ABSTRACT:

The strength of intact rock has been used widely for the evaluation of rock properties for excavation. However, the performance of boring and excavation of a rock mass is affected by not only its compression strength but also the comprehensive characteristics including the type and mechanical properties of rock-forming minerals. In this study, three kinds of granites were evaluated based on general properties as the intact rock and micro-scale mechanical properties of the rock-forming minerals contained in rocks. Micro-scale properties such as hardness and Young's modulus of grains of quartz and feldspars were measured by the depth sensing indentation test (DSI test). The DSI test, a kind of indentation hardness test, is the technique that measures continuously indentation load and indentation depth in pushing a very small indenter into a surface of a specimen. As a result, micro-scale mechanical properties of the granite which was difficult to excavate by tunnel boring machine were heterogeneous significantly though those of same kinds of minerals that were contained in common granites were roughly uniform.

1. INTRODUCTION

In the excavation works of rock mass, the intact rock strength is widely used for evaluating the mechanical characteristics of rock and for judging difficulty of excavation. This judgment procedure is based on the view that a rock mass is excavated by doing destruction as rock debris. For example, some empirical formulas that associated the excavation speed of a tunnel boring machine with the strength of rock were obtained [1]. On the other hand, the performance of excavation of a rock mass is affected not only by its compressive strength but also by the comprehensive characteristics including the type and properties of rock-forming minerals and the properties of the rock mass [2]. In actual fact, a trouble that excavation efficiency was deteriorated and the rate of tunnel advance was reduced occurred in a granite rock mass, which one cause was interpreted as the numerous micro cracks in the grains of rock-forming minerals [3]. However, the properties of rock-forming minerals have not been usually considered to estimate the excavation efficiency. Furthermore it is not rare that the micro cracks exists in the granite [4, 5], and it is not possible to evaluate the mechanical properties only by observing the existence of the cracks. It has been used also for evaluating the relationship between rock's sawability and performance of a cutting tool. Best and Jacobson [6] measured the microscopic hardness in the rock matrixes using a micro indenter by penetrating objective rocks with a very low load. They elucidated the importance of scale in the hardness measurement especially for the multi-mineral rocks such as granitic rocks. They recommended that the hardness measured at micro-scale should be used to judge the wear of the rock drilling tools because the wear of such tools operate on very small scale actually. Some authors [7, 8] measured the micro-hardness in mineral grains contained in granitic rocks and discuss the sawability of rocks and the performance of rock tools.

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