Mechanical Characterization of Modeling Clay.

Soil structure governs water and air flow but also affects soil stiffness and strength. Despite their importance, quantitative relations between structure, mechanical and hydraulic properties of soil are still poorly understood. A rigorous “top-down" approach characterizing for example the influence of macropores on soil mechanical and hydraulic properties fails because of the complexity of macropore structure and the heterogeneity of the soil matrix material. This study presents a first step towards a "bottom-up" approach to describe the influence of pore structure on mechanical and hydraulic properties of soils. The idea is to comprehensively characterize mechanical properties of the matrix material to which, in a second step structure, in particular macropores, can be added in a controlled fashion. This way, the influence of macroporosity on soil mechanical and hydraulic behavior can be studied systematically. Mechanical properties of modeling clay as a surrogate for real elasto-viscoplastic soil matrix material were determined using unconfined and confined uniaxial compression tests. Elasto-plastic parameters of the modeling clay (Young's modulus, Poisson's ratio, yield stress, tangent modulus) were determined from one-dimensional stress-strain behavior. Viscosity was determined from one-dimensional stress-strain rate curves. Uniaxial stress strain and stress strain rate tests were modeled using the elasto-viscoplastic constitutive relations implemented in the Finite Element package COMSOL. Measurements show that elasto-viscoplastic deformation behavior of modeling clay can be described reasonably well with uniaxial tests. Uniaxial stress-strain and stress-strain-rate curves could be simulate straightforwardly only using the five material properties obtained from measurements and assuming the clay to deform as an elasto-viscoplastic solid material.