ABSTRACT:

The Particle Flow Code (PFC) is a useful tool for simulating the failure behavior of hard rocks. However, one of the major drawbacks of the modeling is the unrealistically low ratios of simulated compressive strength to tensile strength for hard rock specimens. This means that the straightforward adoption of circular particles cannot fully reproduce the brittle failure of hard rocks. Instead, complex-shaped grain structures should be adopted for the model. The goal of this paper is to clarify the relationship between the irregularity of the clump configuration and the failure behavior of the simulation model. PFC simulations of unconfined compressive tests and Brazilian tensile tests were carried out using a triplet particle clump model, with clumps whose three same-radius particles partially overlap each other. From the simulation results, it was found that both compressive and tensile strength decrease with the increasing irregularity of the clump shape. It is inferred from the number of contact points and the growth of micro-cracks that irregular-shaped clumps have more freedom of movement, in relation to their adjacent clumps, and that this brings about the rapid growth of micro-cracks and lower compressive/tensile strength.

1 INTRODUCTION

The Particle Flow Code (PFC), a numerical simulation code based on the distinct element method (DEM), is a useful tool for simulating the failure behavior of hard rocks. However, one of the major drawbacks of the PFC modeling is the unrealistically low ratios of simulated compressive strength to tensile strength for hard rock specimens. This means that the straightforward adoption of circular particles cannot fully reproduce the brittle failure of hard rocks. Therefore, complexshaped grain structures should be adopted for the model. To overcome this limitation, Cho et al. (2007) introduced clumped-particle geometry. It allowed us to reproduce correct strength ratios (e.g., Funatsu et al. 2008). On the other hand, no clear criterion has been found for determining the size and the configuration of the clumps (Nakashima et al. 2013, 2015).

The goal of this paper is to clarify the relationship between the irregularity of the clump configuration and the failure behavior of the simulation model. Simulations of unconfined compressive tests and Brazilian tensile tests were carried out. The effect of the surface roughness of the clumps was examined by varying the degree of the overlapping of the particles in the clumps. Based on the results, the effect of clump roughness on the compressive and tensile strength of the rocks will be discussed.

2 OUTLINE OF PFC SIMULATION WITH CLUMPED PARTICLE MODEL AND TRIPLET PARTICLE CLUMPS
2.1 Clumped particle model

In the PFC simulation, objects are modeled as an assembly of rigid balls (two-dimensional disks). The balls are connected by normal and shear springs at the contact points and by micro-bonding (parallel bonds), as shown in Figure 1. The clumped particle model is illustrated in Figure 2. A clump is a group of particles that is rigidly connected and that behaves as a single element. Each clump is unbreakable, and adjacent clumps are connected to each other by contact springs and micro-bonding, like the ball-ball connection in the non-clumped model. The clumped particle model enables the reproduction of a large ratio of compressive strength to tensile strength, characteristic of hard rocks.

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