Y. Sieffert

Strength of an Australian Coal Under Low Confinement

Experimental testing of brittle rocks has shown that both brittle and ductile behaviours can be observed, depending on the level of confinement applied to the specimen. In particular, brittle rocks fail in a brittle mode as long as the confining stress falls below the Mogi line (Mogi 1966). Spalling of rocks is associated with brittle failure and is known to occur under low confinement, i.e. in the vicinity of excavation walls (Stacey 1981; Martin et al. 1999; Cai and Kaiser 2013). Indeed, at low confinement, large tension cracks may develop parallel to the excavation boundary when the stress exceeds the crack initiation threshold, which may lead to rapidly propagating instabilities and formation of thin slabs. Such slabs can represent a significant hazard to the workforce in confined mining excavations. Increasing the level of confinement modifies the nature and propagation mechanism of the cracks that develop upon loading: at high confinement, short shear cracks develop and ultimately join to form a macroscopic shear band. Martin et al. (1999) showed that a single set of Hoek–Brown parameters failed to capture the two mechanisms and they distinguished Hoek–Brown frictional (for high confinement) and brittle (for low confinement) sets of parameters. Their proposed brittle criterion falls below the frictional counterpart reflecting a reduction in strength. Recently, Kaiser and Kim (2008) and Amann et al. (2012) proposed a non-convex criterion to capture the strength under both low and high confining pressures. However, some of the data they used involved a large degree of scatter (in Kaiser and Kim 2008) or not many points were obtained in the low confining range (in Amann et al. 2012). Considering the recent findings by Kaiser et al. and the lack of data in the literature about the strength of coal under low confinement, it has been decided to conduct a series of triaxial tests in order to mitigate this gap. Gaining a better understanding of the behaviour of the coal under low confinement is highly relevant for the stability of coal mine excavations.

Hydromechanical modeling of an initial boundary value problem: Studies of non-uniqueness with a second gradient continuum

Abstract A non-uniqueness study for a hydromechanical boundary value problem is performed. A fully saturated porous medium is considered using two different elasto-plastic constitutive equations to describe the mechanical behavior of the skeleton. Both models are based on the Drucker–Prager yield criterion with a hyperbolic hardening rule for the cohesion and friction angle as a function of an equivalent plastic strain. The two constitutive equations taken into account (Plasol and Aniso-Plasol) differ only for the elastic part of the model: isotropic elasticity or cross anisotropic elasticity respectively. A real hydromechanical experiment which consists in a hollow cylinder test on a Boom Clay sample is modeled in two phases. For the first phase (a hydromechanical unloading) non-uniqueness studies are carried out using both constitutive equations. In the second phase, boundary conditions are kept constant to dissipate the excess water pressure. It is shown in the first phase that the time step discretization of the numerical problem has an effect on the initialization of the Newton–Raphson algorithm on a given time step. Different solutions for the same initial boundary value problem can consequently be found. A convergence study is also presented giving an insight into the behavior of the computation during the iterations.

Modelling Hydromechanical Dilation Geomaterial—Cavitation and Localization

This paper presents an extension of the local second gradient model to multiphasic materials (solid particles, air, water) and including the cavitation phenomenon. This new development was made in order to model the response of saturated dilatant materials under deviatoric stress and undrained conditions and possibly, in future, the behaviour of unsaturated soils.

Hydromechanical Modelling of an Initial Boundary Value Problem: Studies of Non-uniqueness with a Second Gradient Continuum

A non-uniqueness study for a hydromechanical boundary value problem is performed. A fully saturated porous medium is considered using an elasto-plastic constitutive equations to describe the mechanical behavior of the skeleton. A real hydromechanical experiment which consists in a hollow cylinder test on a Boom Clay sample is modelled. It is shown that the time step discretisation of the numerical problem has an effect on the initialisation of the Newton-Raphson algorithm on a given time step. Different solutions for the same initial boundary value problem can consequently be found.

Local Second Gradient Models for Thermo-Hydro-Mechanical Coupling in Rock Like Materials

In the design of nuclear waste disposals, an important topic concerns the evolution of an Excavated Damage Zone (EDZ) with a thermal exchanges. In this paper, a new model of local second gradient coupling with a thermo-hydro-mechanical is presented. As for monophasic case, the use of enhanced media induce the objective of the computation but not the uniqueness of the solution. Some classical engineering problems are presented which exhibit several solutions.