The influence of wetting and drying cycles (WDC) on the strength weakening of typical rocks in soft and hard interbedded rock masses is vital to comprehend the mechanism of numerous bank slope ...failures in soft and hard interbedded strata induced by water level fluctuations. How WDC affect the strength of intact rocks in red stratum of the Badong Formation, a typical sliding-prone interbedded stratum in the Three Gorges area, is a topic worthy of comprehensive investigation. In this study, the effects of WDC on the physical and strength parameters of silty mudstone and silty sandstone were examined by performing a series of experiments on a total of 116 rock samples. Models of strength weakening were developed for the two rock types to express the relations between the strength deterioration coefficient (ratio of the weakened and initial value of strength parameters) and the number of WDC. The results indicate that treatment with WDC causes increases in the water absorption rate, and decreases in longitudinal wave velocity, tensile strength, cohesion and internal friction angle for both silty mudstone and silty sandstone samples with the rate of change decreasing with an increasing number of WDC; the variations in physical and strength parameters are larger for silty mudstone compared with silty sandstone; the effects of WDC on tensile strength and cohesion are higher than on internal friction angle for the studied rock types; Intergranular cracks and intragranular cracks were observed to increase with the number of WDC, evolution mode of rock microstructures was proposed, and the main weakening mechanisms induced by WDC was summarized as microcracks growth by mineral expansion and contraction, which weaken the strength of the studied rocks with the impact depending mainly on the mineral contents and microstructures in rocks. Finally, the established strength weakening models were applied in the numerical model to investigate the influence of WDC on the strength of rock mass with silty mudstone and silty sandstone interbedded layers in the Badong Formation. The obtained results provide useful guidance to reasonably evaluate the long-term stability of soft and hard interbedded bank slopes in reservoir area.
•Typical rocks in a interbedded red stratum were sampled from the field.•Experiments were performed on 116 rock samples under wetting and drying cycles.•Effect on tensile strength and cohesion are higher than on internal friction angle.•Mineral expansion, contraction and dissolution induces weakening of rocks.•The test data were used to study strength weakening of interbedded rock mass.
Recent research has paid little attention to the difference in shear strength between discontinuities with different joint wall material (DDJM) and discontinuities with identical joint wall material ...(DIJM) and what are the controlling factors of shear properties of DDJM. Bedding planes between argillaceous limestone and mudstone in the Badong formation in the Three Gorges area of China were investigated using physical model tests. The effect of joint wall material combination on the shear properties of DDJM was first revealed by conducting tests on a series of artificial joint specimens that were made of different joint wall materials and with varying joint surface topographies under different normal stresses. The results indicate that: the peak shear strength of DDJM is not equal to the lower peak shear strength of DIJM for two joint wall materials of the DDJM, even so it is closer to the lower peak shear strength than to the higher one; effect of joint roughness and normal stress on peak shear strength of DDJM is similar with DIJM; effect of joint wall material combination on peak shear strength of DDJM is greater with larger roughness and higher normal stresses. Based on the data acquired from the physical model tests, a new empirical equation to estimate the peak shear strength of DDJM was developed. The capability of the equation was validated by comparing estimates with data from direct shear tests on natural DDJM samples gained from the Badong formation. Finally, the proposed empirical equation was applied to the stability analysis of a rock slope in the Badong formation.
•Discontinuities with different joint wall material (DDJM) were studied.•Physical model tests were performed to investigate shear properties of DDJM.•Effect of joint material combination was first revealed.•A new equation to estimate shear strength of DDJM was proposed and validated.•The proposed equation was applied in the stability analysis of a rock slope.
Recent research has paid little attention to weakening of discontinuities with different joint wall material (DDJM) induced by wetting and drying cycles. Experimental investigations were conducted to ...examine the effects of wetting–drying cycles on weakening of three typical types of natural planar DDJM collected from a sliding-prone formation in the Three Gorges reservoir region of China. Deterioration of the corresponding four types of discontinuities with identical joint wall material (DIJM) were also been studied for comparison. The weakening laws of DDJM were revealed from macro- and microperspectives by testing on mineral compositions, microstructures and water absorption rate of joint walls as well as shear strength of discontinuities. The results indicate that the repeated wetting–drying cycles lead to obvious changes in microstructure and increases of macro water absorption rate for the studied joint walls with the changing degree closely related to their clay mineral contents; the peak shear strengths of DDJM gradually decreases with the wetting and drying cycles for a certain normal stress with the weakening rate between that of the DIJM with two joint wall materials of the DDJM, while closer to that of DIJM with the weak half materials of the DDJM; significant correlations have been found between the variation coefficient of rock microstructure of joint walls and the macrodeterioration coefficient of DDJM; the changing rates of multi-scale parameters before and after six wetting–drying cycles vary obviously, indicating the weakening is slowed down after six wetting–drying cycles for the studied discontinuities.
Highlights
Effects of wetting-drying cycles on weakening discontinuities with different joint wall material were studied.
Deterioration of the discontinuities with identical joint wall material were also been investigated for comparison.
The weakening laws were revealed from macro and micro perspectives.
Significant correlations have been found between the variation coefficient of microstructure of joint walls and the macro deterioration coefficient.
The results could provide a basis to better understand the stability evolution of rock slopes with soft and hard rock interbedded.
Recent research has paid little attention to the shear damage of discontinuities with different joint wall material (DDJM). In this paper, we present an investigation on the evolution of the shear ...behaviour and the damage of three typical types of natural DDJM in a sliding-prone stratum of China. Experimental direct shear tests were performed on 14 pairs of natural DDJM specimens to examine the changes in the shear strengths and surface damages of the DDJM with increasing normal stresses and an increasing number of shear cycles by evaluating surface damages via damage zone distribution, damage area percentage, and variation of joint roughness coefficient (JRC). The results indicate that the differences in the shear damage between the two halves are closely related to the difference in strength of the two joint walls of the DDJM specimens with similar initial JRC values of the two joint surfaces. Simultaneously, parallel numerical direct shear tests were conducted in
PFC3D
. The performance of the numerical modeling was examined by comparing the parameters of shear strength, damage area and damage depth of DDJM specimens with those obtained in the experimental direct shear tests. Then the validated models were used to explore the evolution of the damage depth of DDJM specimens during the shearing process. The results demonstrate that the proposed numerical approach has the ability to reproduce the shear behavior and damage of DDJM reasonably and could be used to examine the internal damage of DDJM which are not easy to investigate via laboratory experiments.
In order to improve land-use efficiency and solve traffic congestion, in recent years, many cities in China have focused on developing urban underground space resources and urban rail transit ...projects. However, there are various hidden risks for the sustainable development of the ecological environment and water resources. In this paper, a comprehensive investigation and analysis of spring water resources are carried out using the example of the karst area of Jinan, which is known as ‘spring city’. The engineering geological and hydrogeological conditions in Jinan are introduced in detail, and the geological causes of springs are analyzed. In addition, the causes of spring flow attenuation are revealed based on the investigation of the flow dynamics of spring water. Based on the current situation of traffic congestion in Jinan, the necessity and development statuses of rail transit construction are analyzed. Then, according to the different stratigraphic structure, limestone roof depth and karst water head depth, the Jinan spring area is divided into three research regions including the shallow limestone area, concentrated spring water area and deep limestone area. The spring protection problems faced by each region during the construction of urban railways are systematically described. In addition, the countermeasures and suggestions for spring protection are presented. This study aims to reduce the impact of urban rail transit construction on Jinan spring water so as to protect the Jinan spring. It also provides the water resources protection experience for urban rail transit construction in similar karst areas.
•A determination method of optimal supporting time against TBM jamming is proposed.•Critical deformation and ultimate stress jointly define the optimal supporting time.•An optimal supporting time can ...ensure the safety of TBM and support simultaneously.•Support measures and parameters have a significant effect on optimal supporting time.•The steel arch has a greater influence on optimal supporting time in the fault core.
TBM (Tunnel Boring Machine) jamming and support failure occur frequently with inappropriate supporting time, particularly when TBM crosses adverse geology. Given this, an integrated determination method of optimal supporting time against TBM jamming was proposed. In this paper, the causes and failure mechanism of large deformation in the tunnel were revealed through X-ray diffraction (XRD) measurement of the rocks. The frictional resistance of TBM shield and the stress of support under different supporting time and measures were investigated by numerical simulation. It was shown that early installation of the support will lead to excessive load on the support, resulting in support failure. Contrarily, delayed installation of the support will lead to excessive tunnel deformation, inducing TBM shield jamming. The optimal supporting time was determined by comprehensively analyzing the critical deformation of TBM jamming and ultimate stress of support. The deformation of surrounding rock in the range of 31.6 mm ∼ 39.9 mm was the optimal support time in this study. Installation support with an optimal supporting time can ensure the safety of TBM and support simultaneously. The range of optimal supporting time decreases continuously with the weakening of support parameters. Specifically, the support measures (bolt + steel arch, shotcrete + bolt) cannot simultaneously ensure the safety of the TBM and support. The research results can effectively guide the prevention of jamming and support design in TBM tunnels.
Most attention-based methods only concentrate along the time axis, which is insufficient for Acoustic Event Detection (AED). Meanwhile, previous methods for AED rarely considered that target events ...possess distinct temporal and frequential scales. In this work, we propose a Multi-Scale Time-Frequency Attention (MTFA) module for AED. MTFA gathers information at multiple resolutions to generate a time-frequency attention mask which tells the model where to focus along both time and frequency axis. With MTFA, the model could capture the characteristics of target events with different scales. We demonstrate the proposed method on Task 2 of Detection and Classification of Acoustic Scenes and Events (DCASE) 2017 Challenge. Our method achieves competitive results on both development dataset and evaluation dataset.
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