Natural geomaterials often exhibit pore size distributions with two dominant porosity scales. Examples include fractured rocks where the dominant porosities are those of the fractures and rock matrix, and aggregated soils where the dominant porosities are those of the micropores and macropores. I will present a framework for this type of materials that covers both steady-state and transient fluid flow responses. The framework relies on a thermodynamically consistent effective stress previously developed for porous media with two dominant porosity scales. I will show that this effective stress is equivalent to the weighted sum of the individual effective stresses in the micropores and macropores, with the weighting done according to the pore fractions. Apart from this feature, some geomaterials such as shale exhibit pronounced anisotropy in their hydromechanical behavior due to the presence of distinct bedding planes. In this talk I will also present a thermo-plastic framework for transversely isotropic materials incorporating anisotropy and thermal effects in both elastic and plastic responses. Computational stress-point simulations under isothermal and adiabatic conditions reveal the importance of anisotropy and thermal effects on the inception of a deformation band. I will show that anisotropy promotes the formation of dilation band across a wide range of bedding plane orientations relative to the direction of loading.
Ronaldo Borja works in theoretical and computational solid mechanics, geomechanics, and geosciences. His research includes the development of multiscale discontinuity framework for crack and fracture propagation utilizing the strong discontinuity and extended finite element methods; solution techniques for multi-physical processes such as coupled solid deformation-fluid diffusion in saturated and unsaturated porous media; stabilized finite element methods for solid/fluid interaction and nonlinear contact mechanics; and nanometer-scale characterization of the inelastic deformation and fracture properties of shales. Ronaldo Borja is the author of a textbook entitled Plasticity Modeling and Computation published by Springer. He serves as editor of two high-impact journals in his field, the International Journal for Numerical and Analytical Methods in Geomechanics published by Wiley, and Acta Geotechnica published by Springer. He has given a number of distinguished lectures at various universities, the latest ones being the 2016 Szeto Wai Lecture at the University of Hong Kong and the 2017 John H. Argyris Honorary Lecture at the University of Stuttgart. Ronaldo Borja is the recipient of the 2016 ASCE Maurice A. Biot Medal for his work in computational poromechanics.