| Several series of tests were conducted in samples of six rocks to determine the relationship of various common parameters used to predict TBM performance, in response to an inquiry into the nature of rock toughness and how it affects mechanical rock. To investigate toughness, physical properties and indention tests were compared to the cuttability of each rock by a disc cutter, since the ultimate indicator of toughness is the degree-of difficulty of achieving rock failure. For each rock type (two limestones, a welded tuff, two granites and a basalt), the same unvarying linear disc cutting tests, physical property tests, and block- and core-indentation tests were conducted, using the same apparatus for each test. Additionally, the indented rock samples were cut open using a diamond saw, to assess the interior damage resulting from the indentation. The disc cutting tests were conducted using a single disc cutter and constant spacing in a linear cutting machine.; The test results show that uniaxial compressive strength and specific penetration (force per unit penetration) from large-block indentation tests both correlate strongly with disc cutter specific penetration; both relationships are logarithmic. None of the other tests, including core indentation, exhibit correlations as high. The damage caused by indentation in large blocks of unfractured rock is much more limited than from indentation of core samples, being restricted to relatively shallow depths below the indentor and producing less rock debris. In addition, damage depth below the indentor increases as rock strength decreases.; These results indicate new, productive directions in TBM performance prediction, and suggest that a comprehensive experimental program (consisting of complete physical properties, cutting tests, and damage assessment) is needed before a better understanding of rock failure can be reached. Rock toughness was determined to be defined by two interrelated criteria: (1) A tough rock has high resistance to penetration by a cutting tool, and (2) A tough rock has low debris (cuttings) yield when it is penetrated to failure. |
