[Lin, Hang; Liu, Taoying; Cao, Ping; Pu, Chengzhi] Cent S Univ, Sch Resources & Safety Engn, Changsha 410083, Hunan, Peoples R China.;[Liu, Taoying; Pu, Chengzhi] Univ South China, Nucl Resource Coll Engn, Hengyang 421001, Hunan, Peoples R China.
[Liu, Taoying] Cent S Univ, Sch Resources & Safety Engn, Changsha 410083, Hunan, Peoples R China.
Rock-like material;Multiple pre-existing flaws;Crack propagation and coalescence;Uniaxial compression
Fracture coalescence, which plays an important role in the behavior of brittle materials, is investigated by loading rock-like specimens with two and three pre-existing flaws made by pulling out the embedded metal inserts in the pre-cured period. Different geometries are obtained by changing the angle of the flaws with respect to the direction of loading and the spacing. With reference to the experimental observation of crack initiation and propagation from pre-existing flaws, the influences of the third pre-existing flaw on the cracking processes was analyzed. It was found during the test that: with the increase of the angle of the rock bridge, the rock specimen takes a turn from wing crack propagation failure to crack coalescence failure, and it will be more obvious with the increase of the prefabricated crack angle. According to the different geometries of pre-existing cracks, seven types of coalescence have been identified based on the nature of the cracks for the specimen with two pre-existing flaws. The multi-crack interaction results in the continuous degradation of the macroscopic mechanical properties of the rock mass. On one hand, it weakens the trend of relative sliding of the coplanar cracks, and on the other hand, it changed the coalescence patterns of the fractured specimen. The research reported here provides increased understanding of the fundamental nature of rock mass failure in compression. (C) 2014 Elsevier B.V. All rights reserved.
Seismic events and blasts generate seismic waveforms that have different characteristics. The challenge to confidently differentiate these two signatures is complex and requires the integration of physical and statistical techniques. In this paper, the different characteristics of blasts and seismic events were investigated by comparing probability density distributions of different parameters. Five typical parameters of blasts and events and the probability density functions of blast time, as well as probability density functions of origin time difference for neighbouring blasts were extracted as discriminant indicators. The Fisher classifier, naive Bayesian classifier and logistic regression were used to establish discriminators. Databases from three Australian and Canadian mines were established for training, calibrating and testing the discriminant models. The classification performances and discriminant precision of the three statistical techniques were discussed and compared. The proposed discriminators have explicit and simple functions which can be easily used by workers in mines or researchers. Back-test, applied results, cross-validated results and analysis of receiver operating characteristic curves in different mines have shown that the discriminator for one of the mines has a reasonably good discriminating performance.
The effect of thermal treatment on the dynamic fracture toughness of Laurentian granite (LG) was investigated in this work. Notched semi-circular bend (NSCB) LG specimens are heat treated at temperatures up to 850 A degrees C. The micro-cracks in the rock samples induced by thermal treatment are examined by scanning electron microscope (SEM). The microscopic observations are consistent with the subsequent P-wave velocity measurements, which shows that the P-wave velocity decreases with the treatment temperature monotonically when the temperature is higher than 250 A degrees C. Dynamic fracture toughness measurements are then carried out on these samples with the dynamic load exerted by a modified split Hopkinson pressure bar (SHPB) system. The relationship between fracture toughness and treatment temperature is investigated. Experimental results show that fracture toughness increases with the loading rate but decreases with the treatment temperature. However, when the heating temperature is below 250 A degrees C and above 450 A degrees C, the dependence of dynamic fracture toughness on the temperature is different from other temperatures, which can be explained by the physical processes at the microscopic level of the rock due to heating. At treatment temperatures below 250 A degrees C, the thermal expansion of grains leads to an increase in the toughness of the rock. At treatment temperatures above 450 A degrees C, the sources of weakness such as grain boundaries and phase transition of silicon are depleted, and as a result the decrease in fracture toughness is not as significant as other treatment temperature ranges.
[Fan, Xiang] Cent S Univ, Sch Resources & Safety Engn, Changsha 410083, Hunan, Peoples R China.;[Fan, Xiang; Kulatilake, P. H. S. W.] Univ Arizona, Rock Mass Modeling & Computat Rock Mech Labs, Tucson, AZ 85721 USA.;[Chen, Xin] China Univ Min & Technol Beijing, Sch Mech & Civil Engn, Beijing 100083, Peoples R China.
[Kulatilake, P. H. S. W.] Univ Arizona, Rock Mass Modeling & Computat Rock Mech Labs, Tucson, AZ 85721 USA.
The method of Random Forest (RF) was used to classify whether rockburst will happen and the intensity of rockburst in the underground rock projects. Some main control factors of rockburst, such as the values of in-situ stresses, uniaxial compressive strength and tensile strength of rock, and the elastic energy index of rock, were selected in the analysis. The traditional indicators were summarized and divided into indexes I and II. Random Forest model and criterion were obtained through training 36 sets of rockburst samples which come from underground rock projects in domestic and abroad. Another 10 samples were tested and evaluated with the model. The evaluated results agree well with the practical records. Comparing the results of support vector machine (SVM) method, and artificial neural network (ANN) method with random forest method, the corresponding misjudgment ratios are 10%, 20%, and 0, respectively. The misjudgment ratio using index I is smaller than that using index II. It is suggested that using the index I and RF model can accurately classify rockburst grade.
International Journal of Rock Mechanics and Mining Sciences,2017年91:139-154 ISSN：1365-1609
[Wang, Weijun; Tang, Jingzhou; Zhao, Yanlin; Wan, Wen] Hunan Univ Sci & Technol, Sch Resource Environm & Safety Engn, Hunan Prov Key Lab Safe Min Tech Coal Mines, Xiangtan, Hunan, Peoples R China.;[Zhao, Yanlin] China Univ Min & Technol, State Key Lab Coal Resources & Safety Min, Xuzhou, Jiangsu, Peoples R China.;[Zhang, Lianyang] Univ Arizona, Dept Civil Engn & Engn Mech, Tucson, AZ 85721 USA.;[Lin, Hang] Cent S Univ, Sch Resources & Safety Engn, Changsha, Hunan, Peoples R China.
Department of Civil Engineering and Engineering Mechanics, University of Arizona, Tucson, AZ, United States
Single fracture;Permeability;Transient pulse test;Surface morphology;Hydro-mechanical coupling
Transient pulse tests were performed on single rock fractures at different confining pressures. A new data analysis method based on polynomial fitting was introduced to investigate the relationship between flow velocity and hydraulic gradient. 3D laser scanning was used to quantify the morphological changes of the fracture surface after the transient pulse test or under the hydro-mechanical coupling effect. The results show that the flow velocity versus hydraulic gradient data gradient shows a nonlinear relationship at very low hydraulic, possibly due to strong solid-water interaction, but becomes approximately linear after the hydraulic gradient is high enough. The permeability of a single fracture is sensitive to the confining pressure. As the confining pressure increases, the permeability first remarkably decreases when the confining pressure is lower than a certain value and then decreases at a much lower speed when the confining pressure is higher than that value. Both mechanical and hydraulic apertures decrease at a decreasing rate with the increase in confining pressure. The effect of fracture roughness on the permeability is related to the magnitude of the confining pressure. Rougher fractures have lower permeability at low confining pressures; but the opposite can be true when the confining pressure is high enough. Roughness is no longer critical to permeability when the confining pressure is over a certain value.