Translational Slides Research Articles

Automatic landslide length and width estimation based on the geometric processing of the bounding box and the geomorphometric analysis of DEMs

Abstract. The morphology of landslides is influenced by the slide/flow of the material downslope. Usually, the distance of the movement of the material is greater than the width of the displaced material (especially for flows, but also the majority of slides); the resulting landslides have a greater length than width. In some specific geomorphologic environments (monoclinic regions, with cuesta landforms type) or as is the case for some types of landslides (translational slides, bank failures, complex landslides), for the majority of landslides, the distance of the movement of the displaced material can be smaller than its width; thus the landslides have a smaller length than width. When working with landslide inventories containing both types of landslides presented above, the analysis of the length and width of the landslides computed using usual geographic information system techniques (like bounding boxes) can be flawed. To overcome this flaw, I present an algorithm which uses both the geometry of the landslide polygon minimum oriented bounding box and a digital elevation model of the landslide topography for identifying the long vs. wide landslides. I tested the proposed algorithm for a landslide inventory which covers 131.1 km2 of the Moldavian Plateau, eastern Romania. This inventory contains 1327 landslides, of which 518 were manually classified as long and 809 as wide. In a first step, the difference in elevation of the length and width of the minimum oriented bounding box is used to separate long landslides from wide landslides (long landslides having the greatest elevation difference along the length of the bounding box). In a second step, the long landslides are checked as to whether their length is greater than the length of flow downslope (estimated with a flow-routing algorithm), in which case the landslide is classified as wide. By using this approach, the area under the Receiver Operating Characteristic curve value for the classification of the long vs. wide landslides is 87.8 %. An intensive review of the misclassified cases and the challenges of the proposed algorithm is made, and discussions are included about the prospects of improving the approach with further steps, to reduce the number of misclassifications.

Identifying hydrological pre-conditions and rainfall triggers of slope failures at catchment scale for 2014 storm events in the Ialomita Subcarpathians, Romania

This paper addresses a regional-scale analysis of the rainfall-induced landslides for 2014 storm events based on detailed hydro-meteorological data set in the Ialomita Subcarpathians. This area is located in the western part of the Curvature Subcarpathians, a complex geological and geomorphic unit in Romania. The high temporal frequency of landslide events in the last decades (1997, 1998, 2005, 2006, 2010, 2012 and 2014) leads us to considerer that these processes play a major role in the evolution of this area’s landscape where the most frequent landforms are rotational slides, translational slides, mudslides and complex movements. The rainy period between April and August 2014 induced numerous flash floods and landslides in this specific area that resulted to severe economic losses estimated to 8 million euros in Dambovita County. Spatially distributed rainfall during the main storm events estimated from adjusted radar-based precipitation was used to investigate the hydro-meteorological conditions that triggered or not landslides in the Ialomita Subcarpathians. Hydrological pre-conditions were assessed by hourly in situ soil moisture measurements at local scale and hydrological modelling at regional scale. ModClark semi distributed model implemented in HEC HMS software that integrates radar data was used to analyse catchment response to the main rainfall event that resulted to landslides in 2014. Analysis between rainfall, soil moisture conditions and direct runoff was performed for identifying the contribution of the hydro-meteorologic conditions to landsliding process in the Ialomita Subcarpathians. A detailed landslide inventory based on field mapping and visual interpretation of satellite and aerial images was completed with information from local authorities and mass media. Despite the limited number of landslide events, this study allows a detailed insight of understanding the influence of rainfall in landslide occurrence in this specific area.

Characterization of Failure Parameters and Preliminary Slope Stability Analysis of the Cedar Canyon Landslide, Iron County, Utah

The 2011 Cedar Canyon landslide, eight miles (12.8 km) east of Cedar City, Utah, severely damaged State Route 14. Bedrock stratigraphy at the site consists of the cliff-forming Straight Cliffs Formation and the underlying slope-forming Tropic Shale and Dakota Formation. The lower part of the Straight Cliffs Formation and all of Tropic Shale and Dakota Formation are covered with a thick deposit of colluvial soil that has accumulated at the base of the steep cliff formed by the Straight Cliffs Formation. The landslide initiated in the colluvium and propagated as a translational slide along the contact between the colluvial soil and underlying bedrock units. The Utah Department of Transportation (UDOT) drilled and installed slope inclinometers in three borings and conducted a geophysical survey from the crest to the toe of the landslide. We collected orientation data for 186 discontinuities from the Straight Cliffs and Dakota Formations to determine the principal joint sets for use in kinematic analysis; tested samples of colluvial soil and bedrock to determine natural water content, density, and shear strength parameters of soil, bedrock, and the soil-bedrock contact; used SLIDE software to perform a preliminary stability analysis; and performed a sensitivity analysis with respect to varying positions of the water table. Results show that a combination of relatively steep colluvial slope (~30°), extra weight from previous rockfalls, and buildup of pore pressure due to a rainstorm prior to failure, contributed to the landslide.

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.

Geotechnical and landslide aspects in weathered granitoid rock masses (Serre Massif, southern Calabria, Italy)

The weathering processes produce important chemical and physical transformations in rock masses producing a decay of their strength and then stability. In this study different methodologies were employed to define the physical-mechanical properties and the minero-petrographical changes at each grade of weathering, useful to determine a geological model of the weathered granitoids involved in mass movements. The paper focuses on a multidisciplinary research on the weathering profiles of the Paleozoic granitoid rocks surrounding the village of Fabrizia located in the Serre Massif (southern Calabria, Italy). Fabrizia lies on an old erosion surface composed of basement rocks made up of weathered granitoids, belonging to the Serre Massif granitoid complex, and overlain by thin eluvial-colluvial deposits.The tectonic uplift of the area combined with Mediterranean climatic conditions, both active at least since the Quaternary, are the main causes of the deep weathering of the plutonic rocks. The weathered profiles of the granitoid complex have been studied through field observations, borehole explorations with geotechnical tests and seismic surveys coupled to minero-petrographical analysis and laboratory tests. Weathered profiles are mainly composed by rock masses varying from completely weathered and residual soil (classes V–VI) to moderately weathered rocks (class III), characterized by a progressive increase of geotechnical properties (e.g., seismic wave velocity, rock quality designation, internal friction angle, uniaxial compressive strength) with a decrease of weathering. The weathered rocks display a wide variety of mass movement. Landslide types are rockfalls for the fresher rocks (class III); translational slides occur between different weathering degrees (classes IV and V) and rotational slides dominate at the top of the slopes and generally involve the weathering grades from class IV to classes V–VI. The shallow horizons are composed of thin eluvial-colluvial deposits and more weathered granitoids (classes V–VI) that may generate dangerous debris flows or debris avalanches during intense and prolonged rainfall. The landslide surface is usually at a depth lower than 2m in the debris flow type, while deeper landslides (depth>2m) are rotational and translational types with sliding surfaces occurring mainly in the transition between the classes IV and V.The field survey coupled to geotechnical, geophysical and minero-petrographical data allowed the recognition of the different weathered horizons and relative geotechnical properties useful to the reconstruction of the reliable geological model. The model represents an important support for the study of the landslide processes and for a sustainable management of the territory. The used approach provided results to set a general reference framework for establishing more precise correlations between mass movement phenomena and weathering processes.

Erosion processes in calanchi in the Upper Orcia Valley, Southern Tuscany, Italy based on multitemporal high-resolution terrestrial LiDAR and UAV surveys

A 125,000m2 calanchi badland in the Province of Siena (Tuscany) was monitored with an unmanned aerial vehicle (UAV) and terrestrial laser scanning over the period of one year. TLS (terrestrial laser scanning) was carried out on two sample slopes, whereas the image acquisition of the UAV covered the entire catchment. In combination with ground control points, the UAV images were used to create orthophotos and 3D point clouds using the Structure from Motion (SfM) software Photoscan. The TLS surface models indicate seasonal differences in erosion and deposition. The surface change measured with SfM showed nearly 6.700m3 of net material loss, resulting from 8.700m3 erosion and 2.000m3 deposition. These values reveal a mean annual surface lowering of 5.3cm for the catchment. Additionally, several geomorphological processes, such as rill erosion, slope wash and translational slides could be detected in the one-year monitoring period. A comparison of TLS and SfM results showed differences in the calculated volumes of mobilised material. These discrepancies resulted from shadowing effects and low point densities of the TLS point clouds.

Halloysite behaving badly: geomechanics and slope behaviour of halloysite-rich soils

AbstractHalloysite-rich soils derived fromin situweathering of volcanic materials support steep stable slopes, but commonly fail under triggers of earthquakes or rainfall. Resulting landslides are slideflow processes, ranging from small translational slides to larger rotational failures with scarps characteristic of sensitive soils. Remoulding of failed materials results in high-mobility flows with apparent friction angles of 10–16°. The materials characteristically have high peak-friction angles (∼25– 37°), low cohesion (∼12–60 kN m−2) and plasticity ( plasticity index ∼10–48%), and low dry bulk density (∼480–1,080 kg m−3) with small pores due to the small size of the halloysite minerals. They remain saturated under most field conditions, with liquidity indexes frequently >1. Remoulded materials have limited cohesion (<5 kN m−2) and variable residual friction angles (15°–35°). Halloysite mineral morphology affects the rheology of remoulded suspensions: tubular minerals have greater viscosity and undrained shear strength than spherical morphologies.

Landslide characteristics and spatial distribution in the Rwenzori Mountains, Uganda

In many landslide-prone regions, data on landslide characteristics remain poor or inexistent. This is also the case for the Rwenzori Mountains, located on the border of Uganda and the DR Congo. There, landslides frequently occur and cause fatalities and substantial damage to private property and infrastructure. In this paper, we present the results of a field inventory performed in three representative study areas covering 114 km2. A total of 371 landslides were mapped and analyzed for their geomorphological characteristics and their spatial distribution. The average landslide areas varied from less than 0.3 ha in the gneiss-dominated highlands to >1 ha in the rift alluvium of the lowlands. Large landslides (>1.5 ha) are well represented while smaller landslides (<1.5 ha) are underrepresented. The degrees of completeness of the field inventories are comparable to those of similar historical landslide inventories. The diversity of potential mass movements in the Rwenzori is large and depends on the dominant lithological and topographic conditions. A dominance of shallow translational soil slides in gneiss and of deep rotational soil slides in the rift alluvium is observed. Slope angle is the main controlling topographic factor for landslides with the highest landslide concentrations for slope angles above 25–30° in the highlands and 10–15° in the lowlands. The undercutting of slopes by rivers and excavations for construction are important preparatory factors. Rainfall-triggered landslides are the most common in the area, however in the zones of influence of the last two major earthquakes (1966: Mw = 6.6 and 1994: Mw = 6.2), 12 co-seismic landslides were also observed.

Landslide phenomena in weathered granitoid rocks of the Fabrizia surroundings (Serre Massif, southern Italy)

This work shows the preliminary data on the weathered granitoids and related landslide phenomena in the area surrounding Fabrizia, Serre Massif (southern Calabria). Fabrizia lies on an old erosion surface composed of basement rocks made up of weathered granitoids, in turn belonging to the Serre Massif granitoid complex, and overlain by thin eluvial-colluvial deposits. The weathered profiles of the granitoid complex have been studied through a visual description, following both qualitative and quantitative criteria. Weathered profiles are mainly composed by rocky masses varying from completely weathered to highly or moderately weathered terms, characterized by Schmidt Hammer values ranging from 0-20 (class V) to 21-43 (class IV-III). The slopes of the Fabrizia surroundings are characterized by widespread mass movements. Typical landslide typologies are rockfalls for the fresher rocks (class III) and translational slides for the more weathered rocks (classes IV and V). The shallow horizons composed both by thin eluvial-colluvial deposits and by more weathered granitoids (class V-VI), may generate dangerous earth flows, debris flow or debris avalanches under intense and extensive rainfalls.

A multi-annual landslide inventory for the assessment of shallow landslide susceptibility – Two test cases in Vorarlberg, Austria

Geomorphological landslide inventories provide crucial input data for any study on the assessment of landslide susceptibility, hazard or risk. Several approaches for assessing landslide susceptibility have been proposed to identify areas particularly vulnerable to this natural hazard. What they have in common is the need for data of observed landslides. Therefore the first step of any study on landslide susceptibility is usually the compilation of a geomorphological landslide inventory using a geographical information system. Recent research has proved the feasibility of orthophoto interpretation for the preparation of an inventory aimed at the delineation of landslides with the use of distinctive signs in the imagery data. In this study a multi-annual landslide inventory focusing on shallow landslides (i.e. translational soil slides of 0–2m in depth) was compiled for two study areas in Vorarlberg (Austria) from the interpretation of nine orthophoto series. In addition, derivatives of two generations of airborne laser scanning data aided the mapping procedure. Landslide scar areas were delineated on the basis of a high-resolution differential digital terrain model. The derivation of landslide volumes, depths and depth-to-length ratios are discussed. Results show that most mapped landslides meet the definition of a shallow landslide. The inventory therefore provides the data basis for the assessment of shallow landslide susceptibility and allows for the application of various modelling techniques.

The relationships between soft-sediment deformation structures and synsedimentary extensional tectonics in Upper Triassic deep-water carbonate succession (Southern Tethyan rifted continental margin — Central Sicily)

We describe soft-sediment deformation structures into the Upper Triassic cherty limestone outcropping in the Pizzo Lupo section (Central Sicily, Italy), pertaining to the deep-water palaeodomain of the Southern Tethyan margin.In the study section, mainly consisting of thin-bedded mudstone/marl alternations with bedded chert intercalations, some lithofacies have been separated on the basis of the abundance of the calcium carbonate/clay content and the overall textural features.The deformational structures, displaying different deformational styles as folded and faulted beds, disturbed layers, clastic dikes, and slumps occur mainly in the deformed horizons that involve marl-dominated lithofacies. Small-scale water-escape structures involve beds with nodular fabric. Synsedimentary faults affect the mud-limestone dominated lithofacies, which are characterized by fault-rotating blocks producing lateral thinning. These bodies appear to have moved coherently along an overall planar surface.We relate these soft-sediment deformations to slump sheets, associated with down-slope sliding of sedimentary masses. The deformation mechanism and driving force for these soft-sediment deformations are due essentially to gravitational instability and dewatering. Detailing, rotational (slump) and translational (glide) slides and water-escape are the main processes causing the distinguished deformational styles.The synsedimentary extensional tectonics that affected the Upper Triassic pelagic deposits was the triggering process responsible for the instability of the seafloor inducing loss of coherence of the unconsolidated sediments on the sea bottom, developing a large number of gravity-driven slides.The analysis of both of these SSDSs and their relationships with the structural scenario allow us to hypothesise that they are seismically-induced.

Creep response in shear of clayey geo-materials under saturated and unsaturated conditions

Translational slides in clays are often characterized by long-lasting intermittent movements associated with the fluctuations of pore-water pressure. Physically-based models designed to support hazard analysis of landslide movements and early warning systems require the integration of time-dependent (viscous) constitutive models for the shear displacements because landslide movements are typically controlled by the viscous behaviour of the clay geo-material. This paper presents an investigation of the viscous response of a clay geo-material under saturated and unsaturated conditions. Creep and relaxation tests have been first carried out on saturated clay samples by means of direct shear box. To gain a conceptual understanding of the viscous response of the clay in shear, mechanical analogues were considered based on combinations of springs and dashpots. Preliminary tests on unsaturated samples were finally carried out to gain a first insight into the viscous response of the clay under unsaturated conditions.

The Verdesca landslide in the Agri Valley (Basilicata, southern Italy): a new geological and geomorphological framework

Abstract. A landslide, to the west of Montemurro (a small village in southern Italy), has recently caused damage to buildings and other infrastructure in an urbanized area; as a result the development of new economic activities has been prohibited. The landslide phenomenon started in the last century and has been studied since the 1990s using classical geotechnical methods; however the sliding body continues to move. This paper presents the results of a study carried out using field surveys, geognostic investigations and TDR (time domain reflectometry) measurements in order to reconstruct the stratigraphy of the sediments involved and to further understand the geological and geomorphological context of the slope. This study is part of a larger multidisciplinary project, the results of which will also be presented in this paper. The landslide (rotational slide in the upper sector, developing into a translational slide in the lower part) affects Quaternary continental clastic deposits resting on a bedrock formed by Tertiary siliciclastic sediments of the Gorgoglione Flysch. TDR measurements did not show any significant movement during the period monitored (January 2013–January 2014). Slip zone geometries were hypothesized using inclinometric measurements taken from previous studies, stratigraphic data and geomorphological interpretations of topographic scarps. Feedback from monitoring will confirm this hypothesis.

Kinematic model of a translational slide in the Cidu section of the Formosan Freeway

This study presents a kinematic model of a translational slide in the Cidu section of the Formosan Freeway in Taiwan, including the associated kinematic processes and the geometry of the deposition. A numerical simulation of the slide was performed using the discrete element method. This kinematic model has been considered the distribution of bedding plane and joints, engineering installations, and mechanical parameters in the development of a numerical model capable of clarifying the kinematic characteristics of this translational slide with high velocities. When the friction coefficient of each particle was set to 0.03, the predicted maximum velocity was 23.6 m/s and the debris reached the other side of the Formosan Freeway. The simulations demonstrated that the three cars involved in the incident and the Formosan Freeway itself were buried at 2 to 4 s during the event (the predicted average velocity was 10.2 m/s) and that the translational slide ceased its movement at 7.5 s, resulting in the formation of a slightly fragmented depositional mass.

Shallow landslide susceptibility assessment in a data-poor region of Guatemala (Comitancillo municipality)

Although landslides are frequent natural phenomena in mountainous regions, the lack of data in emerging countries is a significant issue in the assessment of shallow landslide susceptibility. A key factor in risk-mitigation strategies is the evaluation of deterministic physical models for hazard assessment in these data-poor regions. Given the lack of physical information, input parameters to these data-intensive deterministic models have to be estimated, which has a negative impact on the reliability of the assessment. To address this problem, we examined shallow landslide hazard in Comitancillo municipality, Guatemala. Shallow landslides are here defined as small (less than two or three metre-deep) rotational or translational slides or earth flows. We based our hazard simulation on the stability index mapping model. The model’s input parameters were estimated from a statistical analysis of factors affecting landslides in the municipality obtained from a geodatabase. The outputs from the model were analysed and compared to an inventory of small-scale landslides. The results of the comparison show the effectiveness of the method developed to estimate input parameters for a deterministic model, in regions where physical data related to the assessment of shallow landslide susceptibility is lacking.

Integration of LiDAR data for the assessment of activity in diachronic landslides: a case study in the Betic Cordillera (Spain)

In the procedures to minimize diachronic landslides, data on their temporal evolution and destructive capacities are necessary. For that purpose, remote-detection techniques proved to be highly useful for quantifying the ongoing change in the relief, as well as in comparisons between digital terrain models achieved by Light Detection and Ranging. The methodology presented in this paper includes the supervised merging and comparison of sequential scans, acquired within nearly annual intervals from an irregular terrain, which improves the quality of the results highlighting ground changes. This approach is based on the processing of digital terrain models from point clouds acquired by Terrestrial Laser Scanning to quantify and interpret the landslide displacements. In parallel, it is supported by Global Navigation Satellite Systems, the use of artificial targets and a refined data processing to minimize the uncertainty and improve the precision of the results. This is applied to a large translational slide affecting phyllite rocks in a IV-V degree of weathering settled on the southern slope of Sierra Nevada (south-eastern Spain). During the monitoring period (2008–2010), the slide remained inactive until 2009, followed by a reactivation with displacements in the range −1.80 to 1.20 m along the period 2009–2010, where negative values are downwards from the reference model (2009). The accumulated relative standard deviation between data sets was on the order of 7.5 cm, whereas the threshold to determine a terrain displacement (also avoiding changes due to erosion-accumulation processes) was of 10 cm. When applying this methodology to Airborne Laser Scanning datasets for the years 2008 and 2010, covering zones hidden to the line of sight of the terrestrial technique, a reactivation with similar deformation pattern was found useful to validate the findings, although the detail of changes and quantitative results did not match exactly due to the different accuracy and resolution of both techniques. The reactivation of the slide coincided with a period of intense rains, pointing to this as the triggering factor, with a precipitation threshold of roughly 1000 mm in a period of 4 months, only reached on one occasion throughout in the historical record.

Comparison of Logistic Regression and Random Forests techniques for shallow landslide susceptibility assessment in Giampilieri (NE Sicily, Italy)

The aim of this work is to define reliable susceptibility models for shallow landslides using Logistic Regression and Random Forests multivariate statistical techniques. The study area, located in North-East Sicily, was hit on October 1st 2009 by a severe rainstorm (225mm of cumulative rainfall in 7h) which caused flash floods and more than 1000 landslides. Several small villages, such as Giampilieri, were hit with 31 fatalities, 6 missing persons and damage to buildings and transportation infrastructures. Landslides, mainly types such as earth and debris translational slides evolving into debris flows, were triggered on steep slopes and involved colluvium and regolith materials which cover the underlying metamorphic bedrock. The work has been carried out with the following steps: i) realization of a detailed event landslide inventory map through field surveys coupled with observation of high resolution aerial colour orthophoto; ii) identification of landslide source areas; iii) data preparation of landslide controlling factors and descriptive statistics based on a bivariate method (Frequency Ratio) to get an initial overview on existing relationships between causative factors and shallow landslide source areas; iv) choice of criteria for the selection and sizing of the mapping unit; v) implementation of 5 multivariate statistical susceptibility models based on Logistic Regression and Random Forests techniques and focused on landslide source areas; vi) evaluation of the influence of sample size and type of sampling on results and performance of the models; vii) evaluation of the predictive capabilities of the models using ROC curve, AUC and contingency tables; viii) comparison of model results and obtained susceptibility maps; and ix) analysis of temporal variation of landslide susceptibility related to input parameter changes. Models based on Logistic Regression and Random Forests have demonstrated excellent predictive capabilities. Land use and wildfire variables were found to have a strong control on the occurrence of very rapid shallow landslides.

The influence of bedrock geology and slip surface characteristics on failure mode and mobility: a comparative study of instability patterns in Nigeria

The correlation among bedrock, regolith properties, groundwater table, and slip surface character and their combined effects on the mode and scale of landslides were evaluated. Standard penetration test, laboratory, and DEM-based analyses were integrated to study igneous, metamorphic, and sedimentary areas where instability was common. A clear pattern of the distribution of translational and rotational slides, and their pre- and post-failure mechanisms were established. It was found that factors such as climate, vegetation types and density, tectonic setting, and catchment area were generally similar. The key differences were in the dissimilar underlying bedrock types which weathered to yield soils with distinctive engineering property and in the nature of the sliding surfaces. Whereas the weathering of granitic and gneissic rocks (where groundwater table was 7 m) produced less compressible, fairly-sorted, porous materials. The hydraulic conductivity of the granitic and gneissic-derived soils decreased with depth. At saturation levels above a threshold, the soils derived from the igneous and metamorphic rocks contracted and generated positive pore pressure. Movement on discrete, planar, slicken-sided, fractured, and foliated slip surfaces transformed the slides to flow (translational failures). On the contrary, pronounced dilation and negative pore-water developed in the more porous, less crushable soils from the sedimentary area leading to the dominance of rotational slumps in this region of active erosion.

Effective stress strength testing of peat

It has been approximately 80 years since the first effective stress strength testing of peat was performed using triaxial apparatus. In light of recent unexpected failures of embankments, dykes, foundations and slopes in peat deposits, it is timely to review current laboratory practice and also provoke discussion about the best way forward for understanding and determining the effective stress strength properties of peat. Compared with fine-grained mineral soils, significant fabric and structural differences (including the porous, compressible nature of the organic solids themselves) make the direct application of classical soil mechanics strength models doubtful for peat. Uncertainties and difficulties of effective stress testing of peat using standard strength apparatus are discussed. Compared with triaxial compression, direct simple shear testing appears to provide conservative estimates of the strength parameter values, and the specimen deformation (failure) mode more closely represents that occurring in translational planar slides. It is recommended that the geotechnical profession investigates alternative theoretical frameworks to explain and predict peat strength (and compression) behaviour along with the development of new apparatus and methodologies for determining appropriate parameter values.

High‐resolution studies of double‐layered ejecta craters: Morphology, inherent structure, and a phenomenological formation model

AbstractThe ejecta blankets of impact craters in volatile‐rich environments often possess characteristic layered ejecta morphologies. The so‐called double‐layered ejecta (DLE) craters are characterized by two ejecta layers with distinct morphologies. The analysis of high‐resolution image data, especially HiRISE and CTX, provides new insights into the formation of DLE craters. A new phenomenological excavation and ejecta emplacement model for DLE craters is proposed based on a detailed case study of the Martian crater Steinheim—a well‐preserved DLE crater—and studies of other DLE craters. The observations show that the outer ejecta layer is emplaced as medial and distal ejecta that propagate outwards in a debris avalanche or (if saturated with water) a debris flow mode after landing, overrunning previously formed secondary craters. In contrast, the inner ejecta layer is formed by a translational slide of the proximal ejecta deposits during the emplacement stage that overrun and superimpose parts of the outer ejecta layer. Based on our model, DLE craters on Mars are the result of an impact event into a rock/ice mixture that produces large amounts of shock‐induced vaporization and melting of ground ice, leading to high ejection angles, proximal landing positions, and an ejecta curtain with relatively wet (in terms of water in liquid form) composition in the distal part versus dryer composition in the proximal part. As a consequence, basal melting of ice components in the ejecta at the transient crater rim, which is induced by frictional heating and the enhanced pressure at depth, initiates an outwards directed collapse of crater rim material in a translational slide mode. Our results indicate that similar processes may also be applicable for other planetary bodies with volatile‐rich environments, such as Ganymede, Europa, and the Earth.