Investigation of unloading rock failure under different σ2 facilitates the control mechanism of excavation surrounding rock. This study focused on single-sided unloading tests of granite specimens ...under true triaxial conditions. The strength and failure characteristics were studied with micro-camera and acoustic emission (AE) monitoring. Furthermore, the choice of test path and the effect of σ2 on fracture of unloading rock were discussed. Results show that the increased σ2 can strengthen the stability of single-sided unloading rock. After unloading, the rock's free surface underwent five phases, namely, inoculation, particle ejection, buckling rupture, stable failure, and unstable rockburst phases. Moreover, at σ2≤30 MPa, the b value shows the following variation tendency: rising, dropping, significant fluctuation, and dropping, with dispersed damages signal. At σ2≥40 MPa, the tendency shows: a rise, a decrease, a slight fluctuation, and final drop, with concentrated damages signal. After unloading, AE energy is mainly concentrated in the micro-energy range. With the increased σ2, the micro-energy ratio rises. In contrast, low, medium and large energy ratios drop gradually. The increased tensile fractures and decreased shear fractures indicate that the failure mode of the unloading rock gradually changes from tensile-shear mode to tensile-split one. The fractional dimension of the rock fragments first increases and then decreases with an inflection point at 20 MPa. The distribution of SIF on the planes changes as σ2 increases, resulting in strengthening and then weakening of the rock bearing capacity.
To study the influence of fillings on rock failure. By turning to the Drucker-Prager strength model and cumulative damage criteria, investigations are made, with the nonlinear AUTODYN software, into ...crack propagation behaviors in crack-filled and unfilled specimens under uniaxial dynamic loading. Under investigations are crack initiation position, sequence and angle, and coalescence mode. According to the mode of propagation and coalescence, cracks are divided into three types, i.e., the tensile wing, the antiwing, and the horsetail. The simulation results show that under uniaxial dynamic loading, differences are found in initiation position, angle, and coalescence mode for specimens with cracks filled or otherwise. However, filling does not affect crack initiation sequence. Under the same loading, the damage to filled specimen is less severe than that to the unfilled specimen.
In underground projects such as mining and tunneling, the presence of coal rock columns plays a certain supporting role, and the instability of coal rock columns is often related to their size and ...shape of presence. Therefore, in order to investigate the compressive damage characteristics and damage evolution law of rocks of different sizes, uniaxial compression tests were conducted on sandstones with different height−diameter ratios to explore the mechanical properties and damage characteristics of sandstones with different height−diameter ratios, analyze the connection between acoustic emission ringing count rate, accumulated energy, peak frequency, and b-value changes and height–diameter ratio, and analyze the evolution law of sandstones during damage based on damage variables, and draw the following conclusions. As the height-to-diameter ratio increases, the less affected by the end effect, the rock strength shows a nonlinear decreasing trend, and the decreasing trend becomes slow. The acoustic emission ringing count rate evolved from intermittent to continuous occurrence, showing multiple peaks as the test proceeded. The accumulated energy rises sharply before rupturing after several steps from stable development. As the height-to-diameter ratio increases, the acoustic emission signal before rupture rises more intensively, and the damage is more concentrated. The overall level of the b-value shows an increasing trend, the proportion of acoustic emission high-frequency signal gradually increases, and the development of tiny cracks inside the rock more intensively. Therefore, the sudden change of acoustic emission signal can be used as a precursor of rock damage. The rock damage curve has smaller values in the stable damage phase. With the increase in the height-to-diameter ratio, the non-stable damage stage damage showed a trend of decreasing and then increasing and reached the minimum at L/D = 2.0.
To investigate the mechanical properties and energy evolution laws of rocks under true triaxial unloading conditions, a study was conducted using a true triaxial rock testing system on three ...different types of rocks: coal, sandy mudstone, and siltstone. The study examined the mechanical behavior, failure patterns, and fractal dimensions of these rocks under true triaxial unloading conditions. The tests revealed significant variations in stress–strain curves and failure patterns among the different rock types. Observation indicated that rocks with lower peak strength exhibited higher fractal dimensions and increased fragmentation upon failure. Subsequently, based on the experimental data of siltstone, the impact of the unloading rate and particle size distribution on the energy evolution under true triaxial single-sided unloading paths was further investigated using the three-dimensional particle flow software PFC3D 6.0, revealing the micro-mechanisms of rock energy evolution. The study revealed that when the initial stress unloading level was low, the total energy and strain energy at the peak strength exhibited a strong linear relationship with the unloading rate. Before the stress peak, the dissipative energy was mainly composed of frictional energy. After the stress peak, the dissipative energy consisted of frictional energy, damping energy, and kinetic energy. The heterogeneity of rock significantly affected the distribution of dissipative energy, with an increase in rock heterogeneity leading to a decrease in frictional energy and an increase in kinetic energy.
To study the size effect of rock under quasistatic and dynamic conditions, the changes in compressive strength with the change in specimen size are measured. Cylindrical granite specimens with ...length-diameter ratios in the range of 0.5∼1 are used for uniaxial compression tests using an RMT testing machine and an SPHB impact testing machine. Under quasistatic loading, the failure modes of the specimens with different length-diameter ratios are different. The larger the size of the specimen structure is, the greater the probability of defects such as joints and micro cracks is and the smaller the influence of the specimen on the distribution of a three-dimensional stress state is. The rock strength decreases with increasing length-diameter ratio. Using the improved Weibull formula, the size of the specimen is expressed by the volume, and the calculated rock strength of different volumes is similar to the compressive strength from the quasistatic tests. Under dynamic loading, the dynamic compressive strengths of the specimens with different length-diameter ratios are similar, and the failure mode of the specimens is different from that under quasistatic loading. Soon after a crack appears in a specimen, the specimen splits. As the size of the specimens decreases, the fragments size to approach the millimeter scale. By improving the Weibull distribution formula and considering variation in strain rate caused by the size of the specimen, the dynamic compressive strength of rocks of different volumes is calculated by introducing the critical strain rate and related parameters, and the results are similar to the experimental dynamic compressive strength obtained. The improved Weibull formula based on the strength size effect can accurately describe the quasistatic and dynamic compressive strength laws.
After roadway excavation, the surrounding rock is the main bearing structure for the original stress and forms a supporting ring with the supporting structure. The automatic load-bearing ring is the ...main interactional structure between the surrounding rock and the supporting system. The rock mass strength, stress, and deformation characteristics were considered to establish the theoretical calculation model for the softened broken surrounding rocks. In the model, the strength, stress, and deformations were analyzed for the elastic-plastic softening and broken zones. The analytical solution was obtained for the stress field, displacement field, and plastic damage range of the surrounding rocks. Considering the postpeak strength softening, shearing dilatation effects, original rock stress, and supporting resistance, the roadway bearing structure was divided into a shallow bearing layer and a deep bearing layer. Moreover, the crucial bearing layer was vital to the bearing capacity and stability of the layer. The influence of the shallow bearing layer, deep bearing layer, and crucial bearing layer on controlling the deformation of the surrounding rock was also analyzed. Based on the evolution characteristics of the shallow and deep buried surrounding rock, the weaker the residual strength of the surrounding rock was, the thicker the shallow and deep buried surrounding rock; the thicker the shallow and deep buried layers were, the greater the surface displacement of the surrounding rock; the crucial bearing layer was farther away from the roadway, and the roadway surface displacement was great; the greater the supporting resistance was, the thinner the shallow and deep bearing layers; and the crucial bearing layer was closer to the roadway, and the roadway deformation control was effective. Furthermore, based on the specific calculation example, the influence of the residual stress and supporting measures on the evolution characteristics of shallow and deep layers and roadway deformation was analyzed in detail.
Highly stressed rock masses continuously exchange substances and energy with the outer environment during single-side unloading, which is a damage evolution process with energy dissipation. The true ...triaxial unloading disturbance rock test system was utilized to perform single-side unloading tests of marble specimens at different unloading rates and axial pressures. The unloading failure characteristics of the marble specimens were revealed from the perspective of energy conversion. Results indicated the following: (1) The marble specimen was more likely to be damaged under the single-side unloading condition than under the loading condition, with the unloading failure strength reaching 80% of the loading failure strength. (2) Crack propagation under single-side unloading was intermittent. (3) A high single-side unloading rate corresponded to a high release rate of the energy concentrated inside the rock mass, a small amount of energy dissipated due to second failure, insufficient crack propagation, and occurrence of only tensile failure. (4) A large amount of axial strain energy indicated a great amount of energy aggregated inside the rock mass and the generation of tensile stress due to unloading dilatancy. Consequently, a high critical value for splitting the failure of the rock specimen implied large failure-induced energy dissipation and a high failure degree. The study of failure energy evolution mechanism in the single-side unloading of highly stressed rock masses at different unloading rates will provide great guiding significance in controlling underground engineering disasters and guaranteeing construction safety.
In order to study the creep behavior of deep soft rock, gritstone was chosen as the research subject, and a rock triaxial rheometer (Rock 600-50) and acoustic emission (AE) system (SH-II) were used ...to carry out the grade unloading confining pressure creep test under a high-stress level. The test results showed that the lateral creep behavior of the gritstone was more prominent than the axial creep under the initial high confining pressure. Under the same confining pressure, the creep strain rate (the direction the same as strain) of the gritstone decreases with the increase in axial pressure. As shown by the AE count, AE signals were generated throughout the entire test process, indicating that the creep was a “microdynamic” process. The creep behavior was characterized by a significant confining pressure effect. As the confining pressure was decreased, the degree of creep increases significantly. During the test, the AE energy increased on the whole but decreases during the creep phase. During the entire test process, the overall energy in the constant deviatoric stress grade unloading of the confining pressure was 45% higher than that in the constant axial pressure grade unloading. The degree of failure of the rock was different in these two unloading creep tests, and the constant axial pressure grade unloading of the confining pressure entails greater damage than the constant deviatoric stress grade unloading of the confining pressure. The main reason was that the former had a lower confining pressure level and longer creep process than the latter, and the sample was mainly characterized by creep damage and large cumulative damage, while the latter features mainly unloading damage. Through the inversion of the Burgers constitutive model and nonlinear damage constitutive model for the creep test curve, the nonlinear constitutive equation can better fit the accelerated creep stage, which suggested that this model can describe the accelerated creep characteristics of the high-stress soft rock.
To deeply analyze the failure evolution of surrounding rock during excavation-induced unloading of the high-stress rock mass, a multistage failure model was established based on revealed failure ...patterns. The critical conditions for wing cracks were determined. The slab crack buckling analysis was carried out. The true-triaxial rockburst testing system was used for the miniature model test to study the fracturing evolution of surrounding rocks during excavation-induced unloading of the high-stress rock mass. The research results indicated that harder rock samples had higher compressive strength. Moreover, the smaller peak strains implied more obvious yield/plastic stages of harder rock samples with high confining pressures and softer rock samples with low confining pressures. V-shaped grooves appeared at the beginning of the surrounding rock’s failure while spalling and splitting occurred as the stress increased. Finally, the entire sample’s overall splitting failure was observed, and the borehole bottom bulged upward. The harder rock masses had fewer fractures and higher degrees of failure. There were obvious V-shaped grooves on both sides of the marble cave wall. The tensile failure occurred near the opening surface and shear failure at a far distance. The sandstone's overall failure was related to tensile cracking, and splitting failure occurred far away from the opening surface, which was similar to the in situ failure of surrounding rocks during excavation-induced unloading of the high-stress rock mass. The results obtained are instrumental in the construction safety control and prevention of underground engineering disasters.
To evaluate the effectiveness and safety of transarterial chemoembolization (TACE) combined with immune and targeted therapy in unresectable hepatocellular carcinoma (HCC). Prospective analysis of 23 ...patients with intermediate or advanced primary HCC treated at the Department of Hepatic Surgery, The First Affiliated Hospital of the University of Science and Technology of China from July 2019, including 11 cases treated with TACE alone and 12 cases treated with TACE combined with targeted therapy. The basal indexes of patients in the two groups were compared, and the response during treatment was observed; regular follow-up was performed to assess the efficacy of tumor treatment. Compared with TACE treatment alone, the objective response rate (ORR) was significantly higher in the TACE combined with targeted treatment group (50.0% vs 36.4%), with a higher success rate of surgical conversion (33.3% vs 18.2%) and a significantly longer progression-free survival (PFS) (20.5 ± 2.9 months vs 11.6 ± 2.9 months). Multifactorial regression analysis identified tumor vascular invasion as an independent prognostic factor affecting HCC. No patient experienced catheter retention-related complications during treatment, and there were no intolerable adverse effects. TACE combined with targeted treatment for intermediate to advanced unresectable HCC was effective, with good tumor responsiveness, high surgical conversion rate, and safe and controllable adverse reactions during treatment.
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