Abstract
The influence of block forms on the shear behaviour of soil–rock mixtures with soft blocks (soft S–RMs) can be efficiently investigated by the discrete element method (DEM) on the basis of accurate 3D models accounting for the block breakage. This paper proposes a novel modelling approach, based on the spherical harmonics series, for the generation of 3D block geometries with different forms but same convexity and angularity. An already existing non-overlapping modelling approach was improved, characterized by a reduced computational cost, for the set-up of 3D block DEM models accounting for the block breakage. A number of soft S–RM DEM samples, subjected to numerical direct shear tests, were generated to analyze the influence of block forms and volumetric block proportion VBP on the mesoscopic and macroscopic behaviours. The results showed that the breakage degree is maximum for the spheroidal blocks, followed by the oblate, prolate and blade ones, due to the combined influence of the block frictional sliding and rotation. The shear strength of soft S–RMs is mainly controlled by the block interlocking and breakage, being maximum in the case of spheroidal block samples when the applied normal stress is low and in the case of prolate and blade ones for a high normal stress. It was found that a nonlinear Mohr–Coulomb criterion can provide a good description of the shear strength envelope of soft S–RMs. Soft S–RMs are characterized by a higher friction angle if composed by spheroidal and prolate blocks when the VBP is 40%, due to their elevated block interlocking, and in the case of prolate and blade blocks when the VBP is 60% at the higher normal stress, due to their lower block breakage degree.
Highlights
Soil–rock mixtures (S–RMs) are extremely inhomogeneous geomaterials typically composed of rock blocks with various sizes and a fine-grained soil matrix (Xu 2008)
Two are the discrete element method (DEM) modelling approaches used for reproducing the morphological features of irregular particles on the basis of a sphere assembly: (a) the non-overlapping combination method developed by Xu et al (2016), with no or little overlapping being permitted between any pair of spherical balls; (b) the overlapping combination method proposed by Ai et al (2011) and Ferellec and McDowell (2010), included as a built-in function in the code PFC3D, allowing a large overlapping between two adjacent sphere balls
The breakage degree is maximum for the spheroidal blocks, followed by the oblate, prolate and blade ones
Summary
Soil–rock mixtures (S–RMs) are extremely inhomogeneous geomaterials typically composed of rock blocks with various sizes and a fine-grained soil matrix (Xu 2008). The geometries of the random convex or concave polyhedrons are still simplifying most of the surface details of real blocks, characterized by different morphological features To overcome this limit, a 3D modelling approach was proposed for the generation of a large number of random soft blocks having the same morphological features of the real blocks. To investigate the influence of block forms (e.g. spheroidal, oblate, prolate and blade) on the shear behaviour, eliminating the effect of other morphological features, a 3D modelling approach, characterized by block geometries with different forms but same convexity and angularity, was developed on the basis of SH series of the CT scanned blocks. Soft S–RMs models with four different block forms and two VBP, namely 40% and 60%, were generated and their response during direct shear test simulations was analyzed in detail
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