Granular Discrete Element Method Research Articles

A granular Discrete Element Method for arbitrary convex particle shapes: Method and packing generation

A novel granular discrete element method (DEM) is introduced to simulate mixtures of particles of any convex shape. To quickly identify pairs of particles in contact, the method first uses a broad-phase and a narrow-phase contact detection strategy. After this, a contact resolution phase finds the contact normal and penetration depth. A new algorithm is introduced to effectively locate the contact point in the geometric center of flat faces in partial contact. This is important for a correct evaluation of the torque on each particle, leading to a much higher stability of stacks of particles than with previous algorithms. The granular DEM is used to generate random packings in a cylindrical vessel. The simulated shapes include non-spherical particles with different aspect ratio cuboids, cylinders and ellipsoids. More complex polyhedral shapes representing sand and woodchip particles are also used. The latter particles all have a unique shape and size, resembling real granular particle packings. All packings are analyzed extensively by investigating positional and orientational ordering.

Instationary compaction wave propagation in highly porous cohesive granular media

We study the collision of a highly porous granular aggregate of adhesive $$\upmu $$ m-sized silica grains with a hard wall using a granular discrete element method. A compaction wave runs through the granular sample building up an inhomogeneous density profile. The compaction is independent of the length of the aggregate, within the regime of lengths studied here. Also short pulses, as they might be exerted by a piston pushing the granular material, excite a compaction wave that runs through the entire material. The speed of the compaction wave is larger than the impact velocity but considerably smaller than the sound speed. The wave speed is related to the compaction rate at the colliding surface and the average slope of the linear density profile.

Compositional Effects and Mechanical Parametric Analysis of Outwash Deposits Based on the Randomised Generation of Stone Blocks

Based on the distribution of the stone blocks in outwash deposits, the paper present a modeling method for the random structure of outwash deposits, in which the long axis of the stone blocks is supposed obeying a lognormal distribution. Then numerical experiments of biaxial compression using the granular discrete element method are used in the macro- and micro parametric analysis. The influences of strength of the cementation, the sizes of stone blocks, and the content of stone blocks on the peak compressive and shear strength are discussed. The micromechanical parameters of the outwash deposits are also analyzed. The proposed method offers a supplement to the mechanical characterization of outwash deposits and accounts for the limitation that indoor experiments cannot consider large stone blocks.

Numerical direct shear tests for outwash deposits with random structure and composition

Outwash deposits is a kind of special geological material composed of soil and stone particles. Since the shear behaviors of outwash deposits are mainly dominated by its complicated structure and composition the strength parameters of which are difficult to be determined accurately. In this work, samples of outwash deposits with random structure and composition taking account of the effects of sizes, shapes and distributions of stone blocks were simulated based on the granular discrete element method. The meso-scale parameters of soil and stone particles were first calibrated by comparing numerical predictions with experimental data. Then a series of numerical direct shear tests for the simulated samples were conducted. Results show that the effects of strain hardening of shear stress–displacement curves become increasingly apparent with the increase of stone content. The shear strength of outwash deposits is dominated by the content as well as by the random distribution of stone blocks. There is an approximate linear relationship between the mean value of shear strength and the stone content lying between 30 and 60 $$\%$$ , while a larger fluctuation in shear strength mainly due to the random distribution of stone blocks. Compared with the mean value of shear strength, the maximum fluctuation of internal friction angle reaches up to 4.26 $$^{\circ }$$ while the maximum fluctuation of cohesion approximates up to 1/3 of its mean value. Results also indicate that the improvement of cementation degree between soil particles increases mainly the macro cohesion of outwash deposits but has almost no influence on the macro internal friction angle. And because of a relative higher strength of stone blocks, the shear yield bands of outwash deposits detours significantly around stone blocks and shows numerous local cracks, which frequently accompanies an acute release of strain energy owing to the geometric redistribution of stone blocks. The integration of laboratory investigations and numerical direct shear tests can be an effective approach to determine the statistical mechanic’s parameters of outwash deposits.

Mechanism of the Donghekou landslide triggered by the 2008 Wenchuan earthquake revealed by discrete element modeling

Abstract. The huge Donghekou landslide was triggered by the Wenchuan earthquake in 2008 with about 2.4 × 107 m3 of rock displaced. The landslide is considered as an example of an earthquake-induced "ejection" event, where dislocated slope materials was expelled over a section of the slope, but the kinematic processes are not well understood. We used the 2-D granular discrete element method to characterize the kinematic behavior and mechanics of this "ejection landslide". The initial boundary conditions were applied along the ball–wall contacts by using derived velocities integrated from strong motion data with a duration of 125 s, including the peak acceleration near the Donghekou area. The constraints were primarily determined from the final geometry of the landslide and geological structures to account for the actual landslide characteristics. Simulated results showed that the large local seismic acceleration and a free face under the sliding body, caused by the dip difference between the upper slide face and the natural slope, originated from the activation of the landslide. For the lower sliding body, its kinematic mechanism was changed during sliding. Initially it was a push-type landslide, and then gradually changed to a retrogressive landslide. The eroded bed on the slope during the landslide had the potential of slightly increasing the runout distance from 1435 to 1519 m, and was predicted in the numerical simulation.

Numerical Simulation of Loading Test on Rubble Bed by Granular Discrete Element Method

Based on engineering tests of rubble bed, numerical simulation of loading tests adopted discrete element software PFC2D was designed. The numerical model of rubble bed considering characteristics of shape, particle size, compactness and aggregate gradation was established, and then the engineering tests were reappeared by the means of applying continuous loading. The mechanical mechanism of rubble bed under continuous loading was analyzed based on comparison on stress-strain curve of engineering test with numerical simulation and the settlement and failure law of rubble bed under continuous loading were obtained. The work also discussed the effect of different compactness on bearing capacity of rubble bed and fitted the curve between compactness and bearing capacity, so it can be taken as a reference for analysis on loading compaction and bearing capacity of rubble bed.

Risk analysis of building damage induced by landslide impact disaster

Assessing building damage in a geological landslide is a dynamic process in which the event can be regarded as the combined effect of the landslide impact and the building’s response. In conventional models, a landslide mass is typically assumed to be rigid, and standard energy principles are used to calculate equivalent impact loads. Because the disintegration phenomenon of a landslide mass is being ignored, the conventional method often leads to a larger impact force than observed in reality. Based on a case study of a critical landslide, this paper applies a granular discrete element method to investigate landslide processes. First, macroscopic and meso-structural parameters are calibrated through biaxial simulation tests. Second, energy conversion laws between the impact force, geotechnical strength parameters and the outrun distance of the landslide are studied, providing a more realistic load curve of the landslide impact. Finally, a distressed building model is established through a loading curve analysis to study the structural damage state with different parameters. Numerical results show that building damage can be characterised through an equivalent impact force and that the precise calculation of dissipated energy in a landslide is greatly improved by taking into account both the energy dissipation at the slipping surface and the internal friction in the rock mass. The proposed evaluation factor of the damage degree of buildings shows great promise in analysing landslide motion and assessing the safety of building structures.

Mesomechanical simulation of direct shear test on outwash deposits with granular discrete element method

The mechanical properties of outwash deposits which are taken as unconsolidated geo-materials with the characteristics of non-uniformity, heterogeneity and multiphase have attracted much attention in engineering. According to the results of laboratory direct shear test on the remolded samples, the soil particle parameters of numerical model based on in-situ particle size cumulative curves and 3D granular discrete element method were determined. Then, numerical experiments on different lithology, stone content and gradation composition were conducted. The results show that it is not a flat surface but a shear band that yields in the sample. The curve of particle velocity vs distance from the designed shear surface of test model that is taken as a datum plane in the vertical section of sample shows in “S” shape. The shear disturbance area is about twice the maximum diameter of stone blocks. The greater the stiffness of stone is, the rougher the shear surface is. The shear strength of outwash deposits is largely controlled by lithology and stone content, and the bite force between stone blocks is the root reason of larger friction angle. It is also shown that strain hardening and low shear dilatancy occur under high confining pressure as well as possibility of shear shrinkage. But it is easy to behave shear dilatation and strain softening under low confining pressure. The relationship between particle frictional coefficient and stone content presents an approximately quadratic parabola increase. The strain energy first increases and then drops with the increase of frictional energy. The cohesion increases with soil stiffness increasing but decreases with stone stiffness increasing. Numerical results are consistent with the laboratory test results of remolded samples, which indicate that this method can be a beneficial supplement to determine the parameters of engineering deposit bodies.