Engineering geological characteristics and failure mechanics of Jure rock avalanche, Nepal

Abstract

Introduction

The rock avalanches are a frequent and disruptive phenomenon in the Himalayas and other mountain chains. To minimize future losses, it is essential to investigate the engineering geological causative factors and mechanism of these mass wasting events.

Study area

The present work is aimed at assessing the failure mechanism of the disastrous 2014 Jure rock avalanche along Araniko Highway, Northern Nepal. The event had blocked the Sunkoshi River and blocked an economically significant route to China.

Geotechnical properties and analysis

Initially, rockmass characterization and intact strength attribute were determined for the site to classify the failure zone. The parameters measured and obtained from the field and laboratory were integrated into the analytical models to obtain a conclusive interpretation of the failure mechanism. Structural, kinematic, and key block theory analyses have been carried out for decipher the evolution of the failure zone.

Results and discussion

Rock mass was found to be of fair quality, however, the structural instabilities and the presence of water has led to a progressive failure. Movement of the key block and subsequent sliding of wedges and foot failure appears to be a possible failure mechanism.

Conclusion

The present research explores the contributory engineering geological aspects of the Jure rock avalanche. The investigation results can be used to tackle similar large scale rock avalanches in similar geological terrains and thus minimizing the losses.

Introduction

The landslides significantly affect mountainous countries like Nepal, India, Switzerland, Japan, Canada, New Zealand, USA, and China (Lateltin et al. 2005; Yin et al 2009; Evans et al. 2011; Bhandary et al. 2013; Dhungana et al. 2023; Xiao et al. 2023). Prakasam et al. (2021) estimated the damages caused by the Himalayan landslides to cost more than one billion US Dollar in economic terms along with more than 200 deaths every year, accounting for about 30% of the total such worldwide losses. Nepal is a part of the Himalayan region and due to its rugged topography and tectonics (Dhital 2015), the country is quite susceptible to slope failures and landslides. Over 12,000 small to large scale slope failures occur each year in the country. On 2nd August 2014, a massive landslide occurred at Jure village in Sindhupalchok district on the border of Mankha and Ramche (Ministry of Irrigation 2014). Approximately 5 million m³ of massive rock fragments and debris were deposited. The landslide killed 156 people, injured 27, and displaced 436 people. The massive landslide dammed completely the Sunkoshi River forming a lake of about 3 km long and 300–350 m in width. The landslide buried about 1 km of Araniko Highway, a major highway leading to china. Other amenities such as school, hospital, police station, and post-office were severely affected. The dust of the landslide reached up to 600 m in height which covered the Dabi Khola village area.

There are several articles on different aspects of 2014 Jure landslides and careful analysis reveals several important information about the mechanism of failure. Bhandary et al. (2018) employed Spectral Element Method (SEM) for analysing stability of wet and dry slopes along with pseudo-static loading of slopes. They highlighted that the slopes become unstable when the groundwater table reaches the surface and peak ground acceleration values become greater than 0.2 g and in some cases even 0.1 g. Jaboyedoff et al. (2015a, b) utilised digital elevation model (DEM) extracted from Terrestrial Laser Scanner (TLS) and field investigations to characterize Jure landslide. They were able to investigate the rockfall avalanche volume, scar structure and deposits. The rockslide was predominantly restricted to phyllites, quartzites and sandstones. Some ephemeral springs were also observed below the unstable mass. The rockslides developed in presence of complex structures is accompanied by degradation of rock mass which further reactivates large scale instabilities. Shrestha and Nakagawa (2016) suggested that existence of unstable slopes and occurrence of previous landslides are responsible for Jure landslides. They further studied areas of flood inundation caused by formation of landslide dam and this study can also assist in implementing mitigation measures for establishing residential areas. Tien et al. (2021) used laboratory tests and numerical simulation to clarify the failure mechanism of the deep-seated landslide and process of dam formation. They suggested that the major slip movement occurred along the bedding plane faults of weathered phyllites and schist. And, more importantly the sliding mechanisms of upper and lower slopes differed from each other. The critical pore pressure ratio of the upper slope reached 0.22 to 0.26 due to heavy rainfall and initiated the failure while the dynamic loading process by downward movement of upper block caused the failure in lower slope. Yagi et al. (2021) combined DEM and InSAR data to detect ground movement. They suggested that prolonged period of creep movement led to the formation of thick weathered layer at the Jure landslide site.

The large-scale landslides are becoming more frequent and is a major cause of disaster leading to huge economic losses (Zhou et al. 2010). The fragile geology, rugged topography, construction activities, heavy rainfall and existence of fault zones allows the hill slope to develop strain over time ultimately leading to slope failures. The massive volume of detached mass generated due to intermittent small landslides and large-scale landslides leads to the deposition of large number of landslide deposits. A major problem associated with large landslide deposits is chocking/damming of a part or entire river. The associated problem with choking of river is lake outburst, flooding of nearby areas, reactivation of old landslides etc. In this regard, Iqbal et al. (2018) have assessed the stability analysis and failure mechanism of an active landslide in Xiangjiaba Reservoir Area, China using field investigation, field monitoring and laboratory tests. Based on geological field investigations, direct shear tests and limit equilibrium methods, stability analysis of the Zheng-Gang landslide, China was studied (Zhang et al. 2014). Mebrahtu et al. (2022) used both limit equilibrium and finite element method to assess the stability of a deep-seated landslide in Debre Sina area, Ethiopia. There are few aspects common in all these analyses related to large scale failures, i.e., saturation threshold, groundwater condition, long-term creeping behaviour, development of localised slip planes, seismicity. The stability of the deep seated landslides triggered by rainfall has also been assessed by estimating the rainfall threshold (Wei et al. 2019; Arnold 2006; Tiwari et al 2020; Kainthola et al 2021; Pandey et al. 2022).

Back-analysis is the process by which the nature and development of a landslide is measured through a series of deductions (Hencher and Malone 2008). Procedure of back analysis start with understanding the subsurface conditions of landslides, defining representative cross-sections, and failure surfaces etc. Appropriate stability method and software should consider varying the landslide ensuing parameters of the problematic layer to the factor of safety (FoS ~ 1.0). Back analysis involves analytical and numerical methods to analyse the available data and determine the factors that contributed to the occurrence of the landslide. Analytical methods engaged theoretical equations and empirical relationships to calculate the stability of the slope and assess the factors that drive to the landslide (Gutiérrez-Martín et al. 2019). These methods include limit equilibrium analysis, slope stability parameters, and analytical solutions for simplified slope geometries. The stability of the slope is evaluated by comparing the driving forces with the resisting forces. Similarly, different numerical methods are applied to simulate the behaviour of the slope and asses its stability condition (Griffiths & Lane 2001). These models incorporate the geotechnical properties of the rock mass, boundary & initial conditions. The slopes are subjected to various loading conditions, such as vehicle movement, constructions, rainfall and seismicity forming the critical factors leading to failure. And, estimation of shear strength parameters is a prime issue in landslide management, and inappropriate parameter evaluation often guides to the failure of mitigation strategies (Zuo et al.2022). However, it is difficult to accurately examine these parameters due to the complex geological setting and different failure models. Therefore, back-analysis in problematic landslides can be employed to comprehend the critical attributes more diligently than laboratory testing (Hussain et al. 2010).

Literature has the pertinent collection of massive landslides events recorded in the earth's history (Sartori et al. 2003; Dahal and Hasegawa 2008). The critical factors responsible for such failure and associated mechanisms have been highlighted by numerous workers (Cui et al. 2015; Shrestha and Nakagawa 2016; Liu et al. 2023; Wang et al. 2023; Çelik 2023; Yang et al. 2023). Usually, stability of jointed rock slopes is controlled by their joint persistency, spacing of joint sets, and shear strength of the discontinuity (Panthee et al. 2016). The Vargas rock avalanche (Venezuela) of December 1999 is one of the largest reported landslides in the planet’s history killing nearly 30,000 lives and was triggered by a heavy storm owing to 911 mm (35.9 in) of rain in a few days (Wieczorek et al. 2001). Similarly, the Armero Tragedy (November 1985) of Colombia had engulfed 23,000 lives (Lowe et al.1986). A catastrophic failure of Aratozawa (Japan) induced by high magnitude (7.2) seismicity had displaced nearly 67 million m³ (Miyagi et al. 2011). Further studies delineated this landslide as translational block glide having the failure surface in thin sand strata. In 2011, heavy precipitation in Kii Peninsula (Japan) led to several mass-movement, the mechanism and consequent dam formation process of two such deep-seated failures in Kuridaira and Akatani valleys have been detailed in the work of Tien et al. (2018). Li et al. (2011), highlighted the problem of slope failure at Hsiaolin village (Taiwan), thereafter landslide dam formation and simulated eventual dam breaching. A large-scale failure incident led to the emergence of a 60 m high landslide dam at the bank of Jinsha River (SE Tibetan Plateau) by disposing nearly 4.9 × 10⁷ m³ of debris materials (Li et al. 2011). Additionally, the buckling of planar blocks under the effect of gravitational force had initiated the failure, as confirmed by fieldwork and theoretical examination. In 2020, the Pettimudi landslide (India), an extreme-rainfall triggered cataclysmic mass-movement that had affected an area of 70,125 m² (including tea-plantation) and claimed 66 natives' lives (Achu et al. 2021). A prolonged precipitation event triggered the deadly Malpa rockfall-debris flow (1998) in the Indian Himalaya that invited 221 fatalities, and partially dammed the Kali River by pouring nearly 1 million m³ (Paul et al. 2000).

The previous investigation on Jure landslide lacks an in depth engineering characterization of material and detailed analysis of the mechanism of failure that led to this catastrophic event. Therefore, the present study is aimed at investigating the causes and failure mechanism of the Jure rock avalanche. A detailed visualization of the geomorphic scenario and engineering geological mapping of the research area is essential to comprehend the landslide occurrence. The rockmass characterisation and intact strength attributes determination will enable numerical simulation of the failure zone. Moreover, back analysis of the landslide by analytical and numerical methods will advance the landslide knowledge. The parameters measured and obtained from the field and laboratory were integrated in the analytical and numerical models to obtain the conclusive interpretation. RS² software was used for finite element numerical modelling. The results computed by various methods complementary to each other were then integrated to know the evolution history and mechanism of slope failure.

Study area

The study area is about 83.5 km northeast of Kathmandu at the right bank of the Sunkoshi River, near Barhabise Bazar, Sindhupalchok district, Central Nepal (Fig. 1). The study area is easily accessible by the Araniko Highway and is drained mainly by the tributaries of the Sunkoshi River flowing in NE to SW direction. The climate is subtropical, temperate, and alpine. The average rainfall is 3604.3 mm, of which 80% falls in the monsoon season. A temperature ranges of about 30°–32 °C can be expected during summer, and it varies from about 6°–8 °C during winter months. Geologically, the Nepal Himalaya is divided into five major geological zones: Terai, Sub-Himalaya (Siwaliks), Lesser Himalaya, Higher Himalaya, and Tibetan-Tethys Himalaya (Gansser 1964; Upreti 1999). And, the basic framework of the Himalayas is controlled by three significant thrusts, Main Central Thrust (MCT), Main Boundary Thrust (MBT), and Himalayan Frontal Fault System (HFFS) that dip north and extend east–west throughout the country (Upreti 1999). These three thrusts are considered to come together in a low angle decollement known as the Main Himalayan Thrust (MHT) to the north, deep below the Himalaya.

Fig. 1

Geospatial and geomorphological aspects of the Jure landslide

The study area lies within the Kuncha Formation of Lesser Himalaya (Central Nepal) comprising meta-sandstone, chloritic schist and pelitic schist (Fig. 2). Kuncha formation ranges from the Gandaki Region to the Bagmati–Gosainkund Region. In the Gandaki region, the inner zone of Gorkha is made essentially of the Kuncha formation. In this region, the Kuncha formation appears immensely thick, monotonous succession of dark grey, green-grey, bluish-grey phyllite, phyllitic metasandstone, gritty phyllite, and quartzite. Dhading Dolomite forms hills and ridges around Dhading and varies in thickness from 500 to 1000 m. Metasandstone and phyllite is the main lithology of this formation and belongs to the Mesoproterozoic Era. Nourpul Formation begins with a few centimetres to decimetre green-grey and purple phyllites. And, marble and schist are the main lithologies of this formation with approximately 800 m thickness formed in Mesoproterozoic. Furthermore, Fagfog formation consists of Fagfog quartzite of Paleoproterozoic Era as their main lithology having a thickness of 400 m. This quartzite shows graded bedding, cross-lamination, and spectacular wave and current ripple marks. Fagfog Formation abruptly over the Kuncha Formation and it consists of fine to coarse grained light grey, pale yellow, and white quartz arenites with thin grey-green phyllite alterations or partings. Auden (1937) established a relationship between the Benighat slates and the Jhiku formation of Udaipur in east Nepal with the Blaini and Krol series. It contains very thinly cleaved varieties. Slate, argillaceous dolomite is the main lithology of this formation and it is referred to as Benighat slates. The age of the Benighat slates is Mesoproterozoic. Jhiku carbonate bed lies with Benighat slates. There are some specific zones rich in carbonates classified as calcareous beds or Jhiku carbonates (Stöcklin and Bhattarai 1977). The major anticline axial trace is horizontal and passes along the Sunkoshi River trending NE-SW direction. The foliation plane of limbs dips gently in two different directions (Budhathoki 2016).

Fig. 2

Geological map of study area (modified after Budhathoki (2016))

Jaboyedoff et al. (2015a, b) characterized the Jure avalanche and analyzed the rock fall avalanche volume, scar structure, and deposits. The estimated rockslide volume was approximated to be 5 million m³. In addition, some ephemeral springs seemed to develop below the unstable mass. The rockslide generated the rock avalanche. Rock avalanches are a type of mass movement entailing rapid downward motion of fragmented rocks from a large rock mass mounted over the steep slopes, quite common in this region (Shrestha and Nakagawa 2016; Tien et al 2021; Yagi et al 2021). The landslide activities in Nepal can be attributed to severe climatic, topographic, and geological circumstances (Ministry of Irrigation 2014). The issue has been further aggravated due to Nepal’s active tectonics, steep topography, and anthropogenic activities (Bhandary et al. 2018). Arnold (2006) classified the country under moderate to high landslide hazard zone.

Geotechnical properties and analysis

Rock mass properties

The mapping of fractures in the Jure landslide zone started in 1958 during the construction of Araniko highway. In 1967, 1971, and 1975 landslides were repeatedly observed just below the crown. In 1983, a landslide occurred on the crown, however the toe remained safe. After 1989, landslides activated and regenerated below crown to mid part. The GoogleEarth images since 2012 indicate an increasing rock fall and scarp development activity of the landslide. There was a smaller slide in 2013 at the upper cliff. The slide reactivated suddenly on 2 August 2014 causing a huge collapse. Figure 3 shows the pre and post landslide features of the Jure landslide.

Fig. 3

Google earth images of 2012, 2014, 2016, and 2017 of the study area

The engineering geological mapping was done to assess different parts of the landslide (Fig. 4). Numerous quartz veins are visible, and most are aligned parallel to the foliation. The foliation of the rocks gently dips about 10°-25° toward NW. The other two major joints are almost vertical, joint dipping 70°-80° towards East (J_S1), from which most of the water is flowing down and joint dipping 80°–85° towards southwest (J_S2). The kinematic analysis shows that failure in the lower cliff is unlikely. On the upper cliff, some wedges are formed on the left flank of landslide, implying possible wedge failure and on other parts wedges formed by intersection of joint planes J_S1 and J_S2 were observed which were stable without any external forces. The Jure landslide is a form of mass wasting that includes a wide range of ground movements such as rock falls, deep failure of slopes, and shallow debris flows. Although, the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability. In this landslide, the sliding surface is mostly deep below the maximum rooting depth of trees typically to depths greater than ten meters. The concave scarps at the top and steep areas at the toe can be visually identified from landslides. There are still large cracks present in the upper cliff from where the spring water is coming out (Fig. 5). Moreover, the cross-sectional view of the Jure landslide can be inferred through Fig. 6.

Fig. 4

Engineering Geological map of study area

Fig. 5

Jure rock avalanche showing different parts (view to N)

Fig. 6

Cross section of Jure landslide showing various landslide parts

The Jure landslide is geomorphologically divided into six zones, based on the parameters like slope angle, rock formation, and condition of spring source as shown in (Fig. 7).

Fig. 7

Geomorphological map of Jure landslide showing various failure zones

The crown is full of cracks, from where the spring water is discharging (zone 1). At this section, the slope is found to be 70°-80° with concavity throughout the slip surface. The slippage initiated from upper cliff (zone 2). At this section, the slope is found to be 65°-70° with concavity throughout the slip surface. This segment consists of highly weathered rocks. The exposed scar looks brownish in colour. Zone 3 comprises huge boulders and rock fragments covering the flat land between the Lower and Upper cliff. This section of landslide consists of an abundance of debris material. The slope ranges from 30°-45°. The spring source flowing downwards from top of the slope follows this section along the surface. Zone 4 of the landslide is the remains of the previous slope signifying the low potentiality of failure and the slope of this section is 55°-65°. This section primarily consists of highly fractured rocks at the surface. The spring source that has been disappeared in, again appears at this section forming the distinct gullies. Zone 5 can be described as toe or debris fan deposit area of the landslide comprising colluvium. Zone 6 is the accumulation zone of failed mass that consists of large boulders dispersed in different sections throughout the landslide areas at toe.

An exhaustive field investigation was carried out to record the data pertaining to rock type, discontinuity characteristics, and rock mass. The joint volume (J_v) was calculated from the same data which was further used to calculate RQD and rock mass rating (Table 1).

Table 1 Attributes of rockmass recorded during field investigation

The discontinuity persistence, aperture, roughness, infilling, and weathering along with hydrological conditions were assessed to calculate the rock mass class (Bieniawski 1973). Most of the rock mass fall in the fair to good rock class.

Intact rock properties

The rock samples were tested in the laboratory of University of Leeds, United Kingdom to determine strength parameters of intact rock (Bray 2016). The samples taken were mainly blocks from the landslide debris. Only samples with no obvious discontinuities were chosen for testing, however during the landslide it is likely that some internal fractures may have formed, reducing the strength of these fallen blocks. The nine parameters of intact rock were determined in the laboratory (Table 2).

Table 2 Mean Geotechnical properties of rocks from Jure rock avalanche (Bray 2016)

Kinematic analysis

The three major discontinuities including the foliation plane were identified at the rock avalanche site. There are numerous tension cracks developed above the crown of the landslide, dipping parallel to the hillslope. The geometry of tension cracks is also included in the analysis. According to the locals, there were large tension cracks above the head from where stream water was flowing down. The tension cracks were nearly parallel 155° with near vertical inclination.

Most tension cracks parallel to slope had slightly greater inclination than the slope, denoting minimal chances (~ 6%) of plane failure (Fig. 8). Moreover, no wedge instability can be located considering mean orientation of major discontinuities from the upper cliff. However, two very gentle wedges formed by the intersection of foliation with J_S2 and J_S1 at the western and northern parts of the stereoplot (Fig. 8). Another three very steep (nearly vertical) wedges formed by J_S1 and J_S2, tension crack and J_S1, and tension crack and J_S2 are not daylighted. Additionally, there are no such discontinuities which may be responsible for the toppling failure (Fig. 8). This indicates none of the simple failure modes appear to be feasible. So, the water flowing from the tension crack may have ultimately appended the water pressure along joints, eventually lifting or pushing the wedges. This gives rise to the possibility of generation of newer fracture planes enabling sliding of the slope mass.

Fig. 8

Kinematics analysis for failures, a planar failure, b wedge failure, c toppling failure

Block theory analysis

The block theory considers the geometry of the discontinuities and the slope orientation, in a similar manner to the kinematic analysis. In addition, it determines the “finiteness” of blocks bounded by several discontinuities and free surface along with their removability. According to the key block theory, a rock slope is stable if the weight of the rock above the key block is balanced by the friction and shear resistance of the key block against the rock below it (Fig. 9). If the weight of the rock above the key block exceeds the resistance of the key block, the slope becomes unstable and may fail.

Fig. 9

Key block theory classification of rock blocks (Kulatilake et al. 2011)

One of the main advantages of the key block theory is that it provides a simple and intuitive way to understand the mechanics of rock slope failure. It also allows engineers to identify the key blocks in a rock slope and determine the forces acting on them, which can be used to design mitigation measures to improve the stability of the slope. However, the key block theory has some limitations, as it does not consider the effect of rock jointing and other factors that can affect the stability of a rock slope. Despite these limitations, the key block theory remains a valuable tool for analysing and predicting the stability of rock slopes. The present work delves into key block analysis over the dataset outlined in Table 3. The Key block theory was performed using stereographic projection of joint planes on the upper hemisphere (Fig. 10).

Table 3 The mean orientation and properties of major discontinuities

Fig. 10

Joint pyramids and critical block shapes analysed from block theory analysis

The stereographic projection delineates 14 joint pyramids, representing the 14 different types of block formation from the intersection of discontinuities (Table 4). The factor of safety of such formed blocks in relation to free surface was determined. Out of 14 various shaped and sized key blocks types, 6 were finite and removable (i.e., Joint Planes: 1000, 1100, 1010, 0101, 1101, and 1011) with FoS less than 1. A slight movement in these blocks by any external pressure and loading can cause these blocks to move. Consequently, the overlying blocks become free to move causing the failure to progressively become larger in a chainlike series.

Table 4 Block classification and corresponding FoS

Plane failure analysis

The tension crack in relation to slope was further examined to determine susceptibility of planar failure along the crack. The role of pore water pressure along open joints/ planes were also investigated. The critical tension crack position and its depth was determined based on trace observed at the avalanche. The height of water in the tension crack was gradually raised by hit and trial of 500 different combinations in order to cause failure on the sliding planes. Simultaneously, their corresponding factor of safety was determined. Afterwards, computer simulations for plane failure analysis were performed using the most critical conditions from parameters, as obtained in back analysis (Fig. 11). The calculation shows the slope was marginally stable in absence of any external forces. The dip of the sliding plane, upslope dip amount, and face dip amount were 48°, 32°, and 65° respectively as input parameters in back analysis of the Jure slope. Additionally, critical tension crack depth, position for tension crack, critical slide plane inclination was fed as 41.86 m, 33.60 m, and 51.50°, respectively. The factor of safety of the slope without water and with water in tension crack is calculated to be 1.7 and 0.59, respectively.

Fig. 11

Planar wedge stability analysis of Jure slide

The spring water infiltrates through the tension cracks and J_S1 leading to the alteration and degradation of rock mass. The process also increases the pore water pressure in the tension crack at the slope’s toe. The water pressure in these cracks allowed the block movement by the forces developed on the joint surfaces resulting in tilting of the blocks from the toe and ultimately slope failure. The sensitivity analysis indicates the variation in FoS with change in slope parameters such as slope angle, height, or water percent filled in tension cracks (Fig. 12). The back-analysis using hit and trial approach gave a perfect match with the result obtained from computation in RocPlane software (RocScience Inc.). Furthermore, the sensitivity analysis also fits the real ground condition (Fig. 12).

Fig. 12

Sensitivity analysis for various slope parameters for planar failure

Wedge analysis

The study further investigates wedge failures, as per structural discontinuities and examines wedge surfaces in rock slopes. In this analysis, the tension crack was added to form the rear release surface. The kinematic approach using the discontinuity sets indicated stable geometries of wedge formed by J₁ and J₂. As the hillslope direction varies throughout the slope, various other valid directions of hillslopes are tested to obtain wedge blocks of different shape and size. All the possible combinations of two discontinuities and the slope surface that form a valid wedge were analysed (Fig. 13). The wedges are found to have a maximum weight of 155 tons, and with addition of water force in the most critical wedge formed by J₁ and J₂, the FoS is found to be 0.65 (Fig. 14). The water infiltrating through the joint intersection increases the water pressure in the walls of the joint that pushes the wedge. The development of pore water pressure at the base of the wedge caused the toe of the block to tilt and decrease in plunge which subsequently failed by sliding along the line of intersection, trending 139°.

Fig. 13

Various shaped and sized blocks formed by wedges in Jure

Fig. 14

Wedge stability analysis for most critical wedges

Results and discussion

The primary objective of the study is to undertake geo-engineering investigation and to interpret the failure mechanism of the landslide. The variation in rock mass classes is due to the differing rock quality designation (RQD), spacing of discontinuity, condition of discontinuities, and ground water condition (Bieniawski 1989). The intact physico-mechanical attributes and discontinuities’ shear strength variables were discerned through laboratory and in-situ testing. The Pelitic schist was found softer having fine grained minerals and mica dominating with smooth surface indicating low Rock Mass Rating (RMR = 50) whereas metasandstone has high RMR value (RMR = 64).

The satellite images since 2012 indicate an increasing rock fall and scarp development activity at the incident site (Fig. 3). However, it was difficult to assume the occurrence of such a catastrophic mishap. The records show that no seismic events were recorded prior to the failure, however, rainfall of more than 70 mm/day was recorded two days before the mishap. The natives have noticed muddy water flowing through the landslide mass in the last 3 days before the event. The high groundwater table weakened the critical rock bridges, causing explosion-like fracturing of rocks and the expulsion of the key block, followed by sliding and foot failure of the other rock blocks. The analysis showed that shifting of water level in joints and tension cracks can cause abnormal failure of slope. The rainfall, two days prior to the event, led to increase in water level and is the main reason behind the failure.

The interpretation of the evolution of the 2014 Jure rock avalanche can be seen in Fig. 15. The surface water infiltrates through tension cracks and joint networks over a period of time causing widening of joint apertures owing to freeze–thaw action and ultimately weakening the rock mass through physical disintegration process. The heavy rainfall (70 mm/day), prior to the event, influenced the pore water pressure in impervious and porous pelitic schist rockmass obstructing the flow by infilling materials in tension crack and some of the discontinuities. The impervious boundary allowed high water pressure to be concentrated on the walls of tension cracks and the toe of the blocks at the base of the upper cliff (Fig. 15a). And, this increase in pore water pressure along the structural discontinuities had significantly lowered the shear strength attributes along joints’ surfaces and eventual decrease in factor of safety (FoS) value. Also, the hydrostatic pressure developed in the rockmass owing to heavy rainfall exceeded the static friction threshold of rock blocks and wedges. And, as the concentration is reached maximum around key blocks, the blocks in front are pushed forward. The tilting and pushing of key blocks by water pressure from the joint networks and base of blocks caused the blocks of schist (the key-block) to bulge out which ultimately was thrown outward along the gentle foliation plane (Fig. 15b). With the support removed below, the above hanging rocks became unstable as discussed above in the key-block theory section. And, around 80 m high rock cliff collapsed from the upper cliff on removal of key block support from below (Fig. 15c–f) and the debris moved downhill triggering the mass movement by washout of bedrock in the lower cliff. At this stage, the debris lost its momentum reaching the narrow valley floor where it buried the Araniko Highway, as well as some houses. Finally, the debris and blocks from the avalanche dammed the Sunkoshi River forming a landslide dam. Furthermore, the material forming the crown of avalanche became unstable, when the rock mass constituting the upper cliff was moved downhill. The movement propagates towards the crown after the main failure event happened (Fig. 15d). It can be observed that streams flowing downward above the crown and landslide mass has infiltrated through open joints. There is still a large crack present in the upper cliff from where the spring water is coming out. The tilting of trees in different directions can be observed above the crown. The reactivation of slope during rainfall and in dry season occurs by smaller failures on crown area.

Fig. 15

Evolution and failure mechanism of Jure rock avalanche

The Jure failure appears like a translational rockslide (Varnes 1978), since translational movement of rock occurred along a more-or-less planar or gently undulating surface (Dhital 2014). Also, it looks like a block failure where key blocks formed by intersection of discontinuities were displaced due to increase of water pressure, and subsequently caused the larger failure of overlying mass along tension crack and discontinuities. Therefore, the Jure rock failures seem a complex and a special type of landslide. A very large quantity of saturated substrate material represents an unusual type of landslide, termed as rock avalanche. Many rock avalanches can thus be seen as end members of a continuum of phenomena involving rock failure followed by interaction with saturated substrate (Hungr and Evans 2004). Based on the nature of the failure mechanism and debris movement, the present case can be a rock avalanche.

In Jure, the rock fragments were dropped from about 800 m height to the base of the landslide, so the speed could be approximately 60–70 m/s (Dhital 2014). The rock avalanche is an extremely rapid, massive, flow-like motion of fragmented rock from a large rock slide and rock fall (Hungr et al. 2001). Again, based on the speed of material it should be classified as a rock avalanche. The Jure failure has a volume of 5.05 million m³ and based on volumetric nomenclature a rockfall having a volume of more than 1 million m³ will be classified as a rock avalanche. Generally, these slope failures are complex and will take place in two stages. Dikau et al. (1996) describes the mechanical analysis of a rock avalanche as “a fall or slide of a rock body which during movement progressively loses its cohesion by turning into dry debris and thus continues its advancement as a debris avalanche”. The debris at the base of the slope at Jure is typical of that described by Dikau et al. (1996) as a rock avalanche which has been constrained by a steep valley and formed a landslide lake.

On April 2 1999, Clanwilliam landslide occurred on a south facing slope above Clanwilliam Lake, approximately 13 km west of Revelstoke, B.C. The failed mass is composed of gneissic material, that in the case of Jure is meta-sandstones and Pelitic schist. The published data examination shows that no earthquake events were recorded prior to the Clanwilliam failure. The climatic record showed that pronounced freeze- thaw cycles occurred the day before and on the day of the failure. Only a significant amount of precipitation (20–25 mm/day) was recorded eleven days before the failure. On the other hand, a heavy rainfall was recorded two days before the Jure landslide. The Clanwilliam landslide area exhibits classical morphology with steep rock walls and a valley bottom (Fig. 16a). The Jure area exhibits similar topography and morphology (Fig. 16b), with steep rock slopes and narrow valley bottom formed by river erosion.

Fig. 16

Overview of the Clanwilliam landslide and Jure rock avalanches, a Clanwilliam landslide, b Jure rock avalanche (Google Earth)

In case of Clanwilliam landslide, the preliminary kinematic analysis identified the potential slope failure mechanisms and was followed by limit equilibrium wedge analysis. The block theory analysis was undertaken to identify critical blocks and block shapes within the rockmass. The results of this study were used to perform a preliminary 3D-distinct element analysis that shows simple and complex wedge blocks appear to be a feasible failure mechanism for the Clanwilliam landslide (Grenon and Hadjigeorgiou 2003). It also emphasized that the orientations of discontinuities assigned to a joint set during the kinematic analysis can have a critical role on the stability of a rock slope.

Moreover, freeze–thaw cycles could have influenced the pore water pressure by obstructing some of the discontinuities with ice near the surface. Several workers have suggested such a phenomenon at a range of slope failure scales (Gardner 1983; Haeberli et al. 1997). In case of Jure, rainwater and surface water percolation through tension cracks and discontinuities is capable of increasing water pressure in networks of joints and tension cracks. The back analysis performed using various 2D and 3D analytical and numerical methods are useful in evaluating the role of the different discontinuity sets observed at the Clanwilliam landslide and Jure rock avalanche. Similar results and nature of the failure mechanism can be observed, which is a positive correlation. The Clanwilliam landslide is a progressive failure, where small movement accumulates along discontinuity and rock bridges are broken over a period of many years until a critical state is reached. As stated above, the current theory for the Jure avalanche is also a progressive failure due to high water pressure concentration around key blocks which was thrown outward when the hydrostatic pressure became greater than confining pressure.

Conclusion

The following conclusion can be drawn from the present study:

• 1. Pelitic schist, chlorotic schist and metasandstone are the dominant rock types observed in the study area. The Rock mass rating indicates metasandstone as a good rock mass and pelitic schist a fair rock mass. Additionally, geotechnical attributes like UCS, tensile strength, and friction angle of metasandstone are greater than that of schist.

• 2. Surface water from small streams percolated through the ground following open joints and tension crack over a period of time causing weathering/degradation of rockmass and creating a high pore-water pressure concentration in some area behind the upper cliff.

• 3. Heavy precipitation two days prior to the event increased the hydrostatic pressure that exceeded the static frictional threshold that caused blocks of schist to bulge due to its porous nature. Thus, the high-water table resulted in an explosion like fracturing of rocks and the expulsion of the block that supported the overlying mass.

• 4. Movement of the key block followed by planar sliding of wedges and foot failure appears to be a possible failure mechanism and propagates towards the crown after the main failure event happened.

• 5. No single slope stability analysis method can explain the failure mechanism of such large natural rock slopes. Various methods should be incorporated for better understanding of the behaviour and failure mechanism of rock slopes.

• 6. According to geometry the slope is kinematically stable, however water forces and slope geometry can cause a decrease in factor of safety. Therefore, similar analysis can be replicated for other critical areas for further stability analysis, owing to heavy rainfall for safe slope conditions. Moreover, water channel management around the slope is necessary to prevent future landslide reactivation.