Issue 49

C.Y. Liu et alii, Frattura ed Integrità Strutturale, 49 (2019) 557-567; DOI: 10.3221/IGF-ESIS.49.52 566 As shown in Fig. 7, the brittleness index B L satisfies the degree of brittleness: fine grain granite > coarse grain granite > sandstone > limestone > marble. The brittleness indices B L and B 13 of marble are significantly lower than those of the other four types of rock samples, which is consistent with the experimental results. For the other four types of rocks, the differences of the brittleness index B L are more obvious, making it much easier to quantitatively measure the degree of brittleness and also better reflect the brittleness variation trends of different rock samples. Section 4 further verifies the accuracy and applicability of the brittleness index B L by using the experimental results of the same type and different types of rocks under surrounding rock pressure. Compared with the brittleness index B 13 , B L is applicable to the brittleness evaluation in both cases. C ONCLUSIONS ccurate evaluation of rock brittleness is of great importance in rock engineering, such as underground tunnel excavation and hydraulic fracturing and so on. This paper proposes a new brittleness index evaluation method B L and verifies its effectiveness with experimental results. In addition, it also verifies the accuracy and applicability of the method using the existing experimental data. Through systematic discussion and analysis of rock brittleness, this paper finally obtains the following conclusions: 1. This paper proposes a new brittleness evaluation method B L based on the post-peak stress drop rate and the difficulty of the pre-peak brittle failure. It verifies the accuracy of this index using the experimental results of phyllite rock samples taken from a deep hole of a tunnel in Southwest China. With the experimental results, this paper further compares B L with other brittleness indices B 8 , B 11 and B 12 , and concludes that B L more accurately reflects the experimental results. 2. With the experimental results of different types of rocks under surrounding rock pressure, this paper compares the brittleness indices B 13 and B L and finds that B L accurately reflects the true brittleness variation trend of different rocks under surrounding rock pressure; at the same time, it also explores the brittleness changes of the same type of rocks under different surrounding rock pressures and finds that B L can give a better quantitative description of the rock brittleness than B 13 . 3. By analyzing the core with a surrounding rock pressure of 12MPa, this paper finds that the internal defects of a rock have an important impact on the rock mechanics. In actual engineering, the internal defects of rocks should be used to the advantage of rock engineering, and attention should be paid to make sure these internal defects will not cause any serious engineering disaster. R EFERENCES [1] Altindag, R. (2002). The evaluation of rock brittleness concept on rotary blast hold drills, Journal of the Southern African Institute of Mining and Metallurgy, 102, pp. 61-66. [2] Holt, R.M., Fjær, E., Stenebråten, J.F., Nes, O.M. (2015). Brittleness of shales: relevance to borehole collapse and hydraulic fracturing, Journal of Petroleum Science and Engineering, 131, pp. 200-209. DOI: 10.1016/j.petrol.2015.04.006 [3] Hajiabdolmajid, V., Kaiser, P. (2003). Brittleness of rock and stability assessment in hard rock tunneling, Tunnelling and Underground Space Technology, 18, pp. 35-48. [4] Dursun, A.E., Gokay, M.K. (2016.) Cuttability assessment of selected rocks through different brittleness values, Rock Mechanics and Rock Engineering, 49, pp. 1173-1190. DOI:10.1007/s00603-015-0810-2. [5] Özfırat, M.K., Yenice, H., Şimşir, F., Yaralı, O. (2016). A new approach to rock brittleness and its usability at prediction of drillability, Journal of African Earth Sciences, 119, pp. 94-101. DOI: 10.1016/j.jafrearsci.2016.03.017. [6] Morley, A. (1944). Strength of Material, Longman, Green, London. [7] EL-Ebrashi, M.K., Craig, R.G., Peyton, F.A. (1969). Experimental stress analysis of dental restorations. Part III. The concept of the geometry of proximal margins, Journal of Prosthetic Dentistry, 22, pp. 333-345. [8] Ramsay, J.G. (1967). Folding and Fracturing of Rocks, Mc Graw Hill Book Company, 568. [9] Obert, L., Duvall, W.I. (1967). Rock mechanics and the design of structures in rock / Leonard Obert, Wilbur I. Duvall, Wiley. [10] Tarasov, B., Potvin, Y. (2013). Universal criteria for rock brittleness estimation under triaxial compression, International A