Modeling the evolution of natural cliffs subject to weathering. 2, Discrete element approach

The evolution of slopes subjected to weathering has been modeled by assuming Mohr-Coulomb behavior and by using a numerical approach based on the discrete element method (DEM). According to this method, soil and/or rock are represented by an assembly of bonded particles. Particle bonds are subject to progressive weakening, and so the material weathering and removal processes are modeled. Slope instability and material movement follow the decrease of material strength in space and time with the only assumption concerning the weathering distribution within the slope. First, the case of cliffs subject to strong erosion (weathering-limited conditions) and uniform weathering was studied to compare the results of the DEM approach with the limit analysis approach. Second, transport-limited slopes subject to nonuniform slope weathering were studied. Results have been compared with experimental data and other geomorphologic models from the literature (Fisher-Lehmann and Bakker–Le Heux). The flux of material from the slope is modeled assuming degradation both in space and time

Discussion of “Limit analysis of slopes with cracks : comparisons of results”

The discusser is the author of one of the two papers (Utili, 2013) compared by the authors of “Limit analysis of slopes with cracks: Comparisons of results”. The discusser wishes to highlight here that in his opinion there are a few misconceptions in the note and wishes to ask the authors to please clarify some ambiguities for the benefit of the readers

Discussion of “Stability assessment of slopes with cracks using limit analysis”

The discusser has recently published a paper in Géotechnique titled “Investigation by limit analysis on the stability of slopes with cracks” (Utili 2013) that includes, for the first time (to the discusser’s knowledge), a systematic investigation of the influence of the presence of cracks in uniform slopes for rotational failure mechanisms via the limit analysis upper bound approach. Looking at the discusser’s paper and the paper under discussion (Michalowski 2013), a reader may note that the aim of the two papers is the same, namely, to assess quantitatively the effect of the presence of cracks on the stability of slopes employing the upper bound approach of limit analysis. The discusser’s paper was sent to Géotechnique when the discusser had no knowledge of either the author’s conference paper (Michalowski 2012) or of the paper under discussion published in July 2013. On the other hand, the discusser’s paper was published after the publication of the author’s conference paper (Michalowski 2012). Hence, it can be concluded that the discusser and the author had independently developed an original formulation for the calculation of upper bounds based on rotational failure mechanisms for cracked uniform slopes at similar times

Investigation into the effect of backpressure on the mechanical behavior of methane-hydrate-bearing sediments via DEM analyses

Backpressure has been extensively applied in experimental tests to improve the water saturation of samples, and its effect on the strength of saturated soils has been traditionally regarded as trivial in Soil Mechanics. However, a non-negligible influence of backpressure on the macro mechanical properties of methane-hydrate-bearing-sediments (MHBS) has been surprisingly observed in several recent experiments reported in the literature. This paper aims to shed light on this phenomenon. A theoretical analysis on the microscopic interaction between soil grains and inter-particle methane hydrate (MH) was carried out to highlight how backpressure affects the mechanical properties of the inter-particle MH which in turn affect the macroscopic mechanical behavior of MHBS. The influence of backpressure is accounted for in a new bond contact model implemented into the Distinct Element Method (DEM). Then, a series of DEM biaxial compression tests were run to investigate the link between mechanical properties of MHBS and backpressure. The DEM numerical results show that shear strength, small strain stiffness and shear dilation of MHBS increase with the level of backpressure. As the critical state is approached, the influence of backpressure ceases. Moreover, the elastic modulus and cohesion of MHBS increase linearly while the internal friction angle decreases at a decreasing rate as the backpressure increases. Simple analytical relationships were achieved so that the effect of backpressure on the mechanical properties of MHBS can be accounted in the design of laboratory tests to characterize the mechanical behavior of MHBS

A combined DEM–FEM numerical method for Shot Peening parameter optimisation

A numerical modelling approach capable of simulating Shot Peening (SP) processes of industrial interest was developed by combining the Discrete Element Method (DEM) with the Finite Element Method (FEM). In this approach, shot–shot and shot–target interactions as well as the overall shot flow were simulated efficiently using rigid body dynamics. A new algorithm to dynamically adapt the coefficient of restitution (CoR) for repeated impacts of shots on the same spot was implemented in the DEM code to take into account the effect of material hardening. Then, a parametric study was conducted using the Finite Element Method (FEM) to investigate the influence of the SP parameters on the development of residual stresses. Finally, a two-step coupling method is presented to combine the output of DEM simulation with FEM analyses to retrieve the Compressive Residual Stresses (CRS) after multiple impacts with the aim to evaluate the minimum area required to be modelled to realistically capture the field of residual stresses. A series of such coupled analyses were performed to determine the effect of peening angle and the combination of initial velocity and mass flow rate on CRS

Rockslide and impulse wave modelling in the Vajont Reservoir by DEM-CFD analyses

This paper investigates the generation of hydrodynamic water waves due to rockslides plunging into a water reservoir. Quasi-3D DEM analyses in plane strain by a coupled DEM-CFD code are adopted to simulate the rockslide from its onset to the impact with the still water and the subsequent generation of the wave. The employed numerical tools and upscaling of hydraulic properties allow predicting a physical response in broad agreement with the observations notwithstanding the assumptions and characteristics of the adopted methods. The results obtained by the DEM-CFD coupled approach are compared to those published in the literature and those presented by Crosta et al. (Landslide spreading, impulse waves and modelling of the Vajont rockslide. Rock mechanics, 2014) in a companion paper obtained through an ALE-FEM method. Analyses performed along two cross sections are representative of the limit conditions of the eastern and western slope sectors. The max rockslide average velocity and the water wave velocity reach ca. 22 and 20 m/s, respectively. The maximum computed run up amounts to ca. 120 and 170 m for the eastern and western lobe cross sections, respectively. These values are reasonably similar to those recorded during the event (i.e. ca. 130 and 190 m, respectively). Therefore, the overall study lays out a possible DEM-CFD framework for the modelling of the generation of the hydrodynamic wave due to the impact of a rapid moving rockslide or rock–debris avalanche

Investigation of granular batch sedimentation via DEM–CFD coupling

This paper presents three dimensional numerical investigations of batch sedimentation of spherical particles in water, by analyses performed by the discrete element method (DEM) coupled with computational fluid dynamics (CFD). By employing this model, the features of both mechanical and hydraulic behaviour of the fluid-solid mixture system are captured. Firstly, the DEM–CFD model is validated by the simulation of the sedimentation of a single spherical particle, for which an analytical solution is available. The numerical model can replicate accurately the settling behaviour of particles as long as the mesh size ratio (Dmesh/d) and model size ratio (W/Dmesh) are both larger than a given threshold. During granular batch sedimentation, segregation of particles is observed at different locations in the model. Coarse grains continuously accumulate at the bottom, leaving the finer grains deposited in the upper part of the granular assembly. During this process, the excess pore water pressure initially increases rapidly to a peak value, and then dissipates gradually to zero. Meanwhile, the compressibility of the sediments decreases slowly as a soil layer builds up at the bottom. Consolidation of the deposited layer is caused by the self-weight of grains, while the compressibility of the sample decreases progressively

3D DEM investigation of granular column collapse : evaluation of debris motion and its destructive power

This paper presents a numerical investigation of the behaviour of dry granular flows generated by the collapse of prismatic columns via 3D Distinct Element Method (DEM) simulations in plane strain conditions. Firstly, by means of dimensional analysis, the governing parameters of the problem are identified, and variables are clustered into dimensionless independent and dependent groups. Secondly, the results of the DEM simulations are illustrated. Different regimes of granular motion were observed depending on the initial column aspect ratio. The profiles observed at different times for columns of various aspect ratios show to be in good agreement with available experimental results. Thirdly, a detailed analysis of the way energy is dissipated by the granular flows was performed. It emerges that most of the energy of the columns is dissipated by inter-particle friction, with frictional dissipation increasing with the column aspect ratio. Also, the translational and rotational components of the kinetic energy of the flows, associated to particle rotational and translational motions respectively, were monitored during the run-out process. It is found that the rotational component is negligible in comparison with the translational one; hence in order to calculate the destructive power of a granular flow slide, only the translational contribution of the kinetic energy is relevant. Finally, a methodology is presented to calculate the flux of kinetic energy over time carried by the granular flow through any vertical section of interest. This can be related to the energy released by landslide induced granular flows impacting against engineering structures under the simplifying assumption of neglecting all structure-flow interactions. This represents the first step towards achieving a computational tool quantitatively predicting the destructive power of a given flow at any location of interest along its path. This can be useful for the design of engineering works for natural hazard mitigation. To this end, also the distribution of the linear momentum of the flow over depth was calculated. It emerges that the distribution is initially bilinear, due to the presence of an uppermost layer of particles in an agitated loose state, but after some time becomes linear. This type of analysis showcases the potential of the Distinct Element Method to investigate the phenomenology of dry granular flows and to gather unique information currently unachievable by experimentation

Novel approach for health monitoring of earthen embankments

This paper introduces a novel modular approach for the monitoring of desiccation-induced deterioration in earthen embankments (levees), which are typically used as flood-defense structures. The approach is based on the use of a combination of geotechnical and noninvasive geophysical probes for the continuous monitoring of the water content in the ground. The level of accuracy of the monitoring is adaptable to the available financial resources. The proposed methodology was used and validated on a recently built, 2-km-long river embankment in Galston (Scotland, U.K.). A suite of geotechnical probes was installed to monitor the seasonal variation of water content over a 2-year period. Most devices were calibrated in situ. A novel procedure to extrapolate the value of water content from the geotechnical and geophysical probes at any point of the embankment is shown. Desiccation fissuring degrades the resistance of embankments against several failure mechanisms. An index of susceptibility is proposed here. The index is a useful tool to assess the health state of the structure and prioritize remedial intervention

A bond contact model for methane hydrate-bearing sediments with interparticle cementation

While methane hydrates (MHs) can be present in various forms in deep seabeds or permafrost regions, this paper deals with MH-bearing sediments (MHBS) where the MH has formed bonds between sand grains. A bond model based on experimentally validated contact laws for cemented granules is introduced to describe the mechanical behavior of the MH bonds. The model parameters were derived from measured values of temperature, water pressure and MH density. Bond width and thickness adopted for each bond of the MHBS were selected based on the degree of MH saturation. The model was implemented into a 2D distinct element method code. A series of numerical biaxial standard compression tests were carried out for various degrees of MH saturation. A comparison with available experimental data shows that the model can effectively capture the essential features of the mechanical behavior of MHBS for a wide range of levels of hydrate saturation under drained and undrained conditions. In addition, the analyses presented here shed light on the following: (1) the relationship between level of cementation and debonding mechanisms taking place at the microscopic level and the observed macro-mechanical behavior of MHBS and (2) the relationship between spatial distribution of bond breakages and contact force chains with the observed strength, dilatancy and deformability of the samples. Copyright © 2014 John Wiley & Sons, Ltd