The Geomechanics of the Dangkhar Landslide, Himachal Pradesh, India

: The Dangkhar Landslide is an extremely large landslide located in the Spiti Valley of Himachal Pradesh, India. The landslide is situated in a remote high mountain desert within the Tethys Himalaya at elevations between 3400 m and 5600 m. It is amongst the five largest continental landslides on earth, covering an area of approximately 54 km 2 and having an estimated volume of 15–20 km 3 . Geomechanical evaluations based on the block theory indicate that the Dangkhar Land-slide formed as a result of unfavorable combinations of structural geological features and complex surface morphology. A massive kinematically removable block is created by a regional synclinal flexure that is crosscut and kinematically liberated by bounding side valleys. Three-dimensional block kinematics are necessary to permit the release of the giant block and its sliding along the synclinal flexure. Pseudostatic slope stability sensitivity analyses incorporating estimates of site seismicity and shear strength parameters suggest that earthquake shaking could have triggered instability if the static factor of safety was less than or in the range of about 1.5–1.9. Considering the glacial history of the region, ice debuttressing represents an additional potential triggering mechanism.


Introduction
The extremely large Dangkhar Landslide located in the remote Spiti Valley, Himachal Pradesh, India at elevations from 3400 to 5600 m covers an area of more than 50 km 2 .The Dangkhar Landslide was first recognized during geotechnical investigations to evaluate structural underpinning options for the foundation of the ancient Dangkhar Monastery [1].Subsequent studies identified landslide features of a massive scale, such as lineaments, trenches and vast accumulations of debris [2][3][4].In the present study, the geological, geomorphological and structural conditions of the landslide are elaborated and incorporated into back-analyses.The purpose of the back-analyses is to elucidate the failure mechanism and potential triggering mechanisms of the Dangkhar Landslide.Although landslide events of this scale are rare, their socioeconomic and environmental impacts are potentially devastating.In the context of climate change, it is not inconceivable that modern societies will be exposed to such events.This is particularly pertinent in alpine regions that are undergoing glacial recession and permafrost thawing.The present study has utilized remote, field and laboratory methods to develop a suitable model for performing stability back-analyses.The overall findings provide insights into potential triggering mechanisms as well as the long-term behavioral characteristics of the landslide.

Materials and Methods
Photointerpretive mapping was performed utilizing high-resolution (up to 1.65 m) satellite images from Maxar Technologies and GeoEye satellites that are accessible through Google Earth Pro and Bing maps.Photointerpretive mapping is concentrated on structural lineaments and geomorphological features that are characteristic of landslides.
Several months of field work was performed to confirm and calibrate the photointerpretive mapping and to assess geologic details concerning the surrounding bedrock and slope failure mechanism.Bedrock units were mapped in the field according to the nomenclature of [5], and geomorphic mapping was performed according to the traverse and boundary methods [6].Additionally, soil samples from sag ponds developed within the Dangkhar Landslide were retrieved using hand augers.Organic material contained in the soil samples were subjected to radiocarbon dating in order to estimate the minimum ages of sag pond formation.
All investigative findings were combined in an ArcGIS [7] framework and presented as thematic maps at a scale of 1:15,000.The thematic maps delineate the areal landslide extent and relate geologic and geomorphologic conditions to a causative failure mechanism and landscape evolution processes.
The Advanced Spaceborne Thermal Emission and Reflection Radiometer Global Digital Elevation Model (ASTER GDEM) dataset [8] was utilized to generate the topographic base map and derivative products such as slope inclination, aspect and hillshade maps in MicroDEM [9].Landslide volume estimates include standard techniques such as the average end-area method, formulas suggested by the UNESCO Working Party on World Landslide Inventory [10] and triangulated irregular network (TIN) difference modeling.The ASTER DEM topography and the basal slip surface geometry inferred from geologic cross sections served as input data to estimate the landslide volume and visualize the landslide in 3D.
Block theory [11] was utilized to evaluate the kinematics, failure mode and stability characteristics of the Dangkhar Landslide.Evaluations of landslide triggering mechanisms considered the seismic setting and late Pleistocene to Holocene glacial history.

Geology, Geomorphology and Climate
The Spiti Valley, Himachal Pradesh, India extends approximately 150 km southeastward from the headwaters at Kunzum pass to Khab, where the Spiti River merges with the Satluj River (Figure 1A,B) at elevations ranging from 6750 to 3400 m [12].The Spiti River receives water from several tributaries, exhibiting a parallel to dendritic pattern (Figure 1B).Geomorphologically, the Spiti Valley is divided into upper and lower reaches.The upper Spiti Valley has been sculpted by Quaternary fluvial, paraglacial and mass wasting processes, and it is characterized by a broad, braided channel that is bounded by fluvial and lacustrine terraces and talus mantled slopes [12,13].The lower Spiti Valley is characterized by a meandering river channel that is bounded by steep slopes and tributaries [14].The study area is located at the transition between the upper and lower Spiti Valley at elevations in the range of 3400 m to 5600 m.
Regional bedrock units are dominated by sedimentary to low-grade metamorphic rocks of the Tethyan (Tibetan) Himalaya (Figure 1C), belonging to the Kuling Group and Lilang Supergroup [5,15].
The study area lies in the rain shadow of the High Himalaya and, according to the Köppen-Geiger climate classification, falls in the Dfb or ET zones, exhibiting a cold climate (D) without a dry season (f), and warm summers (b) or Polar (E) Tundra (T) conditions, respectively [16,17].Typically, the area experiences snowy winters and cool summers with approximately 200 cm of snow and 50 mm of rainfall, respectively [18,19].Precipitation is controlled by the Indian southwest Summer Monsoon (ISM) from June to September and westerly disturbances from December to March [20,21] that transport moisture from the Mediterranean, Black and Caspian Seas [22].Temperatures throughout the year vary from −25 • C in the winter to +15 to 30 • C in the summers [14,19].During the late Pleistocene and Holocene, climatic changes resulted in pulses of glacial advances.In the Spiti Valley, maximum glacial advances extended to the boundary between the upper and lower Spiti Valley [12,24].This significant glaciation coincides approximately with the last glacial maximum (LGM) and is referred to as the regional Chandra stage in the Lahaul-Spiti basin.The timing of the LGM in the NW Himalaya is not yet fully reconstructed and is suspected to fall in the timeframe from 30 to 80 ka [25].According to [26], the Spiti Valley attained its desert character about 25 ka ago, and the thick valley glacier of up to 600 m in size retreated rapidly in the neighboring Chandra valley between 23 and 19 ka [27].A regional Holocene glacial advance occurred between 7.6 and 6.8 ka [28], and pollen records from the upper Spiti Valley indicate a glacial advance from about 2.3 to 1.5 ka ago [26].From the past 0.7 ka onwards, colder climatic conditions prevailed [26], constituting the Little Ice Age (LIA) peaking around the end of the 19th century.Glaciation patterns changed over time from valley to side valley and rock glaciers as a result of reduced precipitation [29,30].Glacial moraine and till deposits within the study area record discrete pulses of glacial advances.

Seismotectonics and Landsliding
The Lahaul-Spiti region is one of the most active tectonic regions in India [31].The 1975 Mb 6.2 Kinnaur earthquake and other major recent earthquakes occurred along the Kaurik-Chango Fault [32].The Spiti Valley Fault (SVF) was triggered during the 1975 Kinnaur earthquake [33].The SVF is a major northwest trending active normal fault that traverses the study area and extends along the valley for about 35 km [24,33].Displaced lake sediments dated at 7.8, 7.4, 6.5 and 6.1 ka document the Holocene activity of the SVF [34], and recent seismicity indicates an average of one to two earthquakes per day with magnitudes of two or less.The majority of SVF earthquakes occur at hypocentral depths of 10-25 km, with rare events recorded at depths up to 70 km [33].
The peak ground acceleration (PGA) data for the study area are on the order of 15 to 20% of gravity [35].According to the Vulnerability Atlas of India [31], the study area falls into zone VIII of the Medwedew-Sponheuer-Kárník scale, where PGA values between 25 and 30% of gravity are typically expected.The Global Seismic Hazard Assessment Program estimates that the study area has a 10% chance of exceeding a PGA in the range of 30 to 40% of gravity within the next 50 years [36].As a comparison, ground motions at the During the late Pleistocene and Holocene, climatic changes resulted in pulses of glacial advances.In the Spiti Valley, maximum glacial advances extended to the boundary between the upper and lower Spiti Valley [12,24].This significant glaciation coincides approximately with the last glacial maximum (LGM) and is referred to as the regional Chandra stage in the Lahaul-Spiti basin.The timing of the LGM in the NW Himalaya is not yet fully reconstructed and is suspected to fall in the timeframe from 30 to 80 ka [25].According to [26], the Spiti Valley attained its desert character about 25 ka ago, and the thick valley glacier of up to 600 m in size retreated rapidly in the neighboring Chandra valley between 23 and 19 ka [27].A regional Holocene glacial advance occurred between 7.6 and 6.8 ka [28], and pollen records from the upper Spiti Valley indicate a glacial advance from about 2.3 to 1.5 ka ago [26].From the past 0.7 ka onwards, colder climatic conditions prevailed [26], constituting the Little Ice Age (LIA) peaking around the end of the 19th century.Glaciation patterns changed over time from valley to side valley and rock glaciers as a result of reduced precipitation [29,30].Glacial moraine and till deposits within the study area record discrete pulses of glacial advances.

Seismotectonics and Landsliding
The Lahaul-Spiti region is one of the most active tectonic regions in India [31].The 1975 M b 6.2 Kinnaur earthquake and other major recent earthquakes occurred along the Kaurik-Chango Fault [32].The Spiti Valley Fault (SVF) was triggered during the 1975 Kinnaur earthquake [33].The SVF is a major northwest trending active normal fault that traverses the study area and extends along the valley for about 35 km [24,33].Displaced lake sediments dated at 7.8, 7.4, 6.5 and 6.1 ka document the Holocene activity of the SVF [34], and recent seismicity indicates an average of one to two earthquakes per day with magnitudes of two or less.The majority of SVF earthquakes occur at hypocentral depths of 10-25 km, with rare events recorded at depths up to 70 km [33].
The peak ground acceleration (PGA) data for the study area are on the order of 15 to 20% of gravity [35].According to the Vulnerability Atlas of India [31], the study area falls into zone VIII of the Medwedew-Sponheuer-Kárník scale, where PGA values between 25 and 30% of gravity are typically expected.The Global Seismic Hazard Assessment Program estimates that the study area has a 10% chance of exceeding a PGA in the range of 30 to 40% of gravity within the next 50 years [36].As a comparison, ground motions at the site that could develop based on the Holocene active regional faults shown in Figure 2 were estimated using the surface rupture length-magnitude regressions in [37] and ground motion attenuation relations for the Himalayan region [38,39].These results are generally commensurate with the values cited in the literature, with the exception of the SVF (Table 1).Based on the regression analysis, the SVF has the potential to generate a PGA on the order of 100% of gravity.
(  [13,24,34,40,41]).CF-Chandra Fault; KCF-Kaurik-Chango Fault; KNF-Karcham Normal Fault; Figure 2. Seismotectonic map of study area and surrounding regions (compiled after [13,24,34,40,41]).CF-Chandra Fault; KCF-Kaurik-Chango Fault; KNF-Karcham Normal Fault; LPF-Leo Pargil Fault; MPF-More Plain Fault; SF-Sarchu Fault; SFC-Syarma Fault Complex; SVF-Spiti Valley Fault; TMF-Tso Morari Fault.Moment tensors shown for major large earthquakes (Pentagrams) (KEQ-Kinnaur Earthquake).Table 1.Estimates of the peak ground acceleration for the study area obtained by the ground motion attenuation relations in [38,39].The fault abbreviations are the same as those in Figure 2. Earthquakes are assumed to occur within ruptured fault segments of the respective faults, which are considered to be about one-third of the total fault length [37].Figure 2 also shows several major late Pleistocene and Holocene landslides that occurred along slopes bordering the Spiti and Sutlej valleys [40].The 1.4 km 3 Mane Landslide and the approximately 0.5 km 3 Lingti Landslide [40] occur partly within the study area.The Parechu Landslide (Figure 2) was triggered during the 1975 Kinnaur earthquake, and the most recent major landslide that was triggered by seismic activity along the SVF occurred in 2009 at a location approximately 25 km northwest of the site [42].

Morphologic Features, Geology and Structure
Bedrocks units involved in the Dangkhar Landside include early Jurassic limestone, dolomite, shale and sandstone of the Kuling Group and Lilang Supergroup.Along the opposing valley flank, polished bedrock indicating glacial abrasion are evident, and along both valley flanks, carbonate breccia outcrops locally occupy positions approximately 200 to 450 m above the Spiti Valley floor.The Mane Landslide deposit unconformably overlies the Dangkhar Landslide and it is expressed as a surge against the Dangkhar Landslide deposit.The Dangkhar Landslide mass consists of materials ranging from silt/sand to big rafted blocks that are several tens of meters in size, which largely preserve their original fabric.The bedrock structure includes an open asymmetric synclinal flexure, with an axial surface that trends subparallel to the Spiti Valley and passes through Dangkhar Village (Figure 3a).The inferred basal slip surface follows the shape of this asymmetric synclinal flexure, where the longer limb of the syncline dips towards the valley and is chiefly located within the shale rich Rama Formation.Southwest of this syncline are the sub-parallel Spiti Valley Anticline and Spiti Valley Fault (Figure 3b).The overall synclinal bedrock structure is exposed near the headscarp and the outcrop around Mt. Palangri (Figure 4).
The geomorphologic features of the Dangkhar Landslide include lineaments, ephemeral stream channels, rock glaciers and secondary landslides of varying scale (Figure 5).Mapped lineaments reach lengths of up to one kilometer and generally exhibit a pattern parallel to the Spiti Valley.The lineaments are expressions of internal ruptures (internal shear surfaces and/or secondary landslides) represented by ridges and trenches (Figure 5a,b).The Dangkhar Lake is situated within a closed depression on the mid-slope bench (Figure 5a) in the central part of the Dangkhar Landslide and receives water from ephemeral streams.These ephemeral streams form channels exhibiting a linear pattern parallel to the dendritic drainage pattern, dissecting the Dangkhar Landslide and partly exposing the underlying rock.
Outcrops of rafted bedrock exposed in ephemeral stream channels reveal large-scale slope tectonic structures.These include graben structures with trapped fine-grained sediments (Figure 6a), displaced bedding planes within rock blocks indicative of toppling with associated obsequent scarps expressed on the surface (Figure 6b) and hummocks related to folded rock strata (Figure 6c).The basal slip surface of the Dangkhar Landslide is not sharply defined but occurs as a transition zone [43], deformation belt [44] or gravitational shear zone [45], exhibiting characteristics commonly associated with mylonitic fault zones.An outcrop north of the Dangkhar Village (Figure 6d) exposed by an erosional gully shows this zone of reworked, crudely bedded bedrock material of the Rama Formation, gradually passing into the overlying Dangkhar Landslide deposits.Deposits representing three generations of rock glaciers occur on the Dangkhar Landslide (Figure 7).The first-and second-generation rock glaciers have frontal slopes of 20 to 25 • and vegetation cover, and they are morphologically subdued.The third-generation rock glaciers exhibit 35 to 40 • steep frontal slopes, an overall lobate shape and sharply defined furrows and ridges without vegetation.These rock glaciers are located in the contemporary permafrost zone [47], indicating their active nature.The rock glaciers record advances down to elevations of 4300, 4500 and 4700 m and represent an overall pattern of progressive retreat.The lineaments occurring within the Dangkhar Landslide debris exhibit cross-cutting relations with the rock glacier deposits (Figure 7a).The lineaments cut through the middle rock glaciers but are truncated by the uppermost rock glaciers (Figure 7b).

Block Theory and Analysis
TIN difference modeling of the basal slip surface, approximated by a second-order local polynomial function based on geologic cross sections, and the ASTER GDEM dataset in ArcGIS 10.1 (Figure 8) yielded a volume of 15.3 km 3 .The volume obtained by the average end-area (AEA) method is 15.7 km 3 .Using formulas proposed by the Working Party on World Landslide Inventory [10], assuming a swell factor of 0.25, results in volume estimates of 19.8 km 3 and 24.7 km 3 for pre-and post-failure volumes, respectively.The overall landslide geometry and landslide volume are adopted and used as input planes for a block theory analysis [11] to assess block kinematics and stability.The input parameters include the orientations of the free slope surfaces, the basal landslide slip surfaces and a resultant force vector orientation representing gravitational loading.The curved nature of the basal slip surface is modeled as a composite surface composed of two best fit tangent planes.The overall areal extent of the removable block is shown in Figure 8c.The block theory results depicted in Figure 9 show that the Joint Pyramid (JP) 00 formed in the upper half space of the two tangent planes is a kinematically removable block with a potential failure mode involving sliding along the tangent planes.The side valleys marking the lateral boundaries of the landslide are necessary for kinematic removability.
The overall landslide geometry and landslide volume are adopted and used as input planes for a block theory analysis [11] to assess block kinematics and stability.The input parameters include the orientations of the free slope surfaces, the basal landslide slip surfaces and a resultant force vector orientation representing gravitational loading.The curved nature of the basal slip surface is modeled as a composite surface composed of two best fit tangent planes.The overall areal extent of the removable block is shown in Figure 8c.The block theory results depicted in Figure 9 show that the Joint Pyramid (JP) 00 formed in the upper half space of the two tangent planes is a kinematically removable block with a potential failure mode involving sliding along the tangent planes.The side valleys marking the lateral boundaries of the landslide are necessary for kinematic removability.

Potential Triggering Mechanisms
The triggering mechanism(s) of extremely large landslides are generally subject to significant uncertainty but typically include seismicity, glacial debuttressing or changes in groundwater conditions [48,49].The potential role of seismicity in triggering the Dangkhar Landslide was assessed in a rudimentary pseudostatic framework.Assuming the hillslope is brought to the condition of limiting equilibrium during an earthquake, the pre-earthquake (static) factor of safety (FoS) and yield coefficient (ky) can be estimated as a function of the shear strength and shear plane inclination.In this simple case, the failure mode is assumed to involve planar sliding.Assuming only frictional shear strength, the dynamic factor of safety is calculated according to Equation (1) [50].Based on the estimated seismic accelerations, the FoS for pseudostatic conditions can be estimated using Equation ( 2 where ky is the yield coefficient and β and ϕ are the shear plane inclination and friction angle, respectively.With an average β value of 20° and for ϕ values ranging from 20° to 40°, Figure 10 illustrates the reduction in an initial static factor of safety (ky = 0) as a function of ky.As an example, a basal slip surface inclined at 20° with a friction angle of 30° has a static FoS of

Potential Triggering Mechanisms
The triggering mechanism(s) of extremely large landslides are generally subject to significant uncertainty but typically include seismicity, glacial debuttressing or changes in groundwater conditions [48,49].The potential role of seismicity in triggering the Dangkhar Landslide was assessed in a rudimentary pseudostatic framework.Assuming the hillslope is brought to the condition of limiting equilibrium during an earthquake, the pre-earthquake (static) factor of safety (FoS) and yield coefficient (k y ) can be estimated as a function of the shear strength and shear plane inclination.In this simple case, the failure mode is assumed to involve planar sliding.Assuming only frictional shear strength, the dynamic factor of safety is calculated according to Equation (1) [50].Based on the estimated seismic accelerations, the FoS for pseudostatic conditions can be estimated using Equation (2) [51].
where k y is the yield coefficient and β and ϕ are the shear plane inclination and friction angle, respectively.With an average β value of 20 • and for ϕ values ranging from 20 • to 40 • , Figure 10 illustrates the reduction in an initial static factor of safety (k y = 0) as a function of k y .As an example, a basal slip surface inclined at 20 • with a friction angle of 30 • has a static FoS of 1.59 and a k y of 0.17.The overall results indicate that ground accelerations ranging from about 0.08 g to 0.26 g could result in a limiting state of stability if the static factor of safety were in the range of 1.25 to 1.93.
1.59 and a ky of 0.17.The overall results indicate that ground accelerations ranging from about 0.08 g to 0.26 g could result in a limiting state of stability if the static factor of safety were in the range of 1.25 to 1.93.Glacial debuttressing (Figure 11a,b) represents an additional potential triggering event.Based on the U-shaped valley geometry, the estimated former valley glacier ice thickness is in the order of 400 to 500 m (Figure 11c).The primary mechanisms leading to failure under the scenario of glacier recession include (1) the removal of the physical stabilizing load applied to the hillslope and exposing the future location of the basal slip surface; (2) the elastic rebound of the hillslope, potentially leading to microcracking and weakening of the rock mass; and (3) increases in slope relief and steepness as a result of glacial downcutting and erosion.Extremely large landslides situated within post-glacial valleys represent long-term changes in slope morphology and stress conditions [52,53].However, strong seismic events can trigger catastrophic landslides and promote phases of accelerated displacement [52,54].In almost every case involving extremely large landslides, the trigger is a combination of several circumstances as the slope attains a critical state [55].
The Dangkhar Landslide is classified as a complex slide principally involving movement along a curved synclinal bedding surface.The failure mechanism also corresponds to the mountain slope deformation type proposed in [56].This type describes large-scale gravitational deformations of high mountain slopes with a relief of more than one kilometer, without a fully defined basal slip surface and phases of extremely slow rates of movement, resulting in the encountered morphological features.Large-scale geomorphic features, such as lineaments formed by ridges, trenches and scarps, but also rafted rock blocks and folded strata, are the result of what is generally referred to as slope tectonics [47,57].Such slope tectonic features resemble those of thin-skinned tectonics and are commonly present in extremely large, slowly or episodically moving landslides [48,58].Crosscutting relationships with rock glaciers suggest that deformations have progressed slowly and in an episodic manner.Glacial debuttressing (Figure 11a,b) represents an additional potential triggering event.
Based on the U-shaped valley geometry, the estimated former valley glacier ice thickness is in the order of 400 to 500 m (Figure 11c).The primary mechanisms leading to failure under the scenario of glacier recession include (1) the removal of the physical stabilizing load applied to the hillslope and exposing the future location of the basal slip surface; (2) the elastic rebound of the hillslope, potentially leading to microcracking and weakening of the rock mass; and (3) increases in slope relief and steepness as a result of glacial downcutting and erosion.Extremely large landslides situated within post-glacial valleys represent long-term changes in slope morphology and stress conditions [52,53].However, strong seismic events can trigger catastrophic landslides and promote phases of accelerated displacement [52,54].In almost every case involving extremely large landslides, the trigger is a combination of several circumstances as the slope attains a critical state [55].
The Dangkhar Landslide is classified as a complex slide principally involving movement along a curved synclinal bedding surface.The failure mechanism also corresponds to the mountain slope deformation type proposed in [56].This type describes large-scale gravitational deformations of high mountain slopes with a relief of more than one kilometer, without a fully defined basal slip surface and phases of extremely slow rates of movement, resulting in the encountered morphological features.Large-scale geomorphic features, such as lineaments formed by ridges, trenches and scarps, but also rafted rock blocks and folded strata, are the result of what is generally referred to as slope tectonics [47,57].Such slope tectonic features resemble those of thin-skinned tectonics and are commonly present in extremely large, slowly or episodically moving landslides [48,58].Cross-cutting relationships with rock glaciers suggest that deformations have progressed slowly and in an episodic manner.

Geochronology and Deformation History
A major glacial advance occurred in the Spiti Valley around 28 to 14 ka [20], and based on erosional and depositional geomorphologic features, the upper Spiti Valley was glaciated (Figure 12-Stage 1) during the late Pleistocene [12,24].The Lahaul and Spiti areas experienced rapid glacier recession between 23 and 19 ka ago after the LGM [27].Subsequent glaciations in the Lahaul-Spiti region were restricted to valley side and rock glaciers [30].Cross-cutting geologic relations between lineaments and three generations of rock glaciers within the landslide deposit suggest that landslide deformations continued episodically since its initiation.
During the recession of the Spiti Valley glacier and glaciers retreating on the Dangkhar slope, massive lodgment till followed by fine grained and rounded proglacial outwash material was deposited and cemented into a carbonate breccia [4].The first major catastrophic movement formed the prominent headscarp around Mt. Chokula and the midslope bench.Meltwater lakes, such as Dangkhar Lake, formed in this area, which transported large amounts of debris during Glacial Lake Outburst Floods (GLOFs) that are recorded in the breccia layer [60] (Figure 12-Stage 2).Such isolated, lithified breccia deposits located at higher elevations above the valley floor represent characteristic remnants of the Chandra glacial stage in the NW Himalaya and are found several hundred meters above the present-day valley floor [61].The Mane Landslide in the southern part of the study area has been dated by radiocarbon at 8.7 ka [34,40] and unconformably overlies the Dangkhar Landslide, supporting early or pre-Holocene initiation.After blocking the Spiti River, the Mane paleolake existed for about 2500 years and led to the deposition of lake sediments on the banks of the Spiti River.First-generation rock glaciers are possibly related to a glacial advance in the region at approximately 7.6 to 6.8 ka [28].Over this time period, lineaments developed through the rock glacier deposits as a result of internal shearing and/or secondary landslides (Figure 12   A soil sample containing organic material was retrieved from a closed depression within the second-generation rock glaciers.The depression is an original depositional feature, and the sample was retrieved at a depth of 55 cm.The total depth of sediment within the depression was 130 cm.Radiocarbon dating provided an age of 0.22 to 0.32 ka, with a pmC (percent modern Carbon) value of 102.6 ± 0.3.Assuming a constant rate sedimentation, the estimated minimum age of sediment accumulation within the depression is approximately 0.52-0.75ka.Lineaments cutting through these rock glacier deposits thus record internal shearing and/or secondary landsliding into the late Holocene (Figure 12-Stage 4).The uppermost thirdgeneration rock glacier deposits overlie these lineaments, and currently, rock fall activity around the headscarp area contributes to talus accumulation on the uppermost rock glaciers.Landslide debris terminating at the left bank of the Spiti River suggests that the Dangkhar Landslide continued to move in a slow and coherent way without long runout distances after its initial catastrophic movement.Erosional action by the Spiti River in the toe area, however, might have contributed to removing evidence of early long runout material.Presently, material from the Dangkhar Landside is remobilized by debris flows that are eroded by the Spiti River (Figure 12-Stage 5).

Summary and Conclusions
The Dangkhar Landslide is an extremely large-scale (giant) landslide.Photointerpretive mapping using satellite images together with on-site field mapping campaigns revealed that the landslide is amongst the five known largest terrestrial, non-volcanic landslides worldwide, covering an area of approximately 54 km 2 and having an estimated volume of 15-20 km 3 .The landslide may have been initiated during the late Pleistocene in a major catastrophic sliding event, and ongoing episodic landslide activity led to the formation of slope tectonic structures such as lineaments, scarps and trenches.
The predominant landslide failure mode involves discrete sliding along an open synclinal surface.The lateral releases formed by the Lingti Valley and Spiti Valley bend are kinematically necessary conditions for failure.Based on the seismic site characterization and rudimentary pseudostatic stability analysis, earthquake shaking emanating from regional fault structures represents a potential triggering mechanism, and the SVF (bordering the landslide toe) is considered a highly capable potential seismic trigger.The basal slip surface of the Dangkhar Landslide is chiefly located within shale rich rocks of the Rama Formation, which, together with the asymmetric synclinal flexure geometry, provide favorable conditions for landslide formation.
The glacial history of the region suggests that glacial debuttressing during the late Pleistocene is another potential triggering factor.Numerical simulations could elucidate this triggering mechanism; however, this would require further information concerning relevant geotechnical parameters.Erosional and depositional remnants of a Pleistocene valley glaciation preserved along the toe area of the Dangkhar Landslide and the Mane Landslide in the southern part of the study area (which unconformably overlies the Dangkhar Landslide) indicate early or pre-Holocene landslide initiation.Cross-cutting relations between lineaments and rock glaciers occurring on the landslide deposit show that the landslide experienced episodes of movement until recent times.Secondary, younger landslides have developed within the Dangkhar Landslide and are concentrated in the toe and head areas.Over time, the landslide experienced glacio-fluvial reworking and redeposition, as documented in lithified carbonate breccia deposits.To a certain extent, these processes continue today.
In a global and regional context, the timing of the Dangkhar Landslide falls in the cluster of Pleistocene to Holocene landslides worldwide and within the northwestern Himalaya [20,52].The clustering of northwestern Himalayan landslides is related to phases of deglaciation and intensified monsoon phases (IMPs) (Figure 13).The monitoring and awareness of slope instabilities in alpine regions will increasingly be necessary in the future to implement early warning systems and mitigation strategies [62].Furthermore, laboratory experiments on hard and soft rock material will contribute to the understanding of landslide mechanics occurring in heterogeneous rock masses [63].
Geotechnics 2024, 4, FOR PEER REVIEW 15 Furthermore, laboratory experiments on hard and soft rock material will contribute to the understanding of landslide mechanics occurring in heterogeneous rock masses [63].Funding: This research received no external funding.

Figure 1 .
Figure 1.The regional setting.(A) The geographic location of the study area within Himachal Pradesh, India.(B) The Spiti Valley with its major rivers and villages.(C) A simplified tectonic map of the Himalaya (modified from [23]).

Figure 1 .
Figure 1.The regional setting.(A) The geographic location of the study area within Himachal Pradesh, India.(B) The Spiti Valley with its major rivers and villages.(C) A simplified tectonic map of the Himalaya (modified from [23]).

Figure 3 .
Figure 3. (a) A geologic map of the study area showing bedrock units and surficial deposits supplemented by morphological and structural features.The locations of the details discussed in Figures 5 and 7 are highlighted.(b) A geologic cross section through the study area.

Figure 3 .
Figure 3. (a) A geologic map of the study area showing bedrock units and surficial deposits supplemented by morphological and structural features.The locations of the details discussed in Figures and 7 are highlighted.(b) A geologic cross section through the study area.

Figure 4 .
Figure 4.A photograph of the Dangkhar Landslide looking in the northwestern direction, highlighting the overall bedrock structure.

Figure 5 .
Figure 5.The major geomorphologic features of the Dangkhar Landslide.(a) A Google Earth Pro satellite image of large-scale lineaments downhill of the Dangkhar Lake.(b) A field photograph of the typical morphology formed by the lineaments.

Figure 4 .Figure 4 .
Figure 4.A photograph of the Dangkhar Landslide looking in the northwestern direction, highlighting the overall bedrock structure.

Figure 5 .
Figure 5.The major geomorphologic features of the Dangkhar Landslide.(a) A Google Earth Pro satellite image of large-scale lineaments downhill of the Dangkhar Lake.(b) A field photograph of the typical morphology formed by the lineaments.

Figure 5 .
Figure 5.The major geomorphologic features of the Dangkhar Landslide.(a) A Google Earth Pro satellite image of large-scale lineaments downhill of the Dangkhar Lake.(b) A field photograph of the typical morphology formed by the lineaments.

Figure 7 .
Figure 7. Cross-cutting relations between rock glaciers and lineaments.(a) Bing maps satellite image of rock glaciers beneath Mount Chokula and cross-cutting relations with lineaments.(b) Field photograph of uppermost rock glaciers overriding lineaments.3.3.2.Block Theory and AnalysisTIN difference modeling of the basal slip surface, approximated by a second-order local polynomial function based on geologic cross sections, and the ASTER GDEM dataset in ArcGIS 10.1 (Figure8) yielded a volume of 15.3 km 3 .The volume obtained by the average end-area (AEA) method is 15.7 km 3 .Using formulas proposed by the Working Party on World Landslide Inventory[10], assuming a swell factor of 0.25, results in volume estimates of 19.8 km 3 and 24.7 km 3 for pre-and post-failure volumes, respectively.

Figure 7 .
Figure 7. Cross-cutting relations between rock glaciers and lineaments.(a) Bing maps satellite image of rock glaciers beneath Mount Chokula and cross-cutting relations with lineaments.(b) Field photograph of uppermost rock glaciers overriding lineaments.

Figure 7 .
Figure 7. Cross-cutting relations between rock glaciers and lineaments.(a) Bing maps satellite image of rock glaciers beneath Mount Chokula and cross-cutting relations with lineaments.(b) Field photograph of uppermost rock glaciers overriding lineaments.

3. 3 . 2 .
Block Theory and Analysis TIN difference modeling of the basal slip surface, approximated by a second-order local polynomial function based on geologic cross sections, and the ASTER GDEM dataset in ArcGIS 10.1 (Figure 8) yielded a volume of 15.3 km 3 .The volume obtained by the average end-area (AEA) method is 15.7 km 3 .Using formulas proposed by the Working Party on World Landslide Inventory [10], assuming a swell factor of 0.25, results in volume estimates of 19.8 km 3 and 24.7 km 3 for pre-and post-failure volumes, respectively.

Figure 9 .
Figure 9. Kinematic and mode analyses for Dangkhar Landslide.Failure mode of removable block is given in parentheses.

Figure 9 .
Figure 9. Kinematic and mode analyses for Dangkhar Landslide.Failure mode of removable block is given in parentheses.

Figure 10 .
Figure 10.Reduction in factor of safety according to yield coefficient.

Figure 10 .
Figure 10.Reduction in factor of safety according to yield coefficient.
-Stage 3).Geotechnics 2024, 4, FOR PEER REVIEW 13 time period, lineaments developed through the rock glacier deposits as a result of internal shearing and/or secondary landslides (Figure 12-Stage 3).

Figure 12 .
Figure 12.Interpretive development of Dangkhar Landslide over time (abbreviations of formation names correspond to those in Figure 3).Figure 12. Interpretive development of Dangkhar Landslide over time (abbreviations of formation names correspond to those in Figure 3).

Figure 12 .
Figure 12.Interpretive development of Dangkhar Landslide over time (abbreviations of formation names correspond to those in Figure 3).Figure 12. Interpretive development of Dangkhar Landslide over time (abbreviations of formation names correspond to those in Figure 3).