Issue 49

C.Y. Liu et alii, Frattura ed Integrità Strutturale, 49 (2019) 557-567; DOI: 10.3221/IGF-ESIS.49.52 562 a deep hole in a tunnel in southwest China as the object. Considering the geostress conditions of the rock samples, the conventional triaxial compression test was carried out to verify the accuracy of the new brittleness index B L . This test was carried out using a MTS 815 test machine under different surrounding rock pressures. Considering the different depths at which the rock samples were taken, 6 surrounding rock pressures were set, which are namely 0, 3, 9, 12, 15, 18 and 25MPa. According to the regulations of the International Society for Rock Mechanics, samples were processed into standard specimens with a height of 100mm and a diameter of 50mm. During the experiment, the axial and radial deformations of the sensor were connected to a computer. The surrounding rock pressure was loaded at a rate of 0.1MPa/s, and the axial force is subject to displacement control, with a loading rate of 0.002mm/s. Comparison and verification of rock brittleness indices under different surrounding rock pressures The information on sample loading and deformation is collected and stored by the data acquisition system. The failure results and stress-strain curves of the rock samples are shown in Fig. 2. Fig. 2 shows the fracture characteristics of the rock samples under different surrounding rock pressures. The macroscopic forms of rock failure mainly include shear-sliding failure along the schistose structure, transverse failure along the schistose surface and composite failure sliding along the schistose surface. As the surrounding rock pressure increased, the fracture angle of the rock sample and the roughness of the fracture surface decreased significantly, and the failure mode of the rock also changed. And with the surrounding rock pressure increasing, the post-peak stress drop rate decreased significantly, and the rock tended to transition from being brittle to being ductile. The experimental results indicate that the brittleness index decreases as the surrounding rock pressure increases. In particular, the rock sample under a surrounding rock pressure of 12MPa had a very rough fracture surface. Through observation of the rock sample after the test, it is found that it was due to the internal defects in this rock sample. Fig. 3 shows the variations of different brittleness indices as the surrounding rock pressure increases according to the calculation results in Tab. 2. Based on the experimental results, the rock brittleness index should satisfy the following characteristics: (1) the brittleness anomaly caused by the internal defects of the core under a surrounding rock pressure of 12MPa; (2) the brittleness tends to decrease with the increase of the surrounding rock pressure. From Fig. 5, it can be seen that the brittleness indices B 8 , B 11 , B 12 and B L basically conform to these two characteristics. For the brittleness indices B 6 and B 7 , as the surrounding rock pressure increases, they cannot reflect the transition of the rock from brittleness to ductility. This is because the brittleness index B 6 only considers the post-peak elastic modulus and the pre-peak elastic modulus. For the brittleness index B 9 , it can be clearly seen from Fig. 5 that it does not reflect the brittleness changes. This is because B 9 only considers the effect of the strain state, but ignores the effect of stress on the brittleness index. B 10 only considers the recoverable strain and peak strain before the peak, but ignores the post-peak brittle characteristics. Figure 2: Stress-strain curve of rock samples under different surrounding rock pressures.