An insight into the yielding and normal compression of sand with irregularly-shaped particles using DEM

The micro mechanics of one-dimensional and isotropic normal compression of granular soil have recently been revealed using the discrete element method. By modelling soil grains as spheres and implementing a new crushing model, the authors have previously investigated the influence of fracture mechanism, particle strengths (and distributions), and the size-hardening law on both the normal compression line and resultant particle size distribution; this resulted in a new compression law. In this work, irregular particle shape is introduced, using ‘clumps’ (groups of spherical particles), allowing different relative densities of the same material to be subjected to normal compression. An investigation into the mechanics of yielding is presented, in which the onset of crushing is related to the average particle octahedral shear stress and ‘yield’ is seen to be a function of the available void space. Beyond yield, the normal compression lines for the clumps at different initial densities are examined and compared to that for spheres. The effect of coordination number and particle shape on the normal compression are studied, and in particular the micro mechanics behind the evolution of a fractal particle size distribution are revealed

Investigating geogrid-reinforced ballast: Experimental pull-out tests and discrete element modelling

AbstractThis paper presents an evaluation of the interlocking behaviour of geogrid-reinforced railway ballast. Experimental large box pull-out tests were conducted to examine the interaction between ballast and a biaxial geogrid. The discrete element method (DEM) was then used to model the interaction between the ballast and the geogrid by simulating large box pull-out tests and comparing the findings with the experimental results. Four different shapes of clumps were used to represent each ballast particle in order to obtain an acceptable shape for modelling the railway ballast. The DEM simulation results were shown to provide good predictions of the pull-out resistance and to examine the effect of clump shape on both the pull-out resistance and the distribution of contact forces. Therefore, the calibrated geogrid model and the 8-ball tetrahedral clumps, used as ballast particles, hold much promise for investigating the interaction between geogrids and ballast, and thus, optimising performance

On the micro mechanics of one-dimensional normal compression

Discrete-element modelling has been used to investigate the micro mechanics of one-dimensional compression. One-dimensional compression is modelled in three dimensions using an oedometer and a large number of particles, and without the use of agglomerates. The fracture of a particle is governed by the octahedral shear stress within the particle due to the multiple contacts and a Weibull distribution of strengths. Different fracture mechanisms are considered, and the influence of the distribution of fragments produced for each fracture on the global particle size distribution and the slope of the normal compression line is investigated. Using the discrete-element method, compression is related to the evolution of a fractal distribution of particles. The compression index is found to be solely a function of the strengths of the particles as a function of size

Micro mechanics of critical states for isotropically overconsolidated sand

The discrete element method has been used to investigate the micro mechanics of shearing to a critical state on the loose and dense sides of critical. Isotropic compression has previously been modelled in 3D using a large number of particles and without the use of agglomerates. The same procedure is used here. Particle fracture is governed by the octahedral shear stress within the particle due to the multiple contacts and a Weibull distribution of strengths. Isotropic compression of a silica sand has been simulated to 20 MPa and followed by unloading to a range of stresses before shearing to a critical state, using micro parameters which relate to the silica sand particle strengths. The samples at the lowest stress levels exhibit peak strength and dilation. The sample at the highest stress exhibits contraction and ductile yielding to a critical state. A critical state line is established, which appears to become parallel to the isotropic line in log e-log p space at high stress levels. This paper shows that it is the evolving fractal particle size distribution during isotropic normal compression which governs the behaviour on unloading to different overconsolidation ratios. The micro mechanics of the critical state line are shown to be in the evolving particle size distribution during normal compression, and how such an aggregate behaves when it is unloaded

The handbook for standardised field and laboratory measurements in terrestrial climate-change experiments and observational studies

Climate change is a worldwide threat to biodiversity and ecosystem structure, functioning, and services. To understand the underlying drivers and mechanisms, and to predict the consequences for nature and people, we urgently need better understanding of the direction and magnitude of climate‐change impacts across the soil–plant–atmosphere continuum. An increasing number of climate‐change studies is creating new opportunities for meaningful and high‐quality generalisations and improved process understanding. However, significant challenges exist related to data availability and/or compatibility across studies, compromising opportunities for data re‐use, synthesis, and upscaling. Many of these challenges relate to a lack of an established “best practice” for measuring key impacts and responses. This restrains our current understanding of complex processes and mechanisms in terrestrial ecosystems related to climate change