This study quantifies the effects of macropore spacing on unconfined compressive strength and qualifies the failure mode of rock material. Numerical modeling, with FLAC3D (finite difference) and TNO DIANA (finite element) software, is used to assess changes in unconfined compressive strength and failure modes of specimens containing macropores. Modeling results are verified using data from forty-four unconfined compression tests on 10 cm (4") cubic specimens of Hydrocal, a high-quality plaster. The specimens, both laboratory and numerical, are 10 cm (4") cubes with circular "tunnels" extending through the front and back faces. Results show that unconfined compressive strength increases as macropore center-to-center spacing increases with specimens of the same macroporosity. With increasing macroporosity, unconfined compressive strength decreases with laboratory results having higher scatter than numerical results. The failure modes of the specimens change from tensile to shear failure with increasing macroporosity center-to-center spacing. This change in failure mode explains the low unconfined compressive strength with small center-to-center spacing. At small center-to-center spacing, the macropores act as a single large void and the specimen exhibits relatively low strength and tensile failure. At large center-to-center spacing, the macropores act as individual macropores, resulting in relatively high strengths and shear failure between macropores.
1. BACKGROUND
The influence of microscopic and macroscopic porosity on both strength and stiffness of rock materials is well documented (for example [1] and [2]). In cases of microscopic porosity, it is often assumed the pores are homogeneously distributed throughout a specimen with higher porosity specimens have greater amounts of homogeneously distributed void spaces than specimens with lower porosities. The stress field interactions of the microscopic pores due to uniform loading is very similar and results in a general shear failure with a smooth curve defining changes in strength and stiffness with increasing porosity. This is reflected by the use of a variety of continuum models in rock mechanics. With macroporous (voids visible with the unaided eye) rock materials, this model is not valid. The relative sizes of the macropores compared to the specimen and their typically non-homogeneous distribution will produce non-uniform stress field interactions. This results in mixed mode and/or tensile failures and difficulties in assessing the changes in engineering properties with increasing macroporosity [3]. This study quantifies the effects of macropore spacing on unconfined compressive strength and qualifies the failure mode of rock material. Numerical modeling, with FLAC3D (finite difference) and TNO DIANA (finite element) software, is used to assess changes in unconfined compressive strength and failure modes of specimens containing macropores. Modeling results are verified using data from forty-four unconfined compression tests on 10cm (4") cubic specimens of Hydrocal, a high-quality plaster. The specimens, both laboratory and numerical, are 10 cm (4") cubes with circular "tunnels" extending through the front and back faces.
2. INTRODUCTION
Quantifying the effects of defects on the engineering properties of rock has been investigated by numerous researchers within the engineering community. The shape of defects has ranged from crack-like structures to circles (or spheres) and the size of the defects range from microscopic to macroscopic.
