Issue 49

C.Y. Liu et alii, Frattura ed Integrità Strutturale, 49 (2019) 557-567; DOI: 10.3221/IGF-ESIS.49.52 561 where, ε r and ε p are the residual strain and the peak strain, respectively; σ p and σ r are the peak stress and the residual stress, respectively. Peak strain can reflect the difficulty of brittle failure [23]. In addition, according to previous studies, as the peak strain increases, the rock tends to transition from being brittle to being ductile, i.e., the brittleness index will become lower. Considering this factor, it is proposed that the pre-peak brittleness index B L2 should be the reciprocal of the peak strain. Second, the index of the difficulty of pre-peak brittle failure is as follows: L2 = 1 ε P (2) Based on this, the new rock brittleness index B L considers both the post-peak stress drop rate and the difficulty of pre-peak brittle failure. Finally, the brittleness index B L is expressed as follows: L = L1 L2 (3) In summary, the new rock brittleness evaluation method B L considers the post-peak stress drop rate and also introduces the residual strain to emphasize the final increase of post-peak strain. Moreover, it incorporates the difficulty of brittle failure to characterize the pre-peak brittle characteristics. C OMPARISON AND VERIFICATION OF BRITTLENESS INDICES onsidering the great impact of surrounding rock pressure on rock brittleness in underground engineering, this section will explore the variations of rock brittleness under different surrounding rock pressure conditions. In order to verify the accuracy of the brittleness index B L , Tab. 2 determines the relevant mechanical parameters and calculates the brittleness index B L according to the stress-strain curve in Fig. 4. At the same time, for comparison with other brittleness indices, the brittleness indices B 6 - B 12 and B 14 are selected from Tab. 1 (as B 13 needs initiation stress and initiation strain, it is not selected here for comparison. The following sections will use the data by Chen Guoqing et al. for further comparison). Confining pressure /MPa σ p / (100MPa) ε p /10 -3 σ r / (100MPa) ε r /10 -3 ε R /10 -3 Rupture angle(°) B 6 B 7 B 8 B 9 B 10 B 11 B 12 B 14 B L 0 0.419 2.59 0.268 3.47 1.656 85 0.037 0.963 0.361 0.340 0.639 0.081 0.294 0.996 0.549 3 0.467 2.71 0.363 4.23 2.106 79 -1.549 2.549 0.223 0.561 0.777 0.041 0.194 0.982 0.229 9 0.586 3.39 0.484 4.5 2.799 75 -0.908 1.908 0.174 0.327 0.826 0.034 0.149 0.966 0.208 12 0.605 3.63 0.357 4.9 2.142 70 0.080 0.920 0.410 0.350 0.590 0.094 0.339 0.939 0.436 15 0.658 3.84 0.563 5.26 3.286 70 -1.793 2.793 0.144 0.370 0.856 0.026 0.120 0.939 0.139 18 0.7956 4.51 0.677 6.12 3.837 65 -1.657 2.657 0.149 0.357 0.851 0.028 0.127 0.906 0.126 25 0.982 5 0.8085 7.79 4.117 60 -2.292 3.292 0.177 0.558 0.823 0.032 0.161 0.866 0.099 Table 2 : Conventional triaxial compression experimental data of rock specimens from phyllite Preparation and experimental conditions of rock samples Rock brittleness has a profound effect on the stability of deep buried tunnels. In order to accurately evaluate the brittleness of rocks, the effects of surrounding rock pressure must be considered. This paper takes the phyllite samples obtained from C