A. Fakhimi

Simulation of failure around a circular opening in rock

Abstract A biaxial compression test was performed on a sandstone specimen with a circular opening to simulate a loading-type failure around an underground excavation in brittle rock. The axial force and displacements were monitored throughout the failure process, and microcracking was detected by the acoustic emission technique. To model the observed damage zone around the opening, the distinct element computer program, particle flow code (PFC2D), was used. The numerical model consisted of several circular elements that can interact through contact stiffness, exhibit strength through contact bonds and particle friction, and develop damage through fracture of bonds. For the determination of micro-mechanical parameters needed in the calibration process of the computer program, only the macroscopic parameters of Youngs modulus, Poissons ratio and uniaxial compressive strength were used. It is shown that PFC2D was capable of simulating the localization behavior of the rock and the numerical model was able to reproduce the damage zone observed in the laboratory test.

A model for the time-dependent behavior of rock

Abstract Time-dependent stability of rock structures is studied using a visco-elasto-plastic constitutive model. The model consists of an elasto-plastic Mohr-Coulomb unit in series with a linear viscous unit. Both cohesion and friction are assumed to evolve with plastic and viscous strain in order to incorporate strain softening and time-dependent material degradation. To avoid mesh sensitivity in the numerical analysis, a non-local approach is introduced. The numerical results obtained for the long-term stability of a simulated biaxial test seem to be qualitatively consistent with experimental results reported in the literature.

Experimental and Numerical Study of the Effect of an Oversize Particle on the Shear Strength of Mined-Rock Pile Material

During in situ direct shear testing on the Questa mined-rock pile material, oversize rock fragments were encountered in the shear box. To study the effect of an oversize particle on the measured friction angles and cohesions, both laboratory and numerical direct shear tests were conducted on unsaturated samples of the rock pile material. In the numerical simulation, a hybrid discrete-finite element method was implemented in which the rock pile material was modeled with discrete particles while the shear box was modeled with finite elements. Both the numerical and physical tests suggest an increase in the measured shear strength of the material due to the presence of an oversize particle in the box. More specifically, the presence of an oversize particle causes an increase in friction angle while cohesion in most situations decreases. The location of the oversize particle along the shear plane was found to be important, as it can modify the failure mechanism.

Scaling of the fracture process zone in rock

The zone of microcracks surrounding a notch tip—the process zone—is a phenomenon observed in fracture of quasi-brittle materials, and the characterization of the process zone is the topic of the paper. Specimens of different sizes with a center notch fabricated from a granite of large grain (Rockville granite, average grain size of 10 mm), were tested in three-point bending. Acoustic emissions were recorded and locations of microcracks were determined up to peak load. The results show that both the length and width of the process zone increase with the increase of the specimen size. Furthermore, the suitability of a proposed theoretical relationship between the length and width of the process zone and specimen size was studied experimentally and numerically. The discrete element method with a tension softening contact bond model was used to investigate the development of the process zone with the specimen size. A synthetic rock composed of rigid circular particles that interact through normal and shear springs was tested in the numerical simulations. It was shown that the limiting specimen size, beyond which no further noticeable increase in the length of the process zone is observed, is significantly larger than the limiting specimen size beyond which the width of the process zone shows no size effect.

Development of a Modified in situ Direct Shear Test Technique to Determine Shear Strength Parameters of Mine Rock Piles

A modified direct shear test apparatus was designed and used to measure cohesion and friction angle of rock pile materials. Two test apparatuses were constructed, a 30-cm square metal shear box and a 60-cm square metal shear box. In addition to the shear box, the testing apparatus has a metal top plate, a fabricated roller plate, normal and shear dial gages with wooden supports, and two hydraulic jacks and cylinders with a maximum oil pressure of 70 MPa (10,000 psi) and a load capacity of ten tons. The main difference between the in situ shear box and its conventional laboratory equivalent is that the in situ shear box consists of a single box that confines an excavated block of rock pile material. The lower half of the block consists of the rock pile material underneath the shear plane that is a semi-infinite domain. This modification in the shear test apparatus reduces the time needed for block preparation, helps perform several tests at different levels of the same sample block, and allows for accommodating large shear displacement with no reduction in the area of the shear plane.