Cataclinal Slopes Research Articles

Deformation and collapse of rock slopes considering weak plane orientation

Some slopes experience multiple slides without collapse, while other slopes collapse once they are unstable. The early warning of slope collapse is a difficult but important subject. Considering the influence of weak planes on rock slope deformation and collapse is helpful for interpreting the behavior of deep-seated landslides and designing an early warning system. To investigate the deformation behavior of rock slopes with consideration on weak planes, artificial cemented sand plates were produced and stacked to form physical slope models with different weak plane orientations, where inclined loading was applied to induce the deformation and collapse of the slope. In addition, the deformation of real slopes was examined based on topographic features. The average strain at collapse is referred to as the critical strain, whose value changes for various slopes. Sorted by critical strain in descending order, the slope models include an anaclinal slope with 60° weak planes, an anaclinal slope with 30° weak planes, a cataclinal slope where the 30° weak planes coincide with the slope face, and a cataclinal slope with daylighting 20° weak planes. Similar to the experimental results, anaclinal slopes also present greater average strain values than cataclinal slopes for real slopes. A smaller critical strain implies a higher possibility for slope collapse when unstable. Local deformation does not always lead to collapse, but as the average velocity and the average strain rate of the sliding body increase, or the velocity ratio (VR) between the upper and lower parts of the sliding body approaches 1, a sliding surface inside the slope is likely developing and coalescing. Hence, such deformation features may contribute to a landslide warning system.

Geomorphology and evolution of the Blanes Canyon (NW Mediterranean). New insights from high resolution mapping of vertical cliffs

The geomorphology of the Blanes submarine canyon has been characterized based on the analysis of high-resolution hull-mounted multibeam bathymetry (up to 5 m grid size). Additionally, the Hybrid Remotely Operated Vehicle (H-ROV) Ariane was used to map with unprecedented detail (80 cm grid size) the morphology of vertical rockwalls on the canyon flanks, and the ROV Liropus was used to acquire high-resolution video footage on the same walls. The Blanes Canyon exposes well stratified Miocene and Plio-Quaternary successions dipping towards a SW direction, which are affected by NE-SW and NW-SE oriented fault systems that played an important role in the canyon development. Its structural character is evidenced by the rectilinear trajectories and sharp bends of the canyon axis and rims, the alignment of pockmark fields and the presence of vertical rockwalls. The canyon transversal profile is markedly asymmetric due to the underlying stratigraphy. The eastern flank, corresponding to the cataclinal slope characterized by strata beds dipping in the same direction as the slope, is smooth and mainly dominated by slides and toe gullies. The western flank, corresponding to the anaclinal slope facing opposite to the dip of the strata, is steeper and dominated by a dendritic network of rim gullies. H-ROV bathymetric maps and video footage of vertical walls display a wide variety of fine-scale morphological elements that evidence ongoing mass wasting and gully development as the main mechanisms involved in canyon wall retrogression. The multi-scale study of the Blanes Canyon has allowed a better characterization of the erosive processes involved in the broadening of submarine canyons and retrogression of the walls, thus contributing to the better understanding of the evolution of submarine canyons developed in tectonically controlled stratigraphic bedded sequences.

Complex landslide patterns explained by local intra-unit variability of stratigraphy and structure: Case study in the Tyee Formation, Oregon, USA

Lithology and geologic structure are important controls on landslide susceptibility and are incorporated into many regional landslide hazard models. Typically, metrics for mapped geologic units are used as model input variables and a single set of values for material strength are assumed, regardless of spatial heterogeneities that may exist within a map unit. Here we describe how differences in bedding thickness, grain size, inferred uniaxial compressive strength, and bedding dip control the inherent susceptibility of slopes to deep-seated failure within a single mapped geologic unit - the Tyee Formation of Oregon, USA. The Tyee, which covers over 15,000 km2 and underlies much of the Oregon Coast Range, comprises gently folded alternating beds of sandstone and siltstone deposited as turbidites, forming a 2-km thick Eocene submarine fan which has been uplifted and exhumed through the Cenozoic. Deep-seated landslides are widespread in the Tyee, but form a complex spatial pattern such that landslide density ranges from 0 to 24% of the total landscape area. These slides are often extensive and sufficiently deep to reduce local hillslope gradients, resulting in a strong negative correlation between landslide density and mean local slope. Mean annual precipitation and predicted strong ground motions from Cascadia earthquake scenarios also fail to explain the spatial distribution of deep-seated landslides. Consequently, landslide stability models, which are strongly influenced by landscape slope, pore-water pressure, and seismic acceleration, yield landslide susceptibility maps which are broadly anti-correlated with mapped deep-seated landslide density. Through a multivariable linear regression model, we show that much of the variance in deep-seated landslide density can be explained by variability of intra-unit stratigraphic and structural characteristics, which we measure at 128 sites across two study areas totaling ∼3000 km2. Our results suggest bedding dip is only weakly correlated to landslide density, but strongly influences landslide failure style. Subtle increases in bedding dip, even in the gently folded Tyee Formation, result in a substantially higher likelihood of a landslide being cataclinal, or parallel to bedding. Overall, we find a slight majority of landslides fail within these cataclinal slopes, and that these landslides tend to be larger than non-cataclinal landslides. We also show that the lithological and structural properties that influence landslide susceptibility are distinct for these two populations of landslides. Our results demonstrate how localized, intra-unit, geologic variability can exert strong control on landslide susceptibility and failure style. This suggests that in some locations, landslide hazard models could be significantly improved by incorporating detailed, spatially variable, geologic properties rather than relying solely on generalized geologic map units.

A series of rockslides and gravitational slope deformations aligned along the Kali Gandaki across the Nepal Himalaya

Uplifting mountains are denudated by gravitational deformation, landslides, and glacial or fluvial erosion. We investigated the response of valley slopes to glacial and fluvial erosion along the Kali Gandaki (KG) River, one of the largest rivers that flows through the High Himalaya southward from Tibet to India and diagonally cuts the strikes of gneiss and slate foliations. Satellite images with 2.5 m resolution and 5-m Digital Elevation Models (DEMs) were used for the investigation. Within a 20-km reach of the KG River, we found a series of 11 landslides, which vary from a total area of 0.58 to 12 million m2 each. The foliations generally strike NW–SE and dip at 30° to 50° to the NE, so the left bank slopes are anaclinal and the right bank slopes are cataclinal. On the cataclinal slopes, buckling of beds occurs, and gradual deformation proceeds in slate and two-mica gneiss, while in the more competent calc gneiss, buckling deformation develops into a rock avalanche. Such rock avalanches are represented by the largest Dhampu-Chhoya landslide, which once blocked the KG River and drastically changed the river morphology. Anaclinal slopes of two-mica gneiss were toppled and transformed into rock avalanches on three slopes. The KG River has a prominent fluvial knickpoint, which is currently at an elevation of 1800 m, and it has slope breaks on the left bank that are 200–400 m above the riverbed downstream of the knickpoint. Toppling was observed on slopes higher than slope breaks, which suggests that toppling began before the formation of slope breaks by fluvial incision and that the lower slopes did not adapt to the younger fluvial incision. In contrast, slope breaks are scarcely found on the right bank, probably because slow-moving DGSDs, which followed the river incision, erased them.

Analysis of Stability of Concealed Cataclinal Slopes with Talus Deposits Accumulated at Their Toes with Respect to Biplanar Failure

Talus deposits naturally form to a certain height in the toes of slopes because of the accumulation of collapsed material and small landslides. However, the stability of concealed cataclinal slopes with talus deposits accumulated at their toes (referred to as TDAT-CC slopes) with respect to biplanar failure is rarely studied. In this work, the typical geological profiles associated with TDAT-CC slopes are summarized and generalized and their failure modes are classified. A qualitative analysis of their stability is also carried out. The biplanar failure process is investigated by analyzing the development of plastic failure and displacement deformation using Universal Distinct Element Code. The analysis reveals the penetration process occurring in the biplanar sliding planes and clarifies the compression–shear failure processes taking place in such slopes. A new method for determining the stability of TDAT-CC slopes and predicting the failure surface formed is proposed based on limit-equilibrium theory and a model consisting of sliding mass blocks. Finally, the proposed method is used to calculate the stability of TDAT-CC slopes with deposits of different shapes and strengths and the results compared with the results of numerical simulations. The results obtained using the two methods are generally consistent. The talus deposits at the toe of the slope play the role of a toe buttress and help improve the slope’s stability. The safety factor of the slope increases as the inclination of the toe deposits becomes gentler; it also increases as the height of the toe deposits increases. The method proposed offers a sound basis for investigating biplanar failure in TDAT-CC slopes.

Distribution of gravitational slope deformation and deep-seated landslides controlled by thrust faults in the Shimanto accretionary complex

The Cretaceous Shimanto accretionary complex is a typical accretionary complex uplifted without significant deformation after accretion and in which many rain-induced deep-seated landslides (DLs) have occurred. We undertook a thorough geological and geomorphological investigation of the middle Kii Peninsula, where many rain-induced DLs occurred in 1889 and 2011 during intense and prolonged rainfall associated with typhoons. Geomorphological investigations using 1 m digital terrain models (DTMs) obtained from airborne light detection and ranging (LiDAR) before and after the 2011 typhoon allowed us to examine the microtopography (e.g., small scarps and linear depressions) and to locate deep-seated gravitational slope deformations (DGSDs) and DLs. Low-angle thrust faults with >1 m thick incohesive fault rocks were found to be aligned subparallel to each other with spacings of a few kilometers and control the distribution of DLs and DGSDs. When a thrust fault forms a cataclinal slope, river incision removes the overburden and undercuts and destabilizes the slope. This process causes gravitational deformation, which forms downhill-facing scarps and linear depressions at higher elevations on the slope. Tephrochronology applied to the deposits in the DGSD-induced depressions revealed that such deformation began at least 30,000–50,000 years ago and is ongoing. When a high-angle fault strikes along the line of maximum slope, it separates a volume of the rock mass from the juxtaposing slope and facilitates gravitational slope deformation and ultimately catastrophic failure. A rock mass on a thrust fault that forms a downslope-plunging syncline can also be gravitationally deformed and fail. A crush zone of a large-scale thrust fault is a continuous weak layer that also forms a barrier to groundwater flow, thus leading to a rapid increase in pore-pressure during rainstorms, potentially triggering catastrophic slope failure. Accretionary complexes, which typically contain thrust faults with incohesive fault rocks, thus provide a location for DGSDs and DLs.

Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau

A number of large landslides, which have strongly affected fluvial processes in the upstream catchment of the Minjiang River along the eastern margin of the Tibetan Plateau, Sichuan, were investigated with regard to both geological structure and topographic development. Topographic analysis suggested that there is a major knickpoint that formed not as a result of landslides but via tectonic activity and that the knickpoint propagated upstream, forming an inner gorge and undercutting and destabilizing nearby slopes. The effects of the knickpoint propagation and inner gorge formation on slope stability are dependent on the geometrical relationships between the river and geological structures. When the geological trend is normal or highly oblique to the river axis, landslides generally do not occur, but gigantic rockslides have occurred on one side of the Minjiang valley, which can be attributed to a wedge structure consisting of bedding planes and joints with intersections dipping valleyward and tight folds with hinges plunging valleyward. When the beds are planar, moderately dipping, and strike nearly parallel to the river, buckling deformation commonly occurs on cataclinal slopes, often transforming into catastrophic failure. Landslide dams form another type of knickpoint on the river channel that gradually disappear from the downstream to upstream as a result of river erosion. Our findings strongly suggest that the study of slope development by river incision must consider geological structures and that an understanding of geological structures and river incision history can provide a conceptual model for the prediction and mitigation of geohazards in tectonically active drainage basins.

The Influence of Groundwater on the Sliding and Deposition Behaviors of Cataclinal Slopes

In 2015, Typhoon Soudelor caused a number of slopes to collapse in Wulai District of New Taipei City. One of these landslides took place in the village of Zhongzhi and involved atypical cataclinal slope failure with a rock–soil interface. The remaining rock in the slope and the rock that originally covered it contained vertical joints, so groundwater could have flowed through the joints and influenced landslide behavior. However, few existing studies have examined the influence of upward groundwater flow on slope stability. To fill this gap, this study used physical tests and discrete element method software to conduct relevant investigations. We first conducted tests using the ground water flow and cataclinal slope simulator, in which water can flow out of holes in the platform to simulate upward-seeping groundwater. We used gypsum boards or rhombus-shaped grinding stones to simulate rock with vertical joints and round grinding stones mixed in paste to simulate cohesive regolith. The objective of the tests was to understand the influence of water flow on the landslide behavior of the specimens and the connections between movement behavior and the sequence of sliding between different materials during the landslide. We then reproduced the physical tests using discrete element method software PFC3D (Particle Flow Code 3D Version 4.0 by Itasca, Minneapolis, MN, USA) to display the influence of water flow on specimens, including the weakening of bond strength, decreasing coefficient of friction between particles, and the application of seepage force, as well as uplift and lateral forces caused by water pressure. This process gave us an understanding of the influence of different groundwater conditions on landslide behavior, which facilitates the study of landslide mechanisms and movement behavior. Finally, we applied the water flow influence settings to simulate and examine the Zhongzhi landslide process. Compared to methods that simply reduce the friction coefficients to trigger landslides, our numerical simulation was closer to reality in that in this case a rising water table triggered the landslide.

Investigation of rainfall-induced failure processes and characteristics of cataclinal and anaclinal slopes using physical models

In this study, two types of small-scale physical modeling tests have considered the impact of the infiltration of rainfall and groundwater level in order to investigate the processes involved in rock slope deformation and failure. The study conducted the physical tests under controlled conditions of groundwater level with rock block shape for two rock slope types (cataclinal and anaclinal slopes). Observations obtained during each stage of deformation and failure were used to explain how gravity deformation varies with groundwater conditions on cataclinal and anaclinal slopes, and infer how rainfall and groundwater influence slope failure. Our results indicate that groundwater level is a crucial factor in the deformation failure of slopes. The failure mechanisms of cataclinal slopes differ considerably from those of anaclinal slopes. The infiltration of surface water and groundwater can have a significant influence on rock layer deformation and the speed of failure. Different shapes of rock block have two toppling types of rock slope, the spherical rock model relatively close to flexural toppling type and the triangular rock model relatively similar to block flexural toppling type, respectively. Details of the failure characteristics of cataclinal and anaclinal slope models are discussed in this paper.

Suction and rate-dependent behaviour of a shear-zone soil from a landslide in a gently-inclined mudstone-sandstone sequence in the Sichuan basin, China

Bedding-controlled landslides in alternated mudstone and sandstone beds in cataclinal slopes are common in the Sichuan basin and in the Three Gorges area in China. They are characterised by very gentle dip angles of the beds (<10°), and usually exhibit slow creep movements throughout the year. However, during the rainy season, they can have sudden accelerations and undergo large displacements. In this work, the influence of saturation degree, displacement rate and stress state on the shear strength of the shear-zone soil was investigated. The results of the displacement-controlled ring-shear tests show a rate-weakening behaviour even at low rates of displacement. Dually, the shear-controlled tests show that the displacement rate can increase dramatically in response to a slight increase of the shear stress. The threshold stress that triggers large displacement is equal to the rate-dependent residual shear strength of the soil.

Geologic structure, mechanism, and conditions for rock topples on cataclinal slope, Jinchuan, China

Two rock topples with the opposite dip direction, observed in the Ribangliangzi hill and facing the China Provincial Highway S211, are located on the upper stream of the planned dam of Jinchuan Hydropower Station in China. The new site for submerged part of Highway due to reservoir impoundment is designed to be a tunnel crossing the Ribangliangzi hill. However, at this hill, the stratum facing the Dadu River dips southwestward, whereas the stratum facing the Xinzha Gully dips northeastward. Therefore, it’s regarded as an important slope structure that may lead to an occurrence of landslide directly threatening the future tunnel excavation stability and slope stability along the highway. This paper reports for the geologic structures of the overturned strata, formative processes and mechanism for toppling on cataclinal slopes. It was concluded that the toppling on cataclinal slopes was caused by a combination of toppling and sliding. Furthermore, to analyse the conditions for toppling on cataclinal slope, the cantilever beam model was established and analyzed the minimum strata dip angle required for toppling more prone to occur on cataclinal slope and the critical length of rock stratum. The results reveal that the toppling will be more prone to occur on cataclinal slope while the strata dip angle is greater than 60°even if under a very small external force or without the assistance of external forces.

From incipient slope instability through slope deformation to catastrophic failure — Different stages of failure development on the Ivasnasen and Vollan rock slopes (western Norway)

The long-term evolution of rock slope failures involves different stages, from incipience of slope instability to catastrophic failure, through a more or less long-lasting slope deformation phase that also involves creeping or sliding. Topography, lithology, and structural inheritance are the main intrinsic factors that influence this evolution. Here, we investigate the role of these intrinsic factors on the rock slope failure development of the Ivasnasen and Vollan rock slopes (Sunndal Valley, western Norway) using a multitechnique approach that includes geomorphologic and structural field mapping, kinematic analysis, terrestrial cosmogenic nuclide exposure dating, topographic reconstruction, and deformation quantification. Ivasnasen is a rock slope failure complex with several past rock slope failures and a present unstable rock slope, located on a cataclinal NW-facing slope and developed in augen gneiss. Vollan on the opposite valley side is a deep-seated gravitational slope deformation (DSGSD) affecting the whole mountainside, developed in quartzite in the upper part and micaschist in the lower part. These different lithologies belong to different nappe complexes that were emplaced and folded into a series of syn- and anticlines during the Caledonian orogeny. These folds lead to different lithologies being exposed in different structural orientations on the opposite valley flanks, which in turn leads to different types and evolution of rock slope failures. At Ivasnasen the 45°–55° NW-dipping ductile foliation allowed for a fairly simple planar sliding mechanism for the 1.2millionm3 post-glacial rock slope failure. Failure occurred ca. 3.3ka ago after a short period of prefailure deformation. For the present 2.2millionm3 unstable rock slope at Ivasnasen, a steepening of the foliation at the toe impedes such a mechanism and up to 10m of displacement has not lead to a catastrophic failure yet. The Vollan DSGSD is characterized by a steep major back scarp with its up to 130-m-wide graben, which opened along the subvertical foliation in the quartzite, and a conspicuous array of counter-scarps in the micaschist. The morphologic features are explained by a flexural toppling mechanism in the micaschist, which likely initiated in the lower slope section and propagated retrogressively until reaching the massive quartzite at the back scarp. Flexural toppling cannot solely explain the total along-slope displacement of up to 200m, i.e., an elongation of 28%. Creep along a basal shear zone, which may have developed in the hinge zone of the flexural toppling, combined with shallow valley-dipping joints, likely accounts for the large elongation. Implications of this study for the hazard assessment of unstable rock slopes in Norway include the relatively rapid development of instabilities in cataclinal slopes and the important insights on long-term displacement rates gained from cosmogenic nuclide dating for a better understanding of the evolution of unstable rock slopes.

Deep-seated landslides affecting monoclinal flysch morphostructure: Evaluation of LiDAR-derived topography of the highest range of the Czech Carpathians

The recent launch of national-scale LiDAR-derived topographic data provides an opportunity for (re)evaluating the geomorphic imprint of landslides in forested mountains. A LiDAR-based inventory of deep-seated landslides (DSLs) in the highest part of Czech Outer Western Carpathians (Moravskoslezské Beskydy Mts.) reveals that nearly 20% of the territory is affected by ancient DSLs. Rather than the topography itself, we show that the geological characteristics of flysch rocks control the density, scaling relationships and morphometry of DSLs. They reveal a strong tendency to clustering in areas with favourable structural conditions, such as cataclinal slopes of individual monoclinal ridges formed by thick-bedded sandstones. In comparison with high-mountain areas and regions dominated by weaker lithology (e.g., claystone-dominated flysch), sandstone-dominated monoclinal structure of the Moravskoslezské Beskydy Mts. features predominantly short-travelled DSLs that are arrested on slopes and thus reveal a higher slope gradient than non-landslide terrain. Finally, we propose a conceptual model for the evolution of DSLs on monoclinal flysch morphostructures, particularly where coherent sandstone caps overlie weak claystone/siltstone-dominated flysch. In these conditions, we distinguish between “low” and “high” monoclinal ridges, where the former is capped by relatively thin sandstones overlying claystone formations and sandstones predominate in the structure of the latter. We show that the density of DSLs on the cataclinal/dip slopes is similar on both types of monoclinal ridges, but that major differences exist in the evolution of anaclinal/escarpment slopes. Here, low monoclinal ridges are highly affected by DSLs, but escarpments of high monoclinal ridges are nearly devoid of any DSLs.

Identification of deformation and failure characteristics in cataclinal slopes using physical modeling

This study investigates the deformation characteristics of cataclinal slopes in central Taiwan prior to landslide failure. Field surveys and physical model tests were performed to explain the gravitational deformation characteristics of cataclinal slopes under various conditions and to derive the deformation process and failure characteristics. The results show that the distribution of erosion gullies (different length of the slope mass), the extent of erosion (different thickness of the slope mass), the foliation dip angle, and the geological material critically affect the deformation of cataclinal slope masses in the study area. The results of physical model tests indicate that increasing the foliation dip angle, the thickness and the length of sliding mass, particle size (spacing between foliations) increases the depth of slope deformation. Foliation dip angle is the most critical factor that controls the deformation of slate slopes. When the cataclinal slopes reached maximum deformation, a shear failure and translational slide occurred within a short period. The deformation zone exhibited significant cracking at the scarp and the bulging of the slope toe, which facilitated the infiltration of surface water and groundwater, accelerating the deformation to failure.

Deep-seated gravitational deformation of mountain slopes caused by river incision in the Central Range, Taiwan: Spatial distribution and geological characteristics

Fluvial undercutting can cause the large-scale destabilization of mountain slopes and induce deep-seated gravitational slope deformation (DGSD) that eventually leads to catastrophic failures. This study analyses these processes in the catchment of the Dahan River, northern Taiwan. The area has experienced phases of river incision of up to 600m since the middle to late Pleistocene, and middle Pleistocene paleosurfaces at higher elevations were incised to form V-shaped valleys. A DGSD inventory was compiled by visual interpretation of high-resolution images, and field surveys were conducted to clarify the geological structures associated with DGSDs and major rockslide-avalanches. The observed relationships between DGSDs and the topography modified by long-term river incision show that about 53% of all DGSDs occurred on slopes along the rims of paleosurfaces, which is approximately 2 and 2.5 times as many as in the incised valleys and on the paleosurfaces, respectively. A comparison of the power-law scaling exponents of cumulative frequency–area distributions shows that the frequency of DGSDs along the rims of the paleosurfaces decreases less rapidly with DGSD area than for DGSDs on other slopes. Geological investigations suggest that there are two dominant types of DGSD: one is flexural toppling of slate and argillite with high-angle cleavage, and the other is buckling of alternating beds of sandstone and mudstone on parallel or underdip cataclinal slopes. Catastrophic landslides observed along or below the rims of paleosurfaces were preceded by buckling of alternating beds of sandstone and shale. These beds dipped at 50° to 58°, and each bed was 10−1–100m thick. This study suggests that the peripheral zones of the paleosurfaces may be most susceptible to future catastrophic landslides, particularly on parallel or underdip cataclinal slopes comprising alternating beds of sandstone and mudstone dipping at 50° to 60°. Thus, an understanding of the geological characteristics, spatial distribution of DGSD occurrence, and landscape evolution within a particular area can provide important information for landslide hazard-level zoning.

Pregnant Mechanism and Mechanical Model of Rock Landslides in Chair-Shaped Cataclinal Bank Slopes

The pregnant and sliding mechanism was firstly compared between Vajont and Qianjiangping large rock landslides, which happened in the chair-shaped cataclinal bank slopes. And then the essence of the landslides occurred was deduced. The pregnant and sliding process of the landslides was showed two stages. First, after the first impoundment of the reservoir, the soil and shallow rock at the front of the landslides were slipped and then caused posterior margin of the deep latent sliding surface cracks. So the groundwater supply condition was significantly changed. Second, with the rapidly increase of the groundwater recharge, its pressure on the front of the landslides was also rapidly rise to the critical value which the landslides occurred. Afterwards, the stability mechanical model of the chair-shaped cataclinal bank slopes and critical value formula of the groundwater pressure were established on the basis of the stability mechanical model of general chair-shaped cataclinal slopes, added reservoir water level which was a dynamic load. While working procedures to determine the groundwater pressure was put forward. Last, the theory was certified validity take Liangshuijing rock landslide of the Three Gorges Project area as an example and the monitoring methods and control measures for the landslide were proposed.

A method for the assessment of the influence of bedding on landslide abundance and types

Bedding planes are a known factor that controls the type, abundance and pattern of landslides. Where layered rocks crop out, the geometrical relationships between the attitude of the bedding and the geometry of the terrain is crucial to understand landslide phenomena. Obtaining information on bedding attitude for large areas through field surveys is time-consuming, and resource intensive, hampering the possibility of quantitative investigations on the control of bedding planes on landslides. We propose a GIS-based method to extract information on bedding planes from the analysis of information captured through the visual interpretation of stereoscopic aerial photographs and a digital representation of the terrain. We tested the method in the Collazone study area, Umbria, Central Italy, where we used spatially distributed information on beddings and terrain information obtained from a 10 × 10-m DEM to determine morpho-structural domains. We exploited the morpho-structural terrain zonation, in combination with landslide information for the same area, to investigate the role of beddings in controlling the distribution and abundance of landslides in the study area. We found that beddings condition the location and abundance of relict and deep-seated landslides, most abundant in cataclinal slopes, and do not condition significantly the shallow landslides. We expect the method to facilitate the production of maps of morpho-structural domains in layered geological environments. This will contribute to a better understanding of landslide phenomena and to foster the preparation of advanced landslide susceptibility and hazard models.

Inheritance of ductile and brittle structures in the development of large rock slope instabilities: examples from western Norway

Abstract The high density of slope failures in western Norway is due to the steep relief and to the concentration of various structures that followed protracted ductile and brittle tectonics. On the 72 investigated rock slope instabilities, 13 were developed in soft weathered mafic and phyllitic allochthons. Only the intrinsic weakness of such rocks increases the susceptibility to gravitational deformation. In contrast, the gravitational structures in the hard gneisses reactivate prominent ductile or/and brittle fabrics. At 30 rockslides along cataclinal slopes, weak mafic layers of foliation are reactivated as basal planes. Slope-parallel steep foliation forms back-cracks of unstable columns. Folds are specifically present in the Storfjord area, together with a clustering of potential slope failures. Folding increases the probability of having favourably orientated planes with respect to the gravitational forces and the slope. High water pressure is believed to seasonally build up along the shallow-dipping Caledonian detachments and may contribute to destabilization of the rock slope upwards. Regional cataclastic faults localized the gravitational structures at 45 sites. The volume of the slope instabilities tends to increase with the amount of reactivated prominent structures and the spacing of the latter controls the size of instabilities.

Geological structures of large landslides in Japan

There are many large catastrophic landslides and volcanic collapses in Japan. In this paper, the large landslides (with volumes more than a million cubic metres) are categorised into three groups based on their geological structure, slope morphology, and triggering factors. The first type of landslide occurs on a cataclinal slope where the slope and bedding (or foliation) are dipping in the same direction. The landslide occurring on such a slope may be preceded by scarplets with arcuate trace convex upslope on a plan view. The second type of slide is found on a slope with bedding or foliation that dips steeply in the depth and is toppled towards the valley due to mass rock creep. This kind of rock deformation is recognised to precede many landslides in Japan with its geomorphological expressions of linear depressions and multiple ridges. The third type of landslide is found on the slope with a thick cap rock of limestone and/or greenstone underlain by a chaotic mixture of mudstone with scaly cleavage and rock blocks of sandstone and chert.

Some forms of mountain peaks in the Canadian Rockies controlled by their rock structure

Different mountain shapes in sedimentary sequences in the Canadian Rockies were enhanced by glacial erosion and have been modified post-glacially by gravity-driven slope processes. Slope modification by both glacial erosion and post-glaciation landslides is related to slope geometry and rock structure, particularly bedding dip. Five mountain peak shapes in monoclinal sequences each fall into different ranges of bedding dips. (1) Castellate and (2) matterhorn mountains occur in sub-horizontal beds and their slopes are generally 35–65° and oblique to both bedding and joints. (3) Cuestas develop in gently to moderately dipping beds. Dip slopes and steeper, normal escarpments form their cataclinal and anaclinal sides, respectively, with the dihedral angle between them about 90°. (4) Hogbacks in moderately to steeply dipping beds have similar slope angles on both cataclinal and anaclinal slopes. Cataclinal slopes are either dip slopes or underdip slopes but anaclinal slopes are often steepened escarpments, the dihedral angle between the slopes is usually <90°. (5) Dogtooth mountains occur in steeply dipping to sub-vertical beds and the dihedral angle can be as low as 60°. Slope gradients on a sample of 34 peaks are closely related to landslides, particularly rock slides and rock topples. These landslide modes are controlled by bedding dips.