57 th Canadian Geotechnical Conference 5 th Joint IAH-CNC/CGS Conference Hilton Hotel, October 24-28, 2004 Québec city, Québec, Canada MICROMECHANICAL CONSIDERATIONS OF PARTICLE BREAKAGE USING DISCRETE ELEMENT METHOD A. A. Mirghasemi, Department of Civil Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran E. Seyedi Hosseininia, Department of Civil Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran ABSTRACT Using DEM (Discrete Element Method), a model is presented to simulate the breakage of two-dimensional polygon- shaped particles. In this model, shapes of the particles generated after breakage are predefined and each uniform (uncracked) particle is replaced with smaller inter-connected sub-particles. If the bond between these sub-particles breaks, breakage will happen. With the help of this model, it is possible to study the influence of particle breakage on macro and micro mechanical parameters. In this simulation, the evolution of microstructure in granular assemblies can be seen by tracing of coordination number during the shear process. Also variation of contact normal, normal force and tangential force anisotropy can be tracked. For this purpose, two series of biaxial test simulations (breakage is enabled and disabled) are conducted on assemblies of two-dimensional polygon-shaped particles and the results are compared. It is found that the simulation results are in good agreement with observations obtained from experimental tests. 1. INTRODUCTION Stability of soil structures such as breakwaters and rockfill dams is in concern of shear parameters and the behavior of granular media. In general, shear resistance and behavior of granular materials depends on different factors such as mineralogical composition, particle grading, size and shape of particles, fragmentations of particles and stress conditions. In such high earth structures, the underlying layers bear significant weight of the upper layers, the soil grains in the underlying layers are subjected to significant stress magnitudes. The induced high stresses may cause the particles to be broken. Particle breakage and crushing of large particles to smaller ones, results in changes in grain size (gradation) curve; therefore, the mechanical behavior of granular material alters. In this paper the results of performed tests for simulating particle breakage using Discrete Element Method is presented and the assembly behavior in terms of macroscopic and microscopic parameters is discussed. 2. REVIEW Influence of particle breakage on internal friction angle and deformability of granular materials can be studied using experimental tests such as Triaxial and unconfined compression tests((Marsal 1967, Bertacchi et al. 1970, Fumagali et al. 1970, and Marachi et al. 1972, Ansari & Chandra 1986, Venkatachalam 1993,Varadarajan et al. 2003). Marshal(1967) by performing Triaxial compression tests on coarse granular materials found out that the most important factor affecting both shear strength and compressibility is the phenomenon of fragmentation undergone by a granular body when subjected to change in its state of stress both during uniform compression stage and during deviatoric load application. Also the results showed that in granular media, the compressibility is a consequence of complex phenomenon that takes place as a result of displacements between particles combined with the particle breakage. Varadarajan et al.(2003) have investigated the behavior of two dam site rock materials (Ranjit Sagar and Purulia) in Triaxial compression tests which the former consisted of rounded and the latter angular particles. It is interesting to note that the volume change behavior of two rockfill materials was significantly different from each other. During the shearing stage of the triaxial test, compression, rearrangement and breakage of particles took place. The rounded material exhibited continuous volume compression, while the angular particles dilated and expanded after initial compression in volume. Granular materials provide a high degree of interlocking and cause dilation during shearing. Also they observed that a greater degree of particle breakage occurs with the larger particles because of the greater force per contact (Lame and Whitman 1969). There are two factors governing on the shear resistance as interlocking between particle and particle breakage. The effect of increase in interlocking is to increase the shearing resistance, while the effect of breakage of particles is vice versa. Also it is noted that angular particles are more susceptible to break that rounded particles. Performing such tests on material with large particles in order to study the behavior of materials such as rockfill would be costly due to the required large size of specimen. At the present research, an investigation is made to study the influence of particle breakage on behavior of granular media. In recent years, along with the progress of numerical methods and computer technology, different methods have been used to model breakage of brittle bodies with the help of Discrete Element Method (DEM). Among these methods, are the approach used by Cundall (1978), the method based