Issue 49

C.Y. Liu et alii, Frattura ed Integrità Strutturale, 49 (2019) 557-567; DOI: 10.3221/IGF-ESIS.49.52 563 As shown in Fig. 3, the brittleness index B 14 cannot reflect the brittleness anomaly caused by the internal defects of the sample under a surrounding rock pressure of 12MPa. In addition, it is difficult to quantitatively obtain an accurate rock fracture angle, which may also limit the development of the brittleness index B 13 . Brittleness indices B 8 , B 11 , B 12 and B L can reflect the decrease trend of the brittleness index with the increasing surrounding rock pressure and the brittleness anomaly caused by the internal defects of rock samples under a surrounding rock pressure of 12MPa. The next section will further compare the brittleness indices B 8 , B 11 , B 12 and B L based on the results of the experiment. Figure 3: Variations of the core brittleness indices B L , B 14 and B 6 - B 12 with the surrounding rock pressure From the above analysis, it can be seen that the brittleness index B L proposed in this paper is in good consistency with B 8 , B 11 and B 12 . In order to further compare these brittleness indices, the failure characteristics are analyzed under the surrounding rock pressure of 0-12MPa and 12-25MPa. Fig. 4 shows the test results of the core under the surrounding rock pressure of 0MPa and 12MPa, where the red lines represent the fracture surfaces. As can be seen from Fig. 6, the fracture surface of core a is rougher than that of core b, with a larger fracture angle. In summary, core a is more brittle than core b. The brittleness indices B 8 , B 11 , B 12 and B L are not consistent with the above analysis results. However, the brittleness index B L can better reflect the changes. Fig. 5 shows the test results of cores c, d, and e under the surrounding rock pressure of 15, 18 and 25MPa, where the red lines represent the fracture surfaces. The fracture angels of core c, d and e are 70°, 65° and 60°, respectively. As can be seen from Fig. 7, in the three groups of samples, the roughness of the fracture surface gradually decreases from core c to e, so does the penetration. The fracture surface of core c penetrates the entire core, but those of cores d and e do not penetrate