Search

It is acknowledged that most surrounding rocks of underground engineering are subjected to biaxial static stresses and various dynamic fatigue loads. In most cases, the dynamic loads have a point or local acting area, which affect the failure properties of rocks greatly. To reveal the failure mechanism of rocks under the combined effects of biaxial static stress and dynamic fatigue loads, a series of biaxial compression tests and point/local fatigue-induced failure tests on cubic granite specimens with a side length of 50 mm were conducted out. In the biaxial compression tests (the maximum principal stress σ1 > the intermediate principal stress σ2 > the minimum principal stress σ3 = 0), the biaxial compressive strength of granite presents a monotonous increasing trend with the σ2 increasing from 0 to 40 MPa, while the increasing rate of biaxial compressive strength increases firstly and then decreases with the increase in σ2. The rock failure induced by point/local fatigue loads with 5 kHz frequency was analyzed in this study. It shows that the fatigue life of granite is much shorter than that in the traditional full-section fatigue tests, which increases with the increase in σ2 and decreases as the upper level of the fatigue stress or predefined stress of σ1 increase. From the acoustic emission properties, the failure of rocks under biaxial confinements induced by point/local static or fatigue loads are mainly because of tensile cracks. Meanwhile, the application of a disturbance bar with a smaller cross-sectional area induces rock broken more quickly. It seems that the following measures can be used to improve rock breakage efficiency: increasing the predefined stress in the breaking direction, reducing the lateral confinement, using the breaking bar with a small cross-sectional area, and changing the static loading into the dynamic loading of the breaking force. [ABSTRACT FROM AUTHOR]

High geostresses and stress gradients are the predominant stress conditions in deep excavation-disturbed rock masses. The aim of this study is to determine the triaxial compressive strength properties of hollow cylindrical granite specimens under a radially non-uniform confining stress field with different radial stress gradients determined by coupled external and internal confining stresses. Triaxial compression testing of hollow cylindrical rock specimens was performed to investigate the influence of the radial stress gradient, external confining stress and specimen length-to-diameter (L/D) ratio on the triaxial compressive strength. The experimental results and regressed failure criteria indicate that the triaxial compressive strengths of the hollow cylindrical granite specimens increase with the external confining stresses, but decrease with an increase in the radial stress gradients. The calculated goodness of fit (R2) and root-mean-squared error suggest that the nonlinear failure criterion based on the Hoek-Brown model is more accurate than the linear failure criterion based on the Mohr-Coulomb model for determining the influences of the external confining stress and radial stress gradient on the triaxial compressive strength. In addition, the triaxial compressive strength increases with a decreasing L/D ratio due to the strengthening end effect of the hollow cylindrical granite specimens and the change in the failure pattern of these specimens from shear to slabbing. [ABSTRACT FROM AUTHOR]

The failure modes and peak unloading strength of a typical hard rock, Miluo granite, with particular attention to the sample height-to-width ratio (between 2 and 0.5), and the intermediate principal stress was investigated using a true-triaxial test system. The experimental results indicate that both sample height-to-width ratios and intermediate principal stress have an impact on the failure modes, peak strength and severity of rockburst in hard rock under true-triaxial unloading conditions. For longer rectangular specimens, the transition of failure mode from shear to slabbing requires higher intermediate principal stress. With the decrease in sample height-to-width ratios, slabbing failure is more likely to occur under the condition of lower intermediate principal stress. For same intermediate principal stress, the peak unloading strength monotonically increases with the decrease in sample height-to-width. However, the peak unloading strength as functions of intermediate principal stress for different types of rock samples (with sample height-to-width ratio of 2, 1 and 0.5) all present the pattern of initial increase, followed by a subsequent decrease. The curves fitted to octahedral shear stress as a function of mean effective stress also validate the applicability of the Mogi-Coulomb failure criterion for all considered rock sizes under true-triaxial unloading conditions, and the corresponding cohesion C and internal friction angle φ are calculated. The severity of strainburst of granite depends on the sample height-to-width ratios and intermediate principal stress. Therefore, different supporting strategies are recommended in deep tunneling projects and mining activities. Moreover, the comparison of test results of different σ2/σ3 also reveals the little influence of minimum principal stress on failure characteristics of granite during the true-triaxial unloading process. [ABSTRACT FROM AUTHOR]

True triaxial tests have been carried out on granite, sandstone and cement mortar using cubic specimens with the process of unloading the minor principal stress. The strengths and failure modes of the three rock materials are studied in the processes of unloading σ and loading σ by the newly developed true triaxial test system under different σ, aiming to study the mechanical responses of the rock in underground excavation at depth. It shows that the rock strength increases with the raising of the intermediate principal stress σ when σ is unloaded to zero. The true triaxial strength criterion by the power-law relationship can be used to fit the testing data. The 'best-fitting' material parameters A and n ( A > 1.4 and n < 1.0) are almost located in the same range as expected by Al-Ajmi and Zimmerman (Int J Rock Mech Min Sci 563 42(3):431-439, 2005). It indicates that the end effect caused by the height-to-width ratio of the cubic specimens will not significantly affect the testing results under true triaxial tests. Both the strength and failure modes of cubic rock specimens under true triaxial unloading condition are affected by the intermediate principal stress. When σ increases to a critical value for the strong and hard rocks (R4, R5 and R6), the rock failure mode may change from shear to slabbing. However, for medium strong and weak rocks (R3 and R2), even with a relatively high intermediate principal stress, they tend to fail in shear after a large amount of plastic deformation. The maximum extension strain criterion Stacey (Int J Rock Mech Min Sci Geomech Abstr 651 18(6):469-474, 1981) can be used to explain the change of failure mode from shear to slabbing for strong and hard rocks under true triaxial unloading test condition. [ABSTRACT FROM AUTHOR]

The acoustic emission b value is an important and widely used parameter for the early prediction of rock fractures. In this study, five groups of true triaxial compression tests were conducted on granite specimens to analyze changes in b value during the process of rock failure, and to investigate the b value characteristics of acoustic emission events. First, the acoustic emission events that simultaneously triggered at least four sensors were located using P-wave arrivals and sensor coordinates. Then, considering various intervals of acoustic emission event counts, stress magnitude, and stress proportion, b values were calculated using the values of the maximum amplitude, average amplitude, maximum absolute energy, and average absolute energy of the acoustic emission events. In addition, the goodness of the fitting curves was used to evaluate the fitting reliability of the b values. The results indicated higher accuracy of b value when calculated using the average amplitude setting for intervals of acoustic emission event counts of 200 or greater, stress magnitude of 20 MPa or greater, and stress proportion of 10% or greater. Moreover, the interval of event counts of 200 is suggested as a window parameter for b value calculations, and the b values are observed to exhibit a decreasing trend before fracture for more than 80% of the specimens. Furthermore, the b value tends to decrease with an increase in confining pressure. Thus, the b value can be used as an indicator for validating the stress concentration area, including magnitudes and accumulative probability density distribution of events, which is a beneficial complement to clarifying precursor information of rock mass instability. [ABSTRACT FROM AUTHOR]

Cubic specimens of granite, marble, and sandstone, with side lengths of 50 mm, were prepared in the laboratory. True triaxial compression tests were conducted on each rock type with minor principal stress (σ 3) magnitudes of 10, 20, 30, 50, and 100 MPa and the intermediate principal stress (σ 2) varying from σ 2 = σ 3 to σ 2 = σ 1 (where σ 1 represents the major principal stress). The experimental results demonstrated that the failure strength, fracture dip angle, non-linear behavior and failure mode of these rocks under true triaxial conditions were affected by both σ 2 and σ 3 , as well as by the rock lithology. When σ 3 was kept constant and σ 2 increased, failure strength showed an initial increase followed by a decrease, whereas the fracture dip angle increased monotonically. The non-linear behavior before the peak strength point of granite became more apparent by the increase of σ 3 , whereas σ 2 had an opposite effect on the non-linear behavior of granite under high σ 3 (e.g. 100 MPa). The non-linear behavior before the peak strength point was always found in marble and sandstone for all of the stress state conditions. Analyses of fracture dip angle and failure mode showed that when σ 3 = 10, 20, and 30 MPa and σ 2 / σ 3 ≥ 5, 7.5, and 10, respectively, the failure mode of granite changed from shear to slabbing, whereas marble and sandstone always fractured in the shear direction. This study confirms that rocks may experience slabbing failure under true triaxial stress, and slabbing failure mode is likely to be induced in high-strength and brittle rocks under low σ 3 with the stress ratio of σ 2 / σ 3 exceeding a particular threshold value. [ABSTRACT FROM AUTHOR]

The Split-Hopkinson pressure bar (SHPB) was used to determine the fracture characteristics of a long bar rock specimen with an elliptical cavity under different axial ratios and dip angles. A high speed camera was applied to record the fracturing process of the granite specimen around the cavity. The experimental results showed that the fracture characteristics around the elliptical cavity were closely related to the axial ratio and dip angle. A three-dimensional numerical model was established using LS-DYNA to quantitatively analyze the dynamic stress state around the cavity. The numerical results indicate that the dip angle and axial ratio of the elliptical cavity significantly affected the dynamic stress concentration factor (DSCF), then affected the rock failure. The location of higher DSCF led to a higher possibility of spalling failure. The maximum DSCF remarkably decreased with a decreasing dip angle and increased the axial ratio. In the dynamic loading propagate process, the stress concentration distribution around the cavity formed by a compression stress wave had a certain damaging effect on the destruction of rock around the cavity, and the stress concentration generated by the tensile stress wave was the main factor of the rock fracture, which was most notable in the peak area of the stress concentration. [ABSTRACT FROM AUTHOR]