Micromechanics of soils

The soil can be considered as a collection of grains or aggregates with microstructures, aggregates are composed of particles with interparticle voids, and grains or particles are made of minerals with atomic structures. Thus, the origin of mechanical behaviours/properties of the soil can be investigated by downscaling, and the mechanical modelling of the soil can be conducted by upscaling.

**Topic 1: Micromechanics-based model for granular materials**

Granular materials can be considered as a collection of grains, and the stress-strain relationship for the assembly can be determined by integrating the behaviour of inter-grain contacts in all directions. This micromechanical approach has advantages compared to the conventional modelling approach, i.e. the inherent or initial anisotropy of soils can be characterized by an orientation-dependence, which has a clear physical meaning and can be modelled in a direct way; adhesive forces, cementation can be considered at contact level; and so on.

- Zhao C-F#, Yin Z-Y*, Hicher P-Y (2018). A multiscale approach for investigating the effect of microstructural instability on global failure in granular materials. Int. J. Numer. Anal. Methods Geomech., 42(17): 2065-2094.
- Zhao C-F#, Yin Z-Y*, and Hicher P-Y (2018). Integrating a micromechanical model for multiscale analyses. Int. J. Numer. Meth. Eng., 114(2): 105-127.
- Zhao C-F#, Yin Z-Y*, Misra A, and Hicher P-Y (2018). Thermomechanical formulation for micromechanical elasto-plasticity in granular materials. Int. J. Solids Struct., 138(1): 64-75.
- Yin Z-Y*, Zhao J, Hicher PY (2014). A micromechanics-based model for sand-silt mixtures. Int. J. Solids Struct., 51(6): 1350–1363.
- Yin Z-Y*, Chang CS (2013). Stress-dilatancy behavior for sand under loading and unloading conditions. Int. J. Numer. Anal. Methods Geomech.. 37(8): 855-870.
- Yin Z-Y*, Chang CS, Hicher PY (2010). Micromechanical modelling for effect of inherent anisotropy on cyclic behaviour of sand. Int. J. Solids Struct., 47(14-15): 1933-1951.

**Topic 2: Micromechanics-based model for clay**

By observing the microscopic characteristics of clay and its evolution law in different scales (such as the scales of particles and aggregates), assuming the aggregates as basic mechanical unit to replace the grains of granular materials, the micromechanics-based model of granular materials is extended to clay. Several important features of clay have been studies using this approach: such as inherent and induced anisotropy, the microscopic mechanism of rotation hardening of macro yield surface, the additional strength and damage mechanism for structured soil (sensitive clay, hard clay, cemented soil, etc.).

- Yin Z-Y*, Xu Q, Chang CS (2013). Modeling cyclic behavior of clay by micromechanical approach. ASCE J. Eng. Mech., 139(9), 1305–1309.
- Yin Z-Y*, Chang CS, Hicher PY, Karstunen M (2009). Micromechanical analysis of kinematic hardening in natural clay. Int. J. Plasticity, 25(8): 1413-1435.
- Yin Z-Y*, Chang CS (2009). Microstructural modelling of stress-dependent behaviour of clay. Int. J. Solids Struct., 46(6): 1373-1388.

**Topic 3: Multi-scale modelling of granular materials **

This work is extended from 2D H-model of Prof. F. Nicot and collaborate with him. We developed 3D H-model considering mesoscopic scale, implemented the model into finite element code (ABAQUS/Explicit) with validation for different behaviors from RVE to BVP scales.

- Xiong H, Yin Z-Y*, Nicot F, Wautier A, Miot M, Darve F, Veylon G, Philippe P (2021). A novel multi-scale large deformation approach for modelling of granular collapse. Acta Geotech., DOI: 10.1007/s11440-020-01113-5.
- Xiong H#, Yin Z-Y*, Nicot F (2020). Programming the micro-mechanical model of granular materials in Julia. Adv. Eng. Softw., 145: 102816.
- Xiong H#, Yin Z-Y*, Nicot F(2019). A multiscale second-order work analysis approach for geotechnical structures. Int. J. Numer. Anal. Methods Geomech., 43(6): 1230-1250.
- Xiong H, Nicot F, Yin Z-Y* (2019). From microscale to boundary value problem: using a micromechanically-based model. Acta Geotech., 14(5): 1307–1323.
- Xiong H#, Nicot F, Yin Z-Y (2017). A three-dimensional micromechanically-based model. Int. J. Numer. Anal. Methods Geomech., 41(17): 1669–1686.

**Topic 4: Application of discrete element method and development**

We proposed an efficient method for sample generations with different particle shapes and porosity, proposed coupled FDM-DEM to model the membrane by FDM and grains by DEM for biaxial/triaxial tests, proposed soil-structure interface shear tests under different conditions, etc.

- Wang X, Yin Z-Y, Su D, Wu X, Zhao J (2021). A novel approach of random packing generation of complex-shaped 3D particles with controllable sizes and shapes. Acta Geotech., accepted.
- Zhang J, Wang X, Yin Z-Y*, Liang Z (2020). DEM modeling of large-scale triaxial test of rock clasts considering realistic particle shapes and flexible membrane boundary. Eng. Geol., 279: 105871.
- Li K#, Yin Z-Y*, Cheng Y, Cao P, Meng J (2020). Three-dimensional discrete element simulation of indirect tensile behaviour of a transversely isotropic rock. Int. J. Numer. Anal. Methods Geomech., 44(13): 1812-1832.
- Zhu H, Zhou WH, Jing XY, Yin Z-Y (2019). Observations on fabric evolution to a common micromechanical state at the soil-structure interface. Int. J. Numer. Anal. Methods Geomech., 43(15): 2449-2470.
- Zhu H#, Yin Z-Y* (2019). Grain Rotation-based Analysis Method for Shear Band. J. Eng. Mech. ASCE, 145(10): 04019073.
- Jiang MJ, Yin Z-Y, Shen ZF (2016). Shear band formation in lunar regolith by discrete element analyses. Granul. matter, 18:32.
- Liu YJ#, Li G#, Yin Z-Y*, Dano C, Hicher PY, Xia XH, Wang JH (2014). Influence of grading on undrained behavior of granular materials. CR Mecanique, 342: 85-95.

**Topic 5: Application of molecular dynamics method**

Most recently, we adopted the molecular dynamics method to simulate mechanical behaviours/properties of clay minerals at nano scale.

- Zhang LL, Zheng YY, Wei PC, Diao QF, Yin Z-Y (2021). Nanoscale mechanical behavior of kaolinite under uniaxial strain conditions. Appl. Clay Sci., DOI: 10.1016/j.clay.2020.105961.