Lattice Boltzmann simulation and mesoscopic mechanism analysis of permeability in soil-rock mixtures

The subgrade constructed by soil–rock mixture (SRM) usually suffers from slope instability, subgrade collapse, and uneven settlement, which are closely related to the permeability of the SRM. In order to reveal the internal influence mechanism of the seepage characteristics of the SRM, the SRM models of dense structure (DS), medium-dense structure (MDS), and loose structure (LS) were generated based on the improved Monte Carlo method (IMCM) and the self-made MATLAB program, and the lattice Boltzmann method (LBM) was introduced to study the mesoscopic seepage characteristics of SRM under different influence factors. Finally, the internal mechanism of the influence of rock content rate ( RCR ) on the SRM permeability under different conditions was discussed in detail with a simplified model. The results show that the permeability of the SRM in DS, MDS, and LS cases tends to increase continuously with the increase of RCR when the loose-density degree ( LD ) and rock size ( D R ) are the same. The looser the structure of the SRM samples, the larger the permeability. When the RCR is lower than the threshold value, the larger the D R the slower the increase of the permeability; but when the RCR is higher than the threshold value, the larger the D R the significantly larger the increase of the permeability. In this study, the threshold value is determined to be about 35%. The RCR sensitivity index ( S ) shows an overall trend of increasing, then decreasing, and then slowly increasing with increasing RCR , in which large rocks and MDS have a facilitating effect on the sudden change of SRM permeability. When the soil/rock particle size is constant, the permeability of the simplified model tends to “decrease → increase → decrease” with the increase of RCR . This study provides an IMCM-LBM joint simulation method as a promising option to simulate realistic SRM particles permeability in geotechnical experiments.