method (LEM) is a method of slices widely used for assessing slope stability because of its simplicity and user-friend- liness. It derives FoS with respect to force and moment equilibrium. LEM analyses a slope by cutting it into fine slices and applying appropriate equilibrium equations (equilibrium of forces and/ or moments) to calculate the FoS (Matthews et al., 2014). The common methods of LEM analysis are Ordinary/Fellenius (Fellenius, 1936), Bishop sim- plified (Bishop, 1955), Lowe and Karafiath (Lowe & Karafiath, 1965), GLE/Morgenstern-Price (Mor- genstern & Price, 1965), Spencer (Spencer, 1967), Janbu Simplified and Janbu Corrected (Janbu, 1954), and Corps of Engineer #1 and #2 (USACE, 2003). Among these, the Morgenstern-Price (M- P) method (1965) and Sarma’s method (1973) are advanced ones that account for both force and mo- ment equilibrium and improve the accuracy of FoS on a factual basis. The M-P method involves com- plex equations, and different forms can be found depending on the assumptions made. The FoS in the Morgenstern-Price method is expressed as (Eq. 1) (Fan et al., 2021): (1) where, c’ = effective cohesion, f= effective an- gle of internal friction, σ = total normal stress on the base of the slice, μ = pore water pressure on the base of the slice, α= inclination of the base of the slice, and τ= shear stress on the base of the slice The Finite Element Method (FEM) is an ad- vanced computational technique which enhances traditional LEM by providing a higher degree of realism and deformation visualisations of mate- rials (Matthews et al., 2014). FEM provides in- sights into stress, strain and displacement, which makes it a fundamental tool for analysing the deformation behaviour of slope materials (Cheng Fig. 6. Numerical model setup with mesh and boundary conditions: (a) Site-1 and (b) Site-2. FoS = ∫(c' + (σ – µ) tanφ') sec αdx ∫τdx 131Evaluation of cut slope stability in the Lesser Himalaya of Nepal et al., 2007; Burman et al., 2015). FEM offers several advantages over LEM in slope stability analysis (Griff iths & Lane, 1999) as it does not require prior assumptions regarding the shape and location of the failure surface (Dawson et al., 1999). In this method, failure naturally emerges along a surface where material strength is defi- cient of resisting applied shear stresses. In FEM, the Shear Strength Reduction (SSR) technique has been applied to the Mohr-Coulomb criterion using Phase2 to determine the Strength Reduction Factor (SRF). The Particle Finite Element Method (PFEM) is a robust numerical technique which enables the solution of multi-physics problems involv- ing extensive domain deformations (Oñate et al., 2004; Oñate et al., 2011). The PFEM model en- ables domain particles to move through space us- ing Lagrangian dynamics while nodal variables determine their physical properties (e.g., density and viscosity) throughout the entire simulation period. The algorithm generates dynamic meshes through Delaunay triangulation and alpha shape scheme processes to prevent mesh distortion (Oñate et al., 2011). The Navier–Stokes equations describe the f lu- id body motion and are governed by (Eq. 2 & 3). These equations relate the velocity u = u(x, t) and the Cauchy stress tensor σ = σ(x, t) through princi- ples of momentum balance and mass conservation (Cremonesi et al., 2020) by: (2) (3) where, ρ(x) represents the f luid density, b(x, t) denotes external body forces per unit mass, and D/Dt is the material time derivative. A Lagrang- ian technique reduces the total time derivatives to a local time derivative and vanishes the convec- tive factor in the governing equations. This char- acteristic is inherent in Lagrangian methods like the Particle Finite Element Method (Idelsohn et al., 2008; Idelsohn & Oñate, 2010; Franci et al., 2020). The PFEM is capable of solving complex f lu- id-solid interaction problems and deformation mechanics. It is particularly useful in post-failure landslide analysis as it efficiently deals with ex- tensive deformations, fragmentation and f luid-sol- id interactions. Both modelling sites (Site-1 and Site-2) were simulated in PFEM in terms of two- phase system based on distinct layers of materials identified from ERT and borehole data. The upper soil layer shows plastic behaviour under saturat- ed conditions, whereas the underlying solid rock mass serves as a rigid and non-deforming base in the model. For Site-1, the saturated soil mass was modelled with a density (d) of 2331 kg/m³, an in- ternal friction angle (f) of 14°, and cohesion (c) of 4 kPa, which were used as input parameters for the numerical computations. Results and discussion Limit Equilibrium Method (LEM) The rigorous M-P method is an advanced and highly reliable method that satisfies both force and moment equilibrium (Zheng, 2012; Fan et al., 2021; Shrestha et al., 2023). This method has been adopted in the present modelling process. The safety factors of Site-1 were calculated under different conditions by considering f luctuations in the groundwater level of this area using this meth- od, which is appropriate for analysing circular slip surfaces (Fig. 7). The analysis yielded a FoS of 1.42 under normal dry conditions (Fig. 7a), which is above the recommended minimum FoS of 1.25 for cut slopes, indicating a stable slope. Three dif- ferent groundwater scenarios were analysed with varying depths of 7.0 m, 1.5 m and 0.5 m below the existing groundwater level (GWL). The calcu- lated FoS values for these three different GWLs are 1.25, 0.97 and 0.87, respectively (Fig. 7b, c, d). Higher FoS values were observed with deep- er GWL, and FoS decreased under elevated GWL, which corroborates the frequent cut slope failures observed during the rainy seasons. According to the precipitation records from the Department of Hydrology and Meteorology (DHM, 2024), Nepal, the average annual precipitation at Site-1 is 254.88 mm. A transient analysis of this site for 24 hours with 250 mm of rainfall calculat- ed a FoS of 1.18 after the implementation of gabion walls along with a three-stage drainpipe installa- tion, representing the critical situation of slope during heavy rainfall (Fig. 7e). Considering the same rainfall condition with added static loading of 20 kN/m2 and seismic loading of 0.17 g (hori- zontal) and 0.08 g (vertical), the calculated FoS is 1.10, which is still above the unity, indicating the marginal stability of slope (Fig. 7f ) in the extreme conditions of loading too. There is a 42.98 % varia- tion in FoS for Site-1 while transitioning from dry to saturated conditions. The LEM analysis by M-P method for Site-2 has shown that the FoS in dry condition is 1.18 (1) 𝜌𝜌𝜌𝜌 𝐷𝐷𝐷𝐷𝐷 𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 = div 𝜎𝜎𝜎𝜎 + 𝑏𝑏𝑏𝑏 in Ω𝐷𝐷𝐷𝐷 × (0,𝑇𝑇𝑇𝑇) (2) (3) (1) (2) 𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 + 𝜌𝜌𝜌𝜌 div u = 0 in Ω𝐷𝐷𝐷𝐷 × (0,𝑇𝑇𝑇𝑇) (3) 132 Krishna Kumar SHRESTHA, Kabi Raj PAUDYAL, Dinesh PATHAK, Alessandro FRANCI & Prem Bahadur THAPA (Fig. 8a), which significantly declines to 0.41 un- der saturated condition (Fig. 8b). This slope ex- hibits marginal stability under dry conditions and ultimately transitions into an unstable state in Fig. 7. Limit equilibrium stability analysis of Site-1: (a) dry condition, (b) GWL at 7 m, (c) GWL at 1.5 m, (d) GWL at 0.5 m, (e) transient analysis for 24 hours with 250 mm rainfall, (f) transient analysis with static loading of 20 kN/m2 and seismic loading of 170 g (horizontal) and 80 g (vertical). (Note: The scale for all figures is same as given in Fig. 7a) saturated conditions. A variation of 66 % is found in the FoS for this slope when changing from dry to saturated conditions. 1.418 152.2 m 69 .7 m Safety Factor 0.000 0.571 1.143 1.714 2.286 2.857 3.429 4.000+ 1.256 1 1 152.2 m 69 .7 m Safety Factor 0.000 0.571 1.143 1.714 2.286 2.857 3.429 4.000+ 0.974 1 1 152.2 m 69 .7 m Safety Factor 0.000 0.643 1.286 1.929 2.571 3.214 3.857 4.500+ 29 20 29 00 28 80 28 60 28 40 28 20 28 00 -400 1 1 0.853 152.2 m 69 .7 m Safety Factor 0.000 0.571 1.143 1.714 2.286 2.857 3.429 4.000+ -60 -40 1.176 152.2 m 69 .7 m Safety Factor 0.000 0.857 1.714 2.571 3.429 4.286 5.143 6.000+ -120.00 -17.14 85.71 188.57 291.43 394.29 497.14 600.00 29 00 28 80 28 60 28 40 28 20 28 00 -380 -360 -340 -320 -300 -280 -260 -240 -220 -200 -180 -160 -140 -120 -100 -80 Pore Pressure (kPa) 20.00 kN/m2 1.100 152.2 m 69 .7 m 0.17 0.08 Safety Factor 0.000 0.857 1.714 2.571 3.429 4.286 5.143 6.000+ (a) (b) (c) (d) (e) (f) 133Evaluation of cut slope stability in the Lesser Himalaya of Nepal Finite Element Method (FEM) The FEM analysis for Site-1 calculated an SRF of 1.45 under dry conditions, indicating a stable slope state. In saturated conditions, the SRF de- creased to 0.82 (Fig. 9a-d), which has predicted the critical failure probability. These two conditions for Site-1 clearly show a 43.44 % change in safe- ty factor when the slope gets saturated. Similarly, for Site-2, SRF under dry conditions is 1.15, show- ing a marginal stability of slope. This safety factor changes to 0.39 under saturated conditions. This change in FoS between two conditions is 66.08 %, which is very critical in terms of slope instability. The maximum shear strain values at both slope sites have shown that elevated pore water pressure strongly inf luences slope materials to destabilise. The maximum shear strain at Site-1 increased from 0.0105 to 0.0315 on changing the scenario from dry to saturation, thereby showing a 57.5 % increase in slope material deformation (Fig. 9a, c). Likewise, Site-2 experienced a maximum shear strain of 0.00386 under dry and 0.009 under Fig. 8. Limit equilibrium stability analysis of Site-2: (a) dry condition, (b) saturated condition. Fig. 9. Maximum deformation vectors at Site-1 under dry conditions: (a) shear strain, (b) total displacement and saturated conditions: (c) shear strain, (d) total displacement. Pore Pressure kPa -150.000 0.000 150.000 300.000 450.000 600.000 750.000 900.000 15 6. 0 m 222.4 m Safety Factor 0.300 0.543 0.786 1.029 1.271 1.514 1.757 2.000 2.243 2.486 2.729 2.971 3.214 3.457 3.700+ 0.401 222.5 m 15 6. 1 m Safety Factor 0.000 0.214 0.429 0.643 0.857 1.071 1.286 1.500+ 350 375 400 425 450 Critical SRF: 1.45 152.2 m 69 .7 m 0.00e+000 1.50e-003 3.00e-003 4.50e-003 6.00e-003 7.50e-003 9.00e-003 1.05e-002 Maximum Shear Strain Critical SRF: 1.45 152.2 m 69 .7 m Total Displacement m 0.00e+000 3.00e-003 6.00e-003 9.00e-003 1.20e-002 1.50e-002 1.80e-002 2.10e-002 Critical SRF: 0.82 69 .7 m 152.2 m 0.00e+000 7.00e-004 1.40e-003 2.10e-003 2.80e-003 3.50e-003 4.20e-003 4.90e-003 Maximum Shear Strain Critical SRF: 0.82 69 .7 m 152.2 m Total Displacement m 0.00e+000 1.50e-003 3.00e-003 4.50e-003 6.00e-003 7.50e-003 9.00e-003 1.05e-002 (a) (b) (c) (d) (a) (b) 134 Krishna Kumar SHRESTHA, Kabi Raj PAUDYAL, Dinesh PATHAK, Alessandro FRANCI & Prem Bahadur THAPA saturated conditions (Fig. 10a, c) indicating a 133 % increase in shear strain due to increased pore water pressures within the slope debris mass. Total displacement contours with deformation vectors from FEM results show the zone of failure, its distribution and intensity, thereby expressing deformation behaviour across the slope. At Site-1, the magnitude of displacement contours illustrates that maximum displacement occurs at the top portion of the free face and gradually diminishes downwards. The maximum displacements for Site- 1 and Site-2 under dry conditions are 19.6 mm and 140 mm, respectively (Fig. 9d & Fig. 10d). Particle Finite Element Method (PFEM) To ensure the reliability of the numerical results, a series of mesh convergence tests were performed for different mesh sizes of 4.0 m, 3.0 m, 2.0 m, 1.0 m and 0.5 m consecutively to compare the resulting sediment deposition heights (Fig. 11a). Fig. 10. Maximum deformation vectors at Site-2 under dry conditions: (a) shear strain, (b) total displacement and under saturated conditions: (c) shear strain, (d) total displacement. Fig. 11. Temporal variation of sediment deposition height simulated using different mesh sizes in PFEM analysis: (a) at the base of the slope for Site-1 and (b) at the lower road section for Site-2. Critical SRF: 1.15 222.5 m 15 6. 1 m Maximum Shear Strain 0.00e+000 2.86e-003 5.71e-003 8.57e-003 1.14e-002 1.43e-002 1.71e-002 2.00e-002 Critical SRF: 1.15 222.5 m 15 6. 1 m Total Displacement m 0.00e+000 8.50e-003 1.70e-002 2.55e-002 3.40e-002 4.25e-002 5.10e-002 5.95e-002 Critical SRF: 0.39 222.5 m 15 6. 1 m Maximum Shear Strain 0.00e+000 4.50e-003 9.00e-003 1.35e-002 1.80e-002 2.25e-002 2.70e-002 3.15e-002 Critical SRF: 0.39 222.5 m 15 6. 1 m Total Displacement m 0.00e+000 2.00e-002 4.00e-002 6.00e-002 8.00e-002 1.00e-001 1.20e-001 1.40e-001 0 5 10 15 20 25 30 5 10 15 20 25 30 35 Se di m en t H ei gh t ( m ) Time (s) 0.5m Mesh 1.0m Mesh 2.0m Mesh 3.0m mesh 4.0m Mesh 1 5 1 2 9 6 3 0 0 5 10 20 30 Se di m en t H ei gh t ( m ) 15 Time (s) 0.5 m Mesh 1.0 m Mesh 2.0 m Mesh 3.0 m Mesh 4.0 m Mesh 25 (a) (b) (c) (d) (a) (b) 135Evaluation of cut slope stability in the Lesser Himalaya of Nepal The simulation results showed that mesh sizes of 1.0 m and 0.5 m produced nearly identical results, indicating that a 1.0 m mesh provides sufficient resolution for accurately capturing the dynamic debris f low and deposition patterns of the sliding mass, thereby maintaining computational effi- ciency. For Site-2, the density, internal friction angle and cohesion of the saturated material were de- termined to be 2180 kg/m³, 14° and 6 kPa, re- spectively. A 2D mesh sensitivity analysis was conducted to assess the numerical accuracy of the simulations. By comparing sedimentation height profiles across varying mesh sizes (4.0 m, 2.0 m, 1.0 m and 0.5 m), it was observed that mesh sizes of 0.5 m and 1.0 m yielded nearly identical results (Fig. 11b). The results have confirmed the ade- quacy of the 1.0 m mesh for optimising the runout dynamics and f low morphology without hindering computational efficiency, which aligns with the recommendations from similar PFEM studies by Cremonesi et al. (2011) and Oñate et al. (2014). Fig. 12. Progressive post-failure flow stages at Site-1 simulated using 2D-PFEM: (a) t = 0s (b) t = 13s, (c) t = 40s, and (d) t = 90s. Fig. 13. Progressive post-failure stages at Site-2 simulated using 2D-PFEM: (a) t = 1s, (b) t = 20s, (c) t = 30s, and (d) t = 43s. (a) (b) (c) (d) (a) (b) (c) (d) 136 Krishna Kumar SHRESTHA, Kabi Raj PAUDYAL, Dinesh PATHAK, Alessandro FRANCI & Prem Bahadur THAPA The PFEM simulation at Site-1 has assessed both the initiation of failure and the subsequent transport of residual soil with particular emphasis on the Lesser Himalaya of central and western Ne- pal, where annual rainfall averages approximate- ly 254 mm. The numerical result for this site has provided insights into the landslide dynamics un- der debris-f luid f low conditions. The debris mass progressively accelerated downslope, reaching a peak velocity of 19.6 m/s before impacting the slope base. The computed runout distance is ap- proximately 425 m from the roadway at the slope crest, which ref lects the mobility and energy of the sliding mass. The final deposition profile shows a maximum sediment thickness of 26.1 m at the base of the slope (Fig. 12). The PFEM analysis for Site-2 was performed in both 2D and 3D configurations, which displayed a realistic simulation of the post-failure conditions. Fig. 13 and Fig. 14 illustrate the time-evolved Fig. 14. Progressive post-failure stages at Site-2 simulated using 3D-PFEM: (a) t = 4s, (b) t = 15s, (c) t = 24s and (d) t = 40s. Fig. 15. Temporal evolution of sediment accumulation at four locations along the lower road section of Site-2, based on com- puted results from the 3D PFEM simulation. (Notations P1 to P4 correspond to observation points indicated in Fig. 14d) 0 1 2 3 4 5 6 7 0 5 10 15 20 25 Se di m en t H ei gh t ( m ) Time (s) P1 P2 P3 P4 (a) (b) (c) (d) 137Evaluation of cut slope stability in the Lesser Himalaya of Nepal progression of the sliding mass, from initial de- tachment to eventual deposition and spreading at the lower end. A significant accumulation of failed material was observed at the base of the slope where a motorable road was blocked by the debris mass. The simulation results indicated a maxi- mum deposition thickness of approximately 6.2 m at the point of observation along the lower road section (Fig. 15). Model performance and sensitivity The cut slope modelling approach effectively predicted FoS and identified potential slip sur- faces under varying geological and hydrological conditions using pre-failure analyses via LEM and FEM. The quick estimations from LEM were re- fined through FEM, which accounted for stress– strain behaviour and complex slope geometries. Post-failure conditions simulated through PFEM successfully reconstructed deformation patterns using site-specific geotechnical inputs. The con- sistency between pre- and post-failure results aligns well with field observations, validating the reliability and robustness of the adopted model- ling techniques. The results from LEM and FEM analyses demonstrate that changes in GWL have a signif- icant impact on slope stability at both sites. The slopes remained unstable when the GWL was 1.0 m below the surface but became marginally stable between 2.0 and 6.0 m and reached a stabil- ity condition when the GWL dropped below 7.0 m with FoS exceeding 1.25. The observed pattern was uniform for both methods at Site-1; however, FEM results at Site-2 remained unstable even when the GWL was below 2.0 m (Fig. 16). The analy- sis showed that FoS/SRF values increased steadily with groundwater levels lowering down at both lo- cations, which suggests that deeper groundwater levels increase the stability of slopes. Conversely, a rise in GWL due to infiltration during rainfall led to a reduction in FoS. The rise in GWL induces seepage forces which also align in the direction of potential slope movement and augment the con- tributing factors causing instability (Fredlund et al., 2012). At both sites, increased saturation leads to the loss of matric suction, causing the soil to weaken and become more susceptible to failure (Lu & Godt, 2013). FEM results further indicat- ed that Site-2 is more susceptible to groundwater f luctuations than Site-1. Site-1 undergoes FoS changes of 40.78 % in LEM and 46.47 % in FEM when transitioning from dry to saturated conditions. Similarly, Site- 2 undergoes a change of 61.18 % in LEM and 65.81 % in FEM under the same conditions. The changes in FoS from dry to saturated conditions in both LEM and FEM methods are high and range from 40.78 % to 65.81 % across both sites. Under dry conditions, a change of 3.79 % and 27.32 % occurred in FoS by LEM and FEM at Site-1 and Site-2, respectively, which are dependent on the site-specific geo-material properties (Table 2). Similarly, in saturated conditions, the change in FoS in LEM and FEM for Site-1 and Site-2 are 13.04 % and 36 %, respectively. Table 2. Variation in FoS under dry and saturated conditions for both sites using LEM and FEM methods. Location LEM(dry) LEM (sat.) % change in LEM (dry & sat.) FEM (dry) FEM (sat.) % change in FEM (dry & sat.) % change in LEM (dry) & FEM (dry) % change in LEM (sat.) & FEM (sat.) Site-1 1.476 0.874 40.78 1.42 0.76 46.47 3.79 13.04 Site-2 1.610 0.625 61.18 1.17 0.40 65.81 27.32 36.00 Fig. 16. Comparison of safety factors obtained from LEM and FEM at varying groundwater levels: (a) Site-1 and (b) Site-2. 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 0 1 2 3 4 5 6 7 8 9 10 Fo S/ SR F Ground Water Level (m) FoS from LEM SRF from FEM 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 0 1 2 3 4 5 6 7 8 9 10 Fo S/ SR F Ground Water Level (m) FoS from LEM SRF from FEM (a) (b) 138 Krishna Kumar SHRESTHA, Kabi Raj PAUDYAL, Dinesh PATHAK, Alessandro FRANCI & Prem Bahadur THAPA A comparison of LEM and FEM results under dry conditions revealed a minimal variation of 2.54 % in the safety factor, demonstrating strong agreement between the two methodologies in the absence of groundwater inf luence. This variation increased slightly to 2.74 % under saturated con- ditions, indicating that both methods consistently capture the effects of saturation on slope stabili- ty. This marginal increase can be attributed to the more complex hydro-mechanical processes ac- counted for in FEM, including stress redistribution and strain localisation, which are not addressed in LEM (Dawson et al., 1999; Sheng et al., 2003). Sensitivity analyses of the slopes at both sites were performed to identify the slope parameters that significantly inf luence the slope stability (Fig. 17). This analysis helps in understanding failure mechanisms, evaluating model robustness, sup- porting design decisions and optimising the mon- itoring stages of a slope. The sensitivity analyses for Site-1 (Fig. 17a) and Site-2 (Fig. 17b) showed that the friction an- gle (φ) has the most significant inf luence on slope stability and that an increase in friction angle sig- nificantly increases the FoS. Cohesion (c) also has a positive inf luence on stability, but to a lesser ex- tent. On the other hand, unit weight (γ) has a neg- ative correlation, where an increase in unit weight results in a decrease of FoS. This implies that re- ducing the soil weight or increasing the frictional resistance can greatly enhance the slope stability at Site-1. From these two analyses, it is observed that the change in FoS is non-linear for friction angle and unit weight but is linear for cohesion. The variation in FoS in both analyses is less than 1 %, indicating a strong agreement. Fig. 17. Sensitivity analysis of slope stability parameters: (a) Site-1 and (b) Site-2. Fig. 18. Evaluation of model consistency through forward and backward analyses under parameter variation: (a) Site-1 with constant cohe- sion, (b) Site-1 with constant friction angle, (c) Site-2 with constant cohesion and (d) Site-2 with constant friction angle. 0 0.5 1 1.5 2 2.5 3 3.5 10 20 30 40 50 60 70 80 90 100 Fa ct or o f S af et y -G LE /M or ge ns te rn -P ric e Percent of Range (mean = 50%) Soil : Cohesion (kN/m2) Soil : Phi (deg) Soil : Unit Weight (kN/m3) 0 0.5 1 1.5 2 2.5 3 3.5 30 40 50 60 70 80 90 Fa ct or o f S af et y -G LE /M or ge ns te rn -P ric e Percent of Range (mean = 50%) soil : Cohesion (kN/m2) soil : Phi (deg) soil : Unit Weight (kN/m3) 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 17 19 21 23 25 27 29 31 33 35 Fa ct or o f S af et y (F oS ) Friction angle () Forward Analysis Back Analysis 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 6 9 12 15 18 21 24 27 30 Fa ct or o f S af et y (F oS ) Cohesion (c, kPa) Forward Analysis Back Analysis 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 18 20 22 24 26 28 Fa ct or o f S af et y (F oS ) Friction angle () Forward Analysis Back Analysis 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 25 30 35 40 45 50 Fa ct or o f S af et y (F oS ) Cohesion (c, kPa) Forward Analysis Back Analysis (a) (b) (a) (b) (c) (d) 139Evaluation of cut slope stability in the Lesser Himalaya of Nepal The close agreement between forward and back analyses for both modelling sites demonstrated the robustness and reliability of the numerical models (Fig. 18), confirming that the selected ge- otechnical parameters (c, φ, γ) are well-calibrated and effective in predicting slope stability behav- iour under varying conditions. The maximum shear strain for both slopes was found to be confined at the slip surface, which diminishes gradually towards the ground sur- face. This shear strain achieved its maximum value under saturated conditions for both slopes and was lower in dry conditions. The increase in shear strain is attributed to elevated pore water pressures that decrease the effective stress within the slope materials. The higher strain values un- der dry conditions at Site-1 as compared to Site- 2 indicated that the soil profile at Site-1 exhibits weaker deformation characteristics, possibly due to lower stiffness and higher water retention or f iner soil grains. Site-2 showed lower strain val- ues under dry conditions, which signifies the stable and stiff material behaviour, likely due to its coarse-grained structure and dense packing. Thus, the present study highlights the importance of integrating advanced numerical methods such as FEM for slope stability evaluations, particular- ly in Lesser Himalayan regions which experience seasonal groundwater f luctuations. The capability of FEM to simulate both stress and deformation makes it well suited for analysing residual soil slopes, which often exhibit progressive failure un- der saturated conditions. The pre-failure evaluation of cut slope sites was further simulated using PFEM, with results show- ing a maximum deposition thickness of 26.1 m at the base of Site-1 and approximately 6.2 m at the lower road section of Site-2. These results are in close agreement with field observations and the damage reports provided by the Department of Roads (DoR), Nepal, thereby validating the mod- el ’s performance. The consistency between the nu- merical results and field evidence reinforces the reliability and potential of PFEM for modelling re- al-world slope failures. Previous researchers have also successfully simulated comparable failure mechanisms in their studies on rapid landslides, debris f lows and earth dam failures (Llano-Serna et al., 2016; Zabala & Alonso, 2011). Validation of model Validation is essential for establishing the re- liability of numerical models in slope stability as- sessments (e.g., Trucano et al., 2006; Xiong et al., 2009; Fawaz et al., 2014; Kaczmarek & Popielski, 2019). It involves comparing model predictions with field observations or established analytical solutions to ensure the accuracy and reliability of the model outputs (Kaczmarek & Popielski, 2019). This process is crucial for confirming the suit- ability of models in evaluating slope stability and predicting potential failures. In this study, the va- lidity of the results was enhanced through a com- parative analysis of outcomes from LEM, FEM, and PFEM applied to two slides with similar geo- logical settings. The convergence of results among these methods strengthens the overall validity. A similar comparative validation approach was em- ployed by Pradhan & Siddique (2020), using nu- merical outputs from different methods. The use of slope sections with comparable material prop- erties and boundary conditions across different locations has also been recognised as an effective validation strategy (e.g., Fredlund & Krahn, 1977; Xing, 1988; Griffiths & Marquez, 2007; Mekon- nen, 2021). A similar validation approach has been adopted in the present study using a Slide, Phase2 and PFEM simulations with the same material and boundary conditions. The safety factors obtained from LEM and FEM for varying groundwater levels at Site-1 and Site-2 were cross-plotted to assess their correla- tion. The correlation coeff icient of the best-f it line quantif ies the degree of agreement between the two methods, demonstrating the reliability of the results. A strong correlation reinforces the validity of the derived safety factors (Fig. 19), while discrepancies may indicate methodological limitations or site-specif ic inf luences. The f ind- ings underscore the importance of implementing complementary analytical approaches in geotech- nical solutions. The computed correlation coefficients of the safety factors derived from the Limit Equilibri- um Method (LEM) and the Finite Element Meth- od (FEM) for Site-1 and Site-2 are 0.99 and 0.91, respectively. These values indicate a very strong positive linear relationship between the outputs of the two methodologies. For Site-1, the near-per- fect correlation coefficient (R2=0.99) demon- strates that the safety factors calculated by FEM align closely with those obtained from LEM. This implies that under the given boundary conditions, soil properties, and groundwater regimes, the sim- plified assumptions in LEM are sufficiently repre- sentative of the slope’s actual stability conditions captured by FEM, which accounts for more com- plex stress–strain behaviour and material defor- mation. 140 Krishna Kumar SHRESTHA, Kabi Raj PAUDYAL, Dinesh PATHAK, Alessandro FRANCI & Prem Bahadur THAPA The correlation coefficient (R2 = 0.91) at Site-2 is slightly lower than that of Site-1 but still indi- cates a robust agreement between the two meth- ods. The observed difference could be attributed to site-specific geotechnical complexities such as heterogeneity of material layering, anisotro- py, or non-linear deformation behaviour which are better represented in FEM due to its contin- uum-based formulation. FEM’s capability to sim- ulate progressive failure mechanisms, pore pres- sure redistribution, and localised yielding makes it especially sensitive to such conditions, thereby leading to slight deviations from LEM predictions. Nevertheless, the high correlation coefficients at both sites confirm the mutual consistency and val- idation of the two modelling approaches in evalu- ating slope stability. This supports the application of LEM for preliminary assessments or paramet- ric studies while highlighting FEM’s strength for more detailed and deformation-based analyses, particularly under complex geological or hydro- logical conditions. Conclusions This study has evaluated the cut slopes in the Lesser Himalaya of Nepal that are often inf luenced by groundwater saturation togeth- er with slope geometry and triggering factors. Pre-failure analyses using the Limit Equi- librium Method (LEM) and Finite Element Method (FEM) revealed that saturation sub- stantially reduces the shear strength of slope material, causing safety factors to attenuate by 40.78–65.81 % when transitioning from dry to saturated conditions. The analysis of the modelling sites showed that slopes remained unstable when groundwater levels were with- in 1.0 m from the surface, became marginally stable between 2.0 m and 6.0 m, and achieved stable conditions below 7.0 m depths with a factor of safety exceeding 1.3. Closely aligned results from both LEM and FEM results (<10 % variation) and the good agreement between forward and back analyses (<1 % deviation) validate the robustness and reliability of the adopted numerical modelling techniques. The post-failure Particle Finite Element Method (PFEM) simulations quantified the dynamic failure behaviour which computed the debris velocities of 19.6–23 m/s and runout distanc- es of 305–425 m, thereby providing insights into potential impact zones and downstream risks. An integrated LEM-FEM-PFEM framework has proven to be a suitable approach for ana- lysing slope stability under varying ground- water conditions and identifying critical slope sections. The findings of this research not only decipher failure mechanisms of cut slopes in the Lesser Himalaya but also provide a validated multi-method evaluation in sim- ilar geo-environmental settings. The results also deliver valuable scientific and practical insights for designing resilient infrastructure and reducing cut slope failure hazards. Acknowledgements We are grateful to the Centre for Numerical Methods in Engineering (CIMNE), Spain for providing an oppor- tunity of a research stay to the f irst author in acquiring the knowledge of numerical modelling techniques un- der the Marie Skłodowska-Curie Actions (MSCA) Staff Exchange Project (“LOC3G”) funded by the European Commission. Fig. 19. Correlation between the safety factors derived from the Finite Element Method (FEM; Phase2) and the Limit Equilibrium Method (LEM; Slide) for (a) Site-1 and (b) Site-2. Linear regression lines and coefficients of determination (R²) demonstrate the strong consistency between the two modelling approaches. y = 1.1155x - 0.1961 R² = 0.9948 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 SR F fr om F EM FoS from LEM Data Points Best Fit y = 0.9115x - 0.045 R² = 0.9071 0.3 0.5 0.7 0.9 1.1 1.3 1.5 0.6 0.8 1 1.2 1.4 1.6 Fo S fro m F EM FoS from LEM Data Points Best Fit (a) (b) 141Evaluation of cut slope stability in the Lesser Himalaya of Nepal References Acharya, A. & Dhital, M.R. 2023: Rock mass qual- itative stability aspects of cut-slopes in the Lesser Himalaya of central Nepal. Bulletin of Nepal Geological Society, 40: 93–100. Adhikari, B. R. & Gautam, S. 2022: A review of policies and institutions for landslide risk management in Nepal. Nepal Public Policy Re- view, 2: 93–112. Bhattarai, P., Tiwari, B., Marui, H. & Aoyama, K. 2004: Quantitative slope stability mapping with ArcGIS: prioritize highway maintenance. 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