Development of Cuttability Criteria for Rocks

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9612035 Fragmentation of rock is important for many applications in rock engineering. The excavation of underground facilities is increasingly sought to accommodate the complex demands of today's society, and it is important to develop new ways to safely and cost-effectively remove rock, particularly hard rock. Very few mechanical excavators have been designed to operate in hard rock, basically because of a lack of knowledge of the cutting process. This research will lead to an improved understanding of the basic mechanisms involved in mechanical breakage of rock, and will thus ultimately contribute to improvements in the design and/or mode of operation of rock excavation machines. The main objectives of the proposed research are to develop (1) cuttability criteria and (2) and index test to evaluate the ability to cut rocks using mechanical indentor-type tools (such as disk cutters used on tunnel boring machines, TBMs). The essential mechanism involved in the mechanical breakage of rocks by indentation is the development of a crushed zone as a prelude to the formation of tensile cracks, which eventually cause chipping of the rock. The existence of this crushed zone imposes constraints on the efficiency or even the ability to cut rocks by mechanical means. Indeed, compressive damage of the rock beneath the indentor limits the magnitude of the compressive stress required to initiate and propagate tensile cracks. ***