Granular Slopes Research Articles

Energy dissipation of a sphere rolling up a granular slope: Slip and deformation of the granular surface.

We experimentally investigate the dynamics of a sphere rolling up a granular slope. During the rolling-up motion, the sphere experiences slipping and penetration (groove formation) on the surface of the granular layer. The former relates to the stuck motion of the rolling sphere, and the latter causes energy dissipation due to the deformation of the granular surface. To characterize these phenomena, we measured the motion of a sphere rolling up a granular slope of angle α. The initial velocity v_{0}, initial angular velocity ω_{0}, angle of slope α, and density of the sphere ρ_{s} were varied. As a result, the penetration depth can be scaled solely by the density ratio between the sphere and granular layer. By considering the rotational equationof motion, we estimate the friction due to the slips. Besides, by considering energy conservation, we define and estimate the friction due to groove formation. Moreover, the translational friction is proportional to the penetration depth. Using these results, we can quantitatively predict the sphere's motion including stuck behavior.

Repetitive small seismicity coupled with rainfall can trigger large slope instabilities on metastable volcanic edifices

Quantifying the effect of external forcings like seismicity or rain on slope destabilization is a long-standing and challenging issue. To investigate the respective roles of these forcings, we analyze an unprecedented 10-year long catalog of rockfalls occurring in the crater of the Piton de la Fournaise volcano (La Reunion Island), using statistical tools originally developed for earthquakes. Our analysis reveals the predominant effect of low amplitude repetitive seismicity in the triggering of rockfalls located at a few kilometers from the source, due to progressive damaging of the slope. Moreover, we show that the efficiency and time-delay of this dynamic triggering is controlled by the stability state of the slope, i.e. its closeness to the failure, as observed with lab-experiments on metastable granular slopes. Our results show the need to account for long-term swarm-type seismic activity that can affect the stability of geological structures like slopes and faults, but also buildings.

Substrate-mediated leg interactions play a key role in insect stability on granular slopes.

Locomotion on granular inclines is a subject of high relevance in ecological physics as well as in biomimmetics and robotics. Enhancing stability on granular materials represents a huge challenge due to the fluidization transition when inclination approaches the avalanche angle. Our motivating example is the predator-prey system made of the antlion, its pit, and its prey. Recent studies have demonstrated that stability on granular inclines strongly depends on the pressure exerted on the substrate. In this work we show that for multilegged locomotion, along with pressure, the distance between the leg contacts on the substrate also plays a major role in the determination of the stability threshold. Through a set of model experiments using artificial sliders, we determine a critical distance below which stability is importantly affected by the interactions between the perturbed regions generated by each contact point. A simple model based on the Coulomb method of wedges allows us to estimate a stability criterion based on pressure, interleg distance, and substrate characteristics. Our work suggests that mass to leg-length allometric relationships, as the ones observed in ants, may be an important key in determining the locomotion success of multilegged locomotion on granular inclines.

Scientific exploration of challenging planetary analog environments with a team of legged robots

The interest in exploring planetary bodies for scientific investigation and in situ resource utilization is ever-rising. Yet, many sites of interest are inaccessible to state-of-the-art planetary exploration robots because of the robots’ inability to traverse steep slopes, unstructured terrain, and loose soil. In addition, current single-robot approaches only allow a limited exploration speed and a single set of skills. Here, we present a team of legged robots with complementary skills for exploration missions in challenging planetary analog environments. We equipped the robots with an efficient locomotion controller, a mapping pipeline for online and postmission visualization, instance segmentation to highlight scientific targets, and scientific instruments for remote and in situ investigation. Furthermore, we integrated a robotic arm on one of the robots to enable high-precision measurements. Legged robots can swiftly navigate representative terrains, such as granular slopes beyond 25°, loose soil, and unstructured terrain, highlighting their advantages compared with wheeled rover systems. We successfully verified the approach in analog deployments at the Beyond Gravity ExoMars rover test bed, in a quarry in Switzerland, and at the Space Resources Challenge in Luxembourg. Our results show that a team of legged robots with advanced locomotion, perception, and measurement skills, as well as task-level autonomy, can conduct successful, effective missions in a short time. Our approach enables the scientific exploration of planetary target sites that are currently out of human and robotic reach.

Initiation of flow liquefaction in granular soil slopes: drained versus undrained conditions

Initiation of flow liquefaction in granular soil slopes: drained versus undrained conditions

Rainfall-induced flowslides of granular soil slopes: Insights from grain-scale modeling

Rainfall-induced flowslides in soil slopes remain one of the most destructive natural hazards. To prevent and mitigate the hazard requires a good understanding of soil behavior under the constant shear drained (CSD) stress path. This stress path differs from typical triaxial compression stress paths that have been extensively studied in soil laboratories. Here we present an attempt to explore various behaviors of loose granular packings along different stress paths and the associated micromechanical mechanisms using a robust grain-scale modeling technique. Through systematic simulations of the CSD, constant-volume (CDCV), and conventional drained (CD) shearing stress paths, we show that the behavior under the CSD stress path conforms to critical state concepts at both macro- and micro-scale. The evolutions of the stress ratio (q/p'), coordination number (Z), and fabric anisotropy (ac) along the CSD stress path are highly similar to those along the CD stress path. A substantial plastic strain tends to develop once the second-order work becomes nonpositive along the CSD stress path, while a dramatic drop of shear strength will occur at the vanishing of second-order work along the CDCV stress path. In both cases, the second-order work vanishes at an almost identical stress ratio, which demarcates the lower boundary of the potential instability zone in the stress space. We establish an explicit relationship between this stress ratio and the initial state parameter of the packing. The significance of this relationship lies in its applicability for both CSD and CDCV loading paths and its root in the critical state theory.

Interplay Between Friction and Cohesion: A Spectrum of Retrogressive Slope Failure

AbstractRetrogressive failures occur in slopes consisting of sensitive materials such as snow or quick clay. They can be triggered by a small disturbance at the slope toe, but can cause propagated failure spreading miles away. Understanding the physical mechanism and predicting the retrogressive failure process are particularly important. Previous studies have discussed the failure criteria, the soil properties or the method of numerical modeling of retrogressive slope failure. However, little attention has been paid to the microscopic failure mechanism, especially relating to various possible failure patterns. In this study, multiscale modeling is incorporated to study the physical mechanism of different retrogressive failure patterns, including earth flow, flowslide and spread failure, within a unified framework. Utilizing multiscale analysis, we found that earth flow failure is related to the shear failure of granular materials. In contrast, the development of macroscopic shear bands is accompanied by tensile failure. As shear and tension failures are typical failure mechanisms of frictional and cohesive materials, it is deduced that friction and cohesion effects play key roles in different retrogressive failure patterns. Therefore, the distributions of attractive and repulsive contact forces are explored and a novel parameter η is proposed to quantify the interplay between friction and cohesion. Further analysis proves that η can capture the effect of friction and cohesion and distinguish different retrogressive failure patterns. Finally, a spectrum of retrogressive failures for a granular slope is established, in which the failure mechanism is explained by the changeable dominant effect, that is, frictional or cohesive in soil.

Chart-Based Granular Slope Stability Assessment Using the Modified Mohr–Coulomb Criterion

There are usually two types of slopes in nature: soil and rock. However, the granular slope has gradually come into view and has caused concern. In this paper, a modified Mohr–Coulomb (MC) criterion has been proposed to estimate the strength parameters of medium materials using a new parameter (tau_{0}) instead of the traditional MC parameter (c). Meanwhile, there is a new stability chart of a granular slope whose core is the dimensionless stability number Ns and dimensionless coefficient DR based on a modified MC criterion to estimate the safety factor of a granular slope preliminarily when the geometric and strength parameters of the slope are determined. A practical case is introduced to test the results, showing that the stability chart proposed in this paper is applicable to granular slopes. The research content and the proposed new viewpoints are intended to offer preliminary judgments on the stability analysis of granular slopes and provide guidance for the stability analysis and construction design of granular slope engineering.

Analogical model tests on repeated surficial failure of dry granular slopes confined by retaining walls

In alpine and gorge regions, surficial failure of granular slopes has seriously affected infrastructure construction and the safe operation of transportation networks. This research focuses on dry granular slopes confined by retaining walls. Repeated surficial failure of granular slopes has been explored through indoor physical model tests. The results show that surficial failure presents low frequency and large scale for granular slopes dominated by coarse particles. The mass of debris that crosses retaining walls indicates activity level of granular slopes. When this mass for each surficial failure gradually decreases, it is highly likely that large-scale surficial failure will occur. As the amount of debris deposited on the slope increases, it is probable that the debris mass falling during a massive sliding event will be many times that of the previous sliding event. In engineering practice, it is necessary during road cleaning processes to record the volume of debris accumulated on the road. A continuous increase in the slope angle indicates a high probability of large-scale surficial failure. For each granular slope with a unique particle composition, there is a critical change rate for early warning of surficial failure, which can be determined by long-term monitoring.

Seismic behaviour of granular slope under railway embankment in large-scale shaking table test

This study investigates shear zone development in a coarse-grained soil slope beneath a railway embankment under seismic action. Clarifying the dynamic stress and residual strain responses under different seismic excitations reveals the slope instability mechanism. A typical coarse-grained soil slope under construction of the Sichuan-Tibet Railway Railway was selected for large-scale shaking table model testing. The evolution of slope response parameters (dynamic displacement, acceleration, dynamic strain and residual strain, and dynamic earth pressure) was monitored. The seismic damage modes and resulting deformations of the embankment and slope were analysed. The evolution characteristics of a shear zone were described; the relationship among acceleration, dynamic stress, and dynamic strain was clarified; and the critical state of earthquake-induced slope instability was defined. The main conclusions are as follows: under seismic excitation, the slope slip surface forms mostly near the interface between the sand and breccia layers, and a weakness zone exists between the subgrade and slope surface. Under high-energy seismic excitation (i.e., PGA ≥ 0.4 g), positive peak acceleration is more suitable for assessing the elevation amplification effect. Considering the cumulative strain distribution contour maps, the growth characteristics in the dynamic stress peak area can describe the development of a shear slip zone. Acceleration amplification factors can reflect slope responsiveness, but the energy contained in the seismic waves determines deformation. When the ratio of dynamic to shear stress exceeds 2, the dynamic strain in the slope slip zone increases considerably with shear zone extension and slope instability. With increasing dynamic stress, the growth characteristics of residual strain exhibit stability, transition and failure states.

Numerical investigation of the effect of slope angle and height on the stability of a slope composed of sandy soil

Slope stability issues are very frequently in civil engineering applications and are commonly encountered in huge and significant projects such as dams, highways and tunnels, etc. This study examines the impact of slope height (H) and slope angle (β) on the stability of loose granular soil slope underlined by dense granular soil layer. For this purpose, a number of finite element analyses were conducted on a full-scale soil slope. The computer program Plaxis 2D code was used which is based on the finite element method (FEM). The granular soils were described by non-linear hardening soil model, which is an advanced elastoplastic stress-strain constitutive soil model. Six different slope angles were investigated and for each angle, six different heights were chosen. In total, twelve series of numerical analyses were executed. In the first six series, it was assumed that there is no surcharge load on the top of the slope. In the second six series, the effect of surcharge load was also investigated. According to numerical analysis results, it was found that the slope height and slope angle have a considerable effect on the safety factor (FS) of the slope. It was also noticed that most of the failure surfaces of the slope were circular and classified as face slope failure. It was observed that by increasing slope height and the slope angle, the FS of the slope decreases and vice versa. In addition, curves and charts have been proposed to easily estimate the FS of loose granular soil slope.

Traversing Steep and Granular Martian Analog Slopes With a Dynamic Quadrupedal Robot

Celestial bodies, such as the Moon and Mars are mainly covered by loose, granular soil, which is a notoriously challenging terrain to traverse with wheeled robots. Here, we present experimental work on traversing steep, granular slopes with the dynamically-walking quadrupedal robot SpaceBok. To adapt to the challenging environment, we developed passive-adaptive, planar feet and optimized studs to reduce sinkage and increase traction. Single-foot experiments revealed that a surface area of 110 cm2 per foot reduces sinkage to an acceptable level for the 22 kg robot, even on highly collapsible soil. Implementing several 12 mm studs increases traction by 22% to 66% on granular media compared to stud-less designs. Together with a terrain-adapting walking controller, we validate — for the first time — static and dynamic locomotion on Mars analog slopes of up to 25°(the maximum of the testbed). We evaluated the performance between point- and planar feet and static and dynamic gaits for safety, velocity, and energy consumption. We show that dynamic gaits are energetically more efficient than static ones, but are riskier on steep slopes. Our tests also revealed that energy consumption with planar feet increases drastically as slope inclination approaches the soil’s angle of repose. Point feet are less affected by slippage due to their excessive sinkage but, in turn, are prone to instabilities and tripping. Based on our findings, we present safe and energy-efficient, global, path-planning strategies for negotiating steep Martian topography.

Atomic step disorder on polycrystalline surfaces leads to spatially inhomogeneous work functions

Structural disorder causes materials’ surface electronic properties, e.g., work function (ϕ), to vary spatially, yet it is challenging to prove exact causal relationships to underlying ensemble disorder, e.g., roughness or granularity. For polycrystalline Pt, nanoscale resolution photoemission threshold mapping reveals a spatially varying ϕ=5.70±0.03 eV over a distribution of (111) vicinal grain surfaces prepared by sputter deposition and annealing. With regard to field emission and related phenomena, e.g., vacuum arc initiation, a salient feature of the ϕ distribution is that it is skewed with a long tail to values down to 5.4 eV, i.e., far below the mean, which is exponentially impactful to field emission via the Fowler–Nordheim relation. We show that the ϕ spatial variation and distribution can be explained by ensemble variations of granular tilts and surface slopes via a Smoluchowski smoothing model wherein local ϕ variations result from spatially varying densities of electric dipole moments, intrinsic to atomic steps, that locally modify ϕ. Atomic step-terrace structure is confirmed with scanning tunneling microscopy (STM) at several locations on our surfaces, and prior works showed STM evidence for atomic step dipoles at various metal surfaces. From our model, we find an atomic step edge dipole μ=0.12 D/edge atom, which is comparable to values reported in studies that utilized other methods and materials. Our results elucidate a connection between macroscopic ϕ and the nanostructure that may contribute to the spread of reported ϕ for Pt and other surfaces and may be useful toward more complete descriptions of polycrystalline metals in the models of field emission and other related vacuum electronics phenomena, e.g., arc initiation.

Distributed Optical Fiber Sensor Applications in Geotechnical Monitoring.

We report the experimental application of distributed optical fiber sensors, based on stimulated Brillouin scattering (SBS), to the monitoring of a small-scale granular slope reconstituted in an instrumented flume and subjected to artificial rainfall until failure, and to the monitoring of a volcanic rock slope. The experiments demonstrate the sensors’ ability to reveal the sudden increase in soil strain that foreruns the failure in a debris flow phenomenon, as well as to monitor the fractures in the tuff rocks. This study offers an important perspective on the use of distributed optical fiber sensors in the setting up of early warning systems for landslides in both rock and unconsolidated materials.

Experimental study on interaction between rock granular flow and different structural forms of rigid retaining walls

Rock granular flow is a kind of rock debris aggregate produced by landslides, which can be greatly destructive due to its high velocity, long distance and large impact force. In this paper, dynamic responses and energy dissipation capacities of different shapes rigid retaining walls (rectangle, angular, arc-shaped) under the impact of rock granular flow were investigated. The experiments primarily focused on the influences of granular flow mass and slope angle on motion characteristics of rock granular flow and impact force of retaining wall. In addition, the relationship between the energy dissipation capacity and the structural form of the retaining wall were explored through quantitative analysis peak acceleration of different shapes retaining wall. According to the experimental results, substantial increases both in the impact velocity of rock granular flow and in the maximum resultant of impact force of retaining wall with increasing rock granular flow mass and slope angle were observed under the coupling effects of the granular flow mass and slope angle. Moreover, the impact force at different height of the rectangle retaining wall varied evidently with slope angle, but was mainly concentrated in the lower half of the retaining wall. In addition, the energy dissipation capacity of arc-shaped retaining wall was the best among those rigid retaining walls. This study can contribute to a better understanding of the flow-structure interaction and provide guidelines for the engineering application of the new retaining wall.

Measurement of surface deformation and cohesion of a granular pile under the effect of centrifugal force

An experimental apparatus measuring free-surface deformation of a centrifuged granular pile is developed. By horizontally rotating a quasi two-dimensional granular pile whose apex is located at the vertical rotation axis, the resultant force of gravity and centrifuge yields the deformation of the granular pile. In this setup, centrifugal force depends on distance from the rotation axis whilst gravitational force is constant everywhere. Therefore, free-surface deformation by various centrifuge degrees can be systematically examined using this apparatus. In the system, a small unit consisting of a camera and computer is rotated with the granular sample to record the rotation-induced deformation. To evaluate the validity of the system, deformation of a rotated water surface is first measured and analyzed. The obtained data are properly explained by the theoretical parabolas without any fitting parameter. Next, we measure the deformation of non-cohesive and cohesive granular piles using the developed apparatus. Both granular samples show the significant deformation of granular pile and finally develop steep granular slopes on the side walls. However, details of the deformation processes depend on the cohesion strength. To quantitatively characterize the difference, the effective strength by cohesion and granular local-slope variations are analyzed based on the experimental results.

Summary and Trend Analysis of Research Progress on Granular Slope Geological Hazards

There are a large number of granular slope mixture geological disasters in the alpine mountainous areas in western China. Their structural characteristics are significantly different from the general slopes. They have the characteristics of poor self-stability, high suddenness and strong recurrence. With the continuous development of the western region of the country, such geological disasters have received close attention from scholars at home and abroad. This article first summarized and analyzed the basic characteristics of the typical domestic granular mixtures slope accidents, and understood the basic characteristics of disaster. Then the domestic and foreign research progress from the aspects of governance measures were summarized. Finally, based on the summary of typical granular mixtures slope disasters and research status, the existing problems in the current research were analyzed, and the key points and difficulties of future research had prospected. The research results have important theoretical significance for the treatment of granular slope.

Spatiotemporal slope stability analytics for failure estimation (SSSAFE): linking radar data to the fundamental dynamics of granular failure

Impending catastrophic failure of granular earth slopes manifests distinct kinematic patterns in space and time. While risk assessments of slope failure hazards have routinely relied on the monitoring of ground motion, such precursory failure patterns remain poorly understood. A key challenge is the multiplicity of spatiotemporal scales and dynamical regimes. In particular, there exist a precursory failure regime where two mesoscale mechanisms coevolve, namely, the preferred transmission paths for force and damage. Despite extensive studies, a formulation which can address their coevolution not just in laboratory tests but also in large, uncontrolled field environments has proved elusive. Here we address this problem by developing a slope stability analytics framework which uses network flow theory and mesoscience to model this coevolution and predict emergent kinematic clusters solely from surface ground motion data. We test this framework on four data sets: one at the laboratory scale using individual grain displacement data; three at the field scale using line-of-sight displacement of a slope surface, from ground-based radar in two mines and from space-borne radar for the 2017 Xinmo landslide. The dynamics of the kinematic clusters deliver an early prediction of the geometry, location and time of failure.

A theoretical formulation of dilatation/contraction for continuum modelling of granular flows

Abstract

Centroid migration on an impacted granular slope due to asymmetric ejecta deposition and landsliding

ABSTRACT For a fundamental understanding of terrain relaxation occurring on sloped surfaces of terrestrial bodies, we analyse the crater shape produced by an impact on an inclined granular (dry-sand) layer. Owing to asymmetric ejecta deposition followed by landsliding, the slope of the impacted inclined surface can be relaxed. Using the experimental results of a solid projectile impact on an inclined dry-sand layer, we measure the distance of centroid migration induced by asymmetric cratering. We find that the centroid migration distance xmig normalized to the crater minor-axis diameter Dcy can be expressed as a function of the initial inclination of the target tan θ, the effective friction coefficient μ, and two parameters K and c that characterize the asymmetric ejecta deposition and oblique impact effect: xmig/Dcy = K tan θ/[1 − (tan θ/μ)2] + c, where K = 0.6, μ = 0.8, and c = −0.1 to 0.3. This result is consistent with a previous study that considered the effect of asymmetric ejecta deposition. The obtained results provide fundamental information for analysing the degradation of sloped terrain on planetary surfaces, such as crater-shape degradation due to the accumulation of micro-impacts.