Experimental study on energy dissipation and failure characteristics of rock at depth under non-hydrostatic pressure condition

Deep buried rock is often in a non-hydrostatic initial ground stress field, and the different stress states lead to different rock response under dynamic loading. To study the dynamic behavior of rocks under coupled non-hydrostatic pressure and dynamic loading, the dynamic compression tests under different initial stress are carried out by using the split Hopkinson pressure bar (SHPB) experimental system, and the influence of different non-hydrostatic pressure on energy dissipation and failure characteristics of rock is analyzed. Based on the test results, it is found that both axial and lateral confining pressure have a significant impact on the rock dynamic strength. Taking 8 MPa as the critical point, the rock dynamic strength first increases and then decreases with the increase of axial confining pressure, but the rock dynamic strength will continue to increase with the increase of lateral confining pressure. The rock dissipative energy gradually decreases with the increase of axial confining pressure, but the trend of dissipative energy changes with confining pressure is complicated. When the axial confining pressure is low, the dissipative energy decreases with the increase of the confining pressure, and when the axial confining pressure is high, the dissipative energy shows a characteristic of increasing first and then decreasing with the increase of lateral confining pressure. According to the apparent and internal damage patterns of the specimens, combined with fractal theory and CT scanning technology, the influence of the damage pattern of rocks under different non-hydrostatic pressure conditions is analyzed. It is found that there is a significant correlation between the apparent cracks of rock and the ratio of axial-confining pressure. When the axial confining pressure ratio is low, there are fewer cracks on the side and end faces of rock. With the increase of the axial confining pressure ratio, the number of apparent cracks of rock increases. Through the analysis of the cross-sectional and vertical slices of the rock, it is found that there are circumferential and radial cracks in the rock simultaneously, and the rock failure is controlled by the penetrating shear cracks. It also can be found that the damage of rocks under non-hydrostatic pressure conditions can be divided into two parts: the central spherical rock mass and the peripheral broken rock mass, and the damage degree of rocks increases when the axial pressure increases, while the elevation of the surrounding pressure makes the damage degree of rocks decrease

Dynamic Tensile Strength of Dry and Saturated Hard Coal under Impact Loading

To evaluate the influence of water content on the hard coal dynamic behavior, the dynamic tensile properties of saturated coal Brazilian disk (BD) samples were studied using a split Hopkinson pressure bar system, and dry samples were also tested as a control group. In the range of impact speeds studied, the tensile strength of the saturated coal is lower than that of the dry specimen. A synchronized triggering high-speed camera was used to monitor the deformation and failure process of dry and saturated coal samples, allowing analysis of the failure stages and mechanism of dynamic BD test, the broken mode was classified into three types, which can be classified into unilateral tensile failure, bilateral or multilateral tensile failure, and shear failure. Finally, fragments smaller than 5 mm in diameter were statistically analyzed. There is less debris in range of 0–5.0 mm for the saturated coal sample than for the dry coal. This study provides some information about the dynamic response of the hard coal for the relevant practical engineering

Stability of Coal Pillar and Roof Movement Characteristics in Roadway Backfill Mining

In order to explore the stability of coal pillar and the characteristics of roof movement during the process of roadway backfill mining (RBM), the 301 backfilling test working face of Ordos Chahasu coal mine is taken as the background. Based on the expansive pressure arch theory, the evolution process of the stope expansive pressure arch in RBM is studied; by establishing a mechanical model for the stability of coal pillars, the interactions between the height, width, and the maximum number of branches are obtained. When the width and height of the branch are both 5 m, the optimal number of the branches is obtained. Then, by establishing a mechanical model for the subsidence of the immediate roof, the process of the immediate roof subsidence is divided into three stages, namely, the formation stage of the local pressure arch, the merge stage of the pressure arch, and the expansion stage of the pressure arch. In addition, using the numerical method, the alternate bearing process of coal pillars and filling bodies and the change of the maximum supporting stress are studied, and the evolution of the pressure arch bearing structure above the stope and the staged subsidence characteristics of the roof are analyzed. The on-site test showed that the coal pillar has a good stability during the mining process. The maximum stress of the coal pillar is 16.5 MPa, and the maximum stress of the filling body is 9 MPa. The maximum settlement of the immediate roof is 102 mm, indicating that the roof control effect is good. This research will play an important role on engineering practice

Cemented Backfilling Mining Technology for Gently Inclined Coal Seams Using a Continuous Mining and Continuous Backfilling Method

To improve the efficiency and reduce costs of cemented-fill mining, we propose a continuous mining and continuous backfilling (CMCB) method based on the coal resources at the Yuxing mine in Inner Mongolia, China, and constructed a complete filling material transportation system. The new technology is suitable for cemented-fill mining of gently inclined coal seams. Numerical simulations were performed to investigate the dynamic migration law of surrounding rock stress using CMCB cemented-fill mining technology, and similar simulations were conducted to analyze the movement characteristics of the coal overburden. The results show that the coal pillars and filling body alternately bear and support each other during the CMCB process, which resolves the contradiction between mining and filling, achieves parallel mining and filling operations, and improves mining efficiency. The new mining mode exerts minimal disturbance to the overlying rock and effectively controls surface deformation. The engineering application of this technique is promising and provides theoretical guidance and technical support for safe and efficient mining of the same type of coal resources

In Situ Stress Effects on Smooth Blasting: Model Test and Analysis

Most of the roadway excavation is completed by the drilling and blasting method. With the increase of buried depth, the existence of ground stress will generate a significant impact on the rock blasting, especially on the smooth blasting. In this study, self-made homogeneous similar materials and digital image correlation methods were used to determine influence of ground stress on the smooth blasting under uniform explosive charge parameters and various in situ stress conditions. The results show that the crack outline after blasting changes from zigzag to straight in shape, and multifractal calculation results of the rupture section between blastholes show that the fracture surface becomes flatter as ground stress increases, which is conducive to roadway formation. The strain and equivalent strain rate obviously decrease as the distance between the blasthole and measuring points increases. The same trend occurs as the confining pressure goes up. Meanwhile, a postexplosion acoustic wave test indicates that confining pressure inhibits damage of the retained rock, which is consistent with strain and equivalent strain rate results. Finally, we discussed the crack propagation mechanism of rock in smooth blasting

Effectiveness and Safety of Moxibustion on Constipation: A Systematic Review and Meta-Analysis

Aim. This study aimed to evaluate the effects and safety of moxibustion in the management of constipation. Background. Constipation is extremely common in clinical practice and causes physical and mental pain to patients. This study aimed to evaluate the effects and safety of moxibustion in the management of constipation. Methods. Studies on moxibustion for constipation published up to November 2019 were searched in PubMed; EBSCO; EMBASE; Cochrane Library; and three Chinese databases, namely, China National Knowledge Internet, Wanfang, and China Biomedical Network. The methodological quality of the included studies was assessed on the basis of the CLEAR NPT system evaluation methods of Boutron. Further meta-analysis was performed using the RevMan 5.3 and Stata 15.0 software. Results. Ten randomized controlled trials involving 760 patients were included in this study. The meta-analysis revealed that, in comparison to western medicine treatment or other Chinese medicine methods (control group), moxibustion (experimental group) had a higher total effective rate (RR = 1.30, 95% CI [1.21, 1.40], P<0.00001), and the clinical effectiveness of the experimental group was higher than that of the control group in any subgroup analysis. The first defecation time of the experimental group was shorter than that of the control group (SMD = −1.36, 95% CI [−2.03, −0.68], P<0.0001). The clinical symptom score of the patients in the experimental group was lower than that in the control group (SMD = −0.65, 95% CI [−1.00, −0.30], P=0.0003). The patients in the experimental group had higher scores on Bristol stool form scale than those in the control group (MD = 0.99, 95% CI [0.48, 1.50], P=0.0001). However, there was no obvious difference in safety between the two groups (RR = 0.38, 95% CI [0.01, 11.8], P=0.58). Conclusions. Moxibustion may have better effect than other treatments on constipation. However, it is not yet possible to assess the safety level of moxibustion therapy, and the quality of the included literature is low, so rigorous studies are warranted. Implications for Nursing and Health Policy. The focus of this study was to evaluate the effectiveness and safety of moxibustion therapy in constipation. This evaluation showed that moxibustion therapy has a good effect on constipation and provides an effective treatment for constipation patients. Whether moxibustion therapy can be used for different syndrome types deserves further discussion