RESUMO
The precise calculation and evaluation of the in-situ rock stress tensor is a crucial factor in addressing the major challenges related to subsurface engineering applications and earth science research. To improve the accuracy of in-situ stress measurement and prediction, an improved overcoring technique involving a measurement circuit, temperature compensation, and calculation method is presented for accurately measuring the in-situ rock stress tensor. Furthermore, an embedded grey BP neural network (GM-BPNN) model is established for predicting in-situ rock stress values. The results indicate that the improved overcoring technique has significantly improved the stress measurement accuracy, and a large number of valuable stress data obtained from many mines have proved the testing performance of this technique. Moreover, the mean relative errors of the prediction results of GM(0, 1) for the three principal stresses all reach 6-30%, and the accuracy of the model fails to meet the requirements. The average relative errors of the prediction results of the BPNN model are all less than 10%, and the model accuracy meets the requirements and has sufficient credibility. Compared with the GM and BPNN models, the embedded GM-BPNN model produces the best results, with mean relative errors of 0.0001-4.8338%. The embedded GM-BPNN model fully utilizes the characteristics of grey theory and BP neural network, which require a small sample size, weaken the randomness of the original data, and gradually approach the accuracy of the model, making it particularly suitable for situations with limited stress data.
RESUMO
The stress measurements determined by both the overcoring (OC) and hydraulic fracturing (HF) methods in the Shuichang iron mine and Sanshandao gold mine were compared and evaluated, respectively. The results indicate that the independent OC and HF data in the two mines reveal the same dominant faulting stress regime. The σH orientations derived from the OC and HF methods in the Shuichang iron mine are dominantly oriented in the N81.1°W-N89.4°W and N77.0°E-N88.0°E, respectively, and the σH orientations yielded from the OC and HF techniques in the Sanshandao gold mine are predominantly in the N30°W-N90°W and N55.5°W-N60.4°W, respectively; hence, the σH orientations obtained by the two different methods in the two mines are comparatively similar. In addition, the shapes of the probability density diagrams using an improved Bayesian regression approach of the three principal stresses measured by the OC and HF methods in the same mine are quite similar, and all the obtained Kolmogorov-Smirnov test p-values are larger than the selected significance level of 0.01, indicating that the stress data interpreted by the two methods approximately follow the same distribution law. Thus, the performance of the two techniques and the reliability of the measured data are satisfactory.
RESUMO
The rise in the connectivity of the fractures is a key task in oil/gas and geothermal exploitation systems. Natural fractures widely exist in underground reservoir sandstone, while the mechanical behavior of rock with fractures subjected to hydro-mechanical coupling loads is far from clear. This paper employed comprehensive experiments and numerical simulations to investigate the failure mechanism and permeability law for sandstone specimens with T-shaped faces subjected to hydro-mechanical coupling loads. The effects of crack closure stress, crack initiation stress, strength, and axial strain stiffness of the specimens under different fracture inclination angles are discussed, and the evolution processes of permeability are obtained. The results show that secondary fractures are created around the pre-existing T-shaped fractures through tensile, shear, or mixed modes. The fracture network causes an increase in the permeability of the specimen. T-shaped fractures have a more significant effect on the strength of the specimens than water. The peak strengths of T-shaped specimens decreased by 34.89%, 33.79%, 46.09%, 39.32%, 47.23%, 42.76%, and 36.02%, respectively, compared with intact specimen without water pressure. With the increase in deviatoric stress, the permeability of T-shaped sandstone specimens decreases first, then increases, reaching its maximum value when macroscopic fractures are formed, after which the stress suddenly decreases. When the prefabricated T-shaped fracture angle is 75°, the corresponding permeability of the sample at failure is maximum, with a value of 15.84 × 10-16 m2. The failure process of the rock is reproduced through numerical simulations, in which the influence of damage and macroscopic fractures on permeability is discussed.
RESUMO
As a portion of intact rock separating joint surfaces, rock bridge plays a significant role in the stability of rock slopes. This paper aims to investigate the effect of different rock bridges on the mechanical properties and failure mode of rock slope by means of the direct shear test and acoustic emission technique. Field conditions were simulated in direct shear tests which were carried out on specimens with rock bridges at different continuity rates, normal stress, arrangements, and joint angles. Experimental results indicate that the strength of specimens is controlled by the rock bridge and the structural plane. The rock bridge contributes to the strength of the specimen, while the through plane weakens the strength of the specimen. The increase of normal stress can weaken the stress concentration near the tip of the rock bridge and improve the shear resistance of the specimen. The different arrangement of rock bridge has little effect on the normal displacement of the specimen, and has a great influence on the shear strength. The shear capacity of the specimen is related to the angle of the crack, and the angle of the crack is approximately proportional to the peak shear strength. For the specimens with different joint occurrence, the mode of crack propagation at the initial stage is basically the same, and the specimen is finally damaged due to the generation of through cracks in the core area of rock bridge. The instantaneous release of the huge energy generated during the experiment along the shear direction is the root cause of the sudden failure of the rock bridge. The formation, aggregation, and transfixion process of rock bridge is of concern and has been experimentally investigated in this paper for the prevention and control of the locked section rock slope with sudden disasters.
RESUMO
BACKGROUND/AIMS: The effects of hypoxia-inducible factor-1α (HIF-1α) on angiogenesis and cardiac function improvement in rats with myocardial infarction (MI) is unknown and our current study was to evaluate whether HIF-1α would be beneficial for angiogenesis and cardiac function improvement in MI rats. METHODS: A mutant of adenovirus HIF-1α (Ad-HIF-1α-Trip) was constructed by three sites mutation (Pro402, Pro564 and Asn803) in HIF-1α. The rat MI model was produced by permanent ligation of left anterior descending artery and 1×109 PFU adenovirus (Ad) vector particles of Ad-Null, Ad- HIF-1a-564/402, Ad- HIF-1a-Trip, 250ng vascular endothelial growth factor (VEGF) in 0.5ml saline or only 0.5 ml saline were injected intramuscularly around the infarct border zone. Real-time PCR, ELISA and western blotting were used to evaluate angiogenesis factors expression. Capillary density and necrotic areas were detected by immunohistochemistry staining and TTC staining, respectively. Cardiac function assessment was done by echocardiography before operation and on day 7, 14 and 28 after MI. Blood samples were drawn for the measurement of cardiac biomarkers, liver function and kidney function. RESULTS: On day 7, compared to the other groups, the expressions of HIF-1α and angiogenesis factors, and the capillary density were all significantly higher in the Ad-HIF-1α-Trip group. However, on day 28, no significant between-group differences were observed. After 72 hours of MI, serum level of cardiac biomarkers and the necrotic areas were significantly lower in the Ad-HIF-1a-Trip group compared to the other groups. Echocardiography showed that on day 7, cardiac functions were significantly reduced in all groups compared to the baseline. Cardiac function in the Ad-HIF-1α-Trip group was decreased less profoundly through day 7 to day 28 compared to the other groups. Importantly, no significant differences in liver and renal function were observed. CONCLUSION: Mutation of Pro402, Pro564 and Asn803 are beneficial for enhancement of the efficacy of HIF-1α. Ad-HIF-1α-Trip is able to improve angiogenesis and cardiac function, which may be a promising avenue for treatment of ischemic heart disease in the future.
