Numerical simulation of UCS test of rocks based on PFC3D modeling

Abstract

Decision making at the early stage of construction design is a million-dollar question. That is why many laboratory tests and loggings are deployed to help understand behavior of rocks. Uniaxial Compressive Strength (UCS) testing is considered to be one of the most important steps during the design stage. However, time and cost of preparing and running UCS test is a constrain in some circumstances. It also requires good quality of rock samples to produce a representative UCS value. In recent development of numerical simulation, Discrete Element Method (DEM) has been evolved and used widely to simulate the characteristic of rock. Rock is represented by a dense and well-connected assembly non uniform size of rigid circular disks (2 dimensions) or spherical particles (3 dimensions) with their points of contact are bonded together. The macroscopic mechanic behavior is represented by microscopic properties of particles stiffness and strength of the bonds. The quality of information for numerical simulation is comparable to the actual laboratory result. The advantages of this method are time saving and low cost in compare to performing laboratory test on actual rock. Extensive calibration is only required to determine micro scale parameters with actual macro-scale results. In addition, laboratory scale simulation results can be used as optimum input parameters in field scale simulations. Many case studies from researchers have shown that proper design and calibration of numerical model will bring huge values in running simulations, such as tunneling excavation and mining activities. In this study, a numerical simulation of UCS is carried out using three-dimensional Particle Flow Code (PFC3D) developed by ITASCA Consulting Group. The work focuses on developing and calibrating of numerical model using the discrete element method adopted by PFC3D. The physical rock specimens tested are Kowloon granite with an experimental UCS value and Young’s modulus at 50% UCS of 189.42 MPa and 22.0 GPa respectively. The test measures the axial stress-strain behavior, UCS value, and Young’s modulus. The numerical UCS value and Young’s modulus at 50% UCS are 192.40 MPa and 25.1 GPa respectively. The simulation model is formed by spherical particles with a diameter ranged from 1.0-1.5 mm. During simulations, micro parameters are calibrated by trial-and-error with the data from physical experiment.