A series of true-triaxial compression tests were performed on red sandstone cubic specimens with a circular hole to investigate the influence of depth on induced spalling in tunnels. The failure ...process of the hole sidewalls was monitored and recorded in real-time by a micro-video monitoring equipment. The general failure evolution processes of the hole sidewall at different initial depths (500 m, 1000 m and 1500 m) during the adjustment of vertical stress were obtained. The results show that the hole sidewall all formed spalling before resulting in strain rockburst, and ultimately forming a V-shaped notch. The far-field principal stress for the initial failure of the tunnel shows a good positive linear correlation with the depth. As the depth increases, the stress required for the initial failure of the tunnels clearly increased, the spalling became more intense; the size and mass of the rock fragments and depth and width of the V-shaped notches increased, and the range of the failure zone extends along the hole sidewall from the local area to the entire area. Therefore, as the depth increases, the support area around the tunnel should be increased accordingly to prevent spalling.
A considerable number of deep engineering cases show that strain rockbursts occur on both sidewalls of circular caverns and ultimately form a symmetrical V-shaped notch. To investigate the occurrence ...process and mechanism of strain rockbursts in deep circular cavern under high stresses, simulation experiments under four different three-dimensional (3D) stress conditions were conducted on cubic granite specimens with a prefabricated circular hole using the true-triaxial electro-hydraulic servo mutagenesis testing system. The rock bursting process was monitored on the sidewalls of the hole and recorded in real-time with a wireless microcamera. The results show that the entire rock bursting process on the sidewalls can be divided into four distinct periods for every 3D stress condition: calm period, pellet ejection period, rock fragment exfoliation period, and rock bursting period. Under the conditions that the vertical stress is constant and the horizontal radial stress is equal to the horizontal axial stress, the rockburst severity of the sidewalls clearly decreases with increasing horizontal stress. When the vertical stress is constant and the horizontal axial stress is low, the rockburst severity clearly decreases with the increase of horizontal radial stress. In contrast, when the horizontal axial stress is high, the rockburst severity becomes more serious with increasing horizontal radial stress. During the process of rock fracture and bursting, the damage zones on both sidewalls form two symmetrical V-shaped notches, and the line connecting the centres of the two V-shaped notches is perpendicular to the maximum principal stress direction. The symmetrical V-shaped failure modes on both sidewalls under the four stress conditions are in line with the statistical relationship of the far-field stress state and failure mode of a deep circular cavern without support (Martin et al. in Can Geotech J 36(1):136–151, 1999).
To review the rockburst proneness (or tendency) criteria of rock materials and compare the judgment accuracy of them, twenty criteria were summarized, and their judgment accuracy was evaluated and ...compared based on the laboratory tests on fourteen types of rocks. This study begins firstly by introducing the twenty rockburst proneness criteria, and their origins, definitions, calculation methods and grading standards were summarized in detail. Subsequently, to evaluate and compare the judgment accuracy of the twenty criteria, a series of laboratory tests were carried out on fourteen types of rocks, and the rockburst proneness judgment results of the twenty criteria for the fourteen types of rocks were obtained accordingly. Moreover, to provide a unified basis for the judgment accuracy evaluation of above criteria, a classification standard (obtained according to the actual failure results and phenomena of rock specimen) of rockburst proneness in laboratory tests was introduced. The judgment results of the twenty criteria were compared with the judgment results of this classification standard. The results show that the judgment results of the criterion based on residual elastic energy (REE) index are completely consistent with the actual rockburst proneness, and the other criteria have some inconsistent situations more or less. Moreover, the REE index is based on the linear energy storage law and defined in form of a difference value and considered the whole failure process, and these superior characteristics ensure its accuracy. It is believed that the criterion based on REE index is comparatively more accurate and scientific than other criteria, and it can be recommended to be applied to judge the rockburst proneness of rock materials.
The effects of high strain rates and low confining pressures on the dynamic mechanical properties of sandstone were investigated experimentally with a modified triaxial split Hopkinson pressure bar ...(SHPB) system. For comparison, dynamic uniaxial compression tests of the sandstone were also conducted by using a conventional SHPB system. The confining pressures used in the dynamic triaxial compression tests are 5, 7.5, 10, 12.5 and 15 MPa, and the strain rate of these tests varied from approximately 40 s−1 to 160 s−1. The results show that the dynamic uniaxial and triaxial compressive strengths of the sandstone will linearly increase with the logarithm of the stain rate. Under approximately the same strain rate, the dynamic triaxial compressive strength will linearly increase with the confining pressure. Although the peak strain has no dependence on the confining pressure, it will generally increase with the strain rate. The secant modulus is independent of the strain rate and confining pressure. Three trend diagrams, which reflect the influences of strain rate or confining pressure on the dynamic mechanical properties of sandstone in the uniaxial and triaxial compression tests, are given, and the correlation between the failure modes of the sandstone specimens and the characteristics of the stress-strain curves is also analyzed.
To study the energy storage and dissipation characteristics of deep rock under two-dimensional compression with constant confining pressure, the single cyclic loading-unloading two-dimensional ...compression tests were performed on granite specimens with two height-to-width (
H/W
) ratios under five confining pressures. Three energy density parameters (input energy density, elastic energy density and dissipated energy density) in the axial and lateral directions of granite specimens under different confining pressures were calculated using the area integral method. The experimental results show that, for the specimens with a specific
H/W
ratio, these three energy density parameters in the axial and lateral directions increase nonlinearly with the confining pressure as quadratic polynomial functions. Under constant confining pressure compression, the linear energy storage law of granite specimens in the axial and lateral directions was founded. Using the linear energy storage law in different directions, the elastic energy density in various directions (axial elastic energy density, lateral elastic energy density and total elastic energy density) of granite under any specific confining pressures can be calculated. When the
H/W
ratio varies from 1:1 to 2:1, the lateral compression energy storage coefficient increases and the corresponding axial compression energy storage coefficient decreases, while the total compression energy storage coefficient is almost independent of the
H/W
ratio.
Spalling and rockburst are two common failure modes in deep hard-rock tunnels and they exhibit a strong correlation. In this paper, the process of rockburst induced by spalling is investigated. The ...uniaxial compressive strength and rockburst tendency of red sandstone are measured. Four different initial stress conditions are set up, and cubic red sandstone samples with a prefabricated hole are tested by a true-triaxial test system. During the experimental process, the failure of the hole sidewall is monitored and recorded in real time by a micro-camera. The process of rockburst induced by spalling damage in deep hard-rock tunnels is reproduced, and the mechanism by which spalling damage on induces rockburst is revealed. In addition, the evolution process and failure characteristics of rockburst induced by spalling damage are analysed. The experimental results indicated that the red sandstone has a moderate rockburst tendency, and its rockburst process can be divided into four periods: the calm period, the small grain ejection & spalling damage period, the slab buckling & fragment ejection period and the violent ejection period. The mechanism of spalling damage on inducing rockburst is mainly embodied in two aspects: promoting large buckling deformations (providing energy for rockburst) and to weakening the strength of the rock mass (creating conditions for the suddenly release of energy). The effect of lateral stress on spalling damage and rockburst is more obvious than that of axial stress, and the severity of rockburst can be significantly reduced by increasing the lateral stress. The diameter of the hole has a strength size effect on the sidewall damage, producing a certain inhibitory effect on the spalling damage and rockburst. Smaller lateral stress corresponds with greater depths of the V-shaped notch and much smaller width-to-length ratios in the rock fragments.
Rock failure generally refers to the process of damaging rock material to the point at which it partially or completely loses its load-carrying capacity. For rock materials, the fracture pattern and ...mechanical properties, including compressive strength, tensile strength, shear strength, and fracture toughness, under dynamic loads are affected by the loading rate/strain rate. In this study, a series of dynamic indirect tensile tests was conducted on sandstone from Changsha, China.
Tension failure of deep surrounding rock is a very common failure mode, which is closely related to the couple of static pre-stress and impact load. Thus, the dynamic tensile strength of pretension ...stressed Brazilian disc (BD) specimens subjected to the impact load was measured at the couple different pretension levels and loading rates with the modified split Hopkinson pressure bar (SHPB) system. Six groups of Linyi sandstone BD specimen were impacted with the loading rates from 400 to 1200 GPa/s under the pretension of 0, 0.48, 1.44, 2.39, 3.45 and 4.30 MPa. The test results reveal the dynamic tensile strength has a very significant linear positive correlation with the loading rate, wherein increases gradually with the loading rate increase, reflecting the obvious rate dependency. The dynamic tensile strength decreased significantly with pretension stress level increase at the same loading rate, showing an obvious dynamic tensile strength weakening effect. Besides, the mechanism of the dynamic tensile strength weakening effect is summarized, wherein the pretension stress level dominates and determines the dynamic tensile strength weakening level, and the impact load induces the appearance of the strength weakening effect.
Tensile failure (spalling or slabbing) often occurs on the sidewall of deep tunnel, which is closely related to the coupled stress state of deep rock mass under high pre-static load and dynamic ...disturbance. To reveal the mechanism of rock tensile failure caused by this coupled stress mode, the Brazilian disc tests were carried on red sandstone under high pre-static load induced by dynamic disturbance. Based on the pure static tensile fracture load of red sandstone specimen, two static load levels (80% and 90% of the pure static tensile fracture load) were selected as the initial high pre-static loading state, and then the dynamic disturbance load was applied until the rock specimen was destroyed. The dynamic disturbance loading mode adopted a sinusoidal wave (sine-wave) load, and the loading wave amplitude was 20% and 10% of the pure static tensile fracture load, respectively. The dynamic disturbance frequencies were set to 1, 10, 20, 30, 40, and 50 Hz. The results show that the tensile failure strength and peak displacement of red sandstone specimens under coupled load actions are lower than those under pure static tensile load, and both parameters decrease significantly with the increase of dynamic disturbance frequency. With the increase of dynamic disturbance frequency, the decrease range of tensile strength of red sandstone increased from 3.3% to 9.4% when the pre-static load level is 80%. While when the pre-static load level is 90%, the decrease range will increase from 7.4% to 11.6%. This weakening effect of tensile strength shows that the deep surrounding rock is more likely to fail under the coupled load actions of pre-static load and dynamic disturbance. In this tensile failure mechanism of the deep surrounding rock, the stress environment of deep sidewall rock determines that the failure mode of rock is a tensile failure, the pre-static load level dominates the tensile failure strength of surrounding rock, and dynamic disturbance promotes the strength-weakening effect and affects the weakening range.
