Gongcheng Kexue Yu Jishu/Advanced Engineering Science

A deep understanding of the loading and unloading rate effect of rock mass mechanical response under mining stress path is an important basis for defining the optimal advancing rate of actual working face. Based on the initial crustal stress environment of Pingdingshan mining area, the evolution law of stress under the condition of protective layer mining with the depth of kilometer was analyzed quantitatively. The research on the mechanical behavior of coal and rock mass under different unloading rates was carried out, which was more in line with the real stress state. At the same time, the comparative analysis with the results of tests without considering the mining was carried out. The results showed that 1) In the conventional triaxial compression test, the strength of the sample was less affected by the unloading rate, and there was no obvious change at 1～4 MPa/min, and only when it reached 5 MPa/min, the strength increased significantly to about 115 MPa. 2) With the increase of unloading rate, the strength of coal and rock mass showed a trend of decreasing, rising and falling again. The strength of coal and rock mass reached the maximum under the unloading rate of 1 MPa/min and 4 MPa/min, and its peak stress was about 64 MPa, which was 12% higher than that of 3 MPa/min sample. 3) The micro cracks could be fully developed and expanded under low unloading rate, and the fracture density of the specimen decreased with the increase of unloading rate, and it was 1.61 times for 1 MPa/min and 5 MPa/min, respectively, and thus the gas drainage efficiency could be improved by properly reducing the mining speed. 4) The volume strain of coal and rock in the whole mining process at different mining speeds not only had the volume compression in the relative initial state, but also had the volume expansion in the failure stage, which could be regarded as the mining characteristics. This was obviously different from that in the relative initial state without considering the mining test, which was always volume compression under mining. The strength of coal and rock was obviously smaller, and the damage degree was greater. The research results can lay a theoretical foundation for similar geological conditions to carry out protective layer mining design.

Microwave-assisted concrete aggregate recycling is a new green technology, with little pollution and low energy generated during the heating process. The influence of moisture content on the microwave heating effect and concrete strength weakening was explored, and the separation effect of aggregate and mortar in concrete after microwave heating was analysed. Different heating paths were utilized to heat the concrete test blocks to obtain their temperature rise characteristics, and the compressive strength of the test blocks with different moisture content after heating was obtained by conducting uniaxial compression tests. The testing results showed that microwave power and moisture content were two important factors affecting the concrete breaking. When the water content of the test block was relatively high, the test block would break under a short irradiation time and low temperature under higher microwave power input. The aggregate-mortar interface debonding occurred, which could reduce the adhesion of mortar on aggregate, and more complete aggregates with low mortar adhesion could be selected from the crushed concrete. The testing results may provide a reference for further development of microwave-assisted concrete recycling technology.

Mountain rivers often carry a large amount of sediment in the process of flash floods and cause severe deformation of the riverbed. In particular, the riverbed of the wide and narrow rivers is adjusted sharply and the water level rises sharply, which can easily increase the risk of flood inundation in the widened rivers. The study of water and sediment transport and riverbed deformation in the evolution of mountain torrents is of great significance for flood control and disaster mitigation. In this paper, two-dimensional numerical experiments of water and sediment dynamics are used to explore the process of water and sediment transport and river bed deformation in the wide and narrow reaches under the conditions of upstream sediment changes. The results show that: when there is sufficient sand from the upstream, the flood carries the sediment in the widening section and scours the narrowing section, and the river bed is lifted up in the process of sediment transport. The main reason is that the saturated sediment transport of the channel leads to the overall siltation of the river bed, which leads to the steep rise of the upstream water level, the increase of the water surface gradient, the increase of the shear stress of the bed surface, and the increase of the sediment transport rate of the river channel, and the increase of the bed surface fluctuation of the wide and narrow reaches. The morphological resistance of the bed surface causes the upstream water level to rise; when the amount of sediment is small, the river bed is mainly scoured, the water level drops, and the water surface gradient tends to slow down.

To effectively evaluate the loss of axial force for the bolts in cable clamp of suspension bridge, the influence factors analyses and application of ultrasonic identification method for cable clamp blot axial force in suspension bridge were carried out. Firstly, the axial force calculation formula was established based on the acoustic elastic effect, and the accuracy of this formula was verified by experiments. Then, effects of the non-stress acoustic time difference and stress coefficient difference on the recognition accuracy was analyzed. Finally, this method was utilized to identify the tension efficiency and the axial force loss during the lifting process. The results show that, 1) The acoustoelastic effect of bolts is obvious and the recognition error of screw axial force is less than 1.1%. 2) Identification errors caused by the coupling state of sensors and the blot geometry and material parameters are 47.7 kN and 43.1 kN, respectively. 3) The stress coefficients of different bolts are different. Identification deviation caused by the difference of stress coefficients between calibrated bolts and tested bolts is 4.75%. 4) With the increase of screw nut tightening degree, the tension efficiency can reach more than 94%, which is 95.93% higher than that before the tightening degree of nut is not controlled. The bolt axial force is seriously lost with the increase of beam weight. The measured minimum average axial force of cable clamps is 424.32 kN, which is only 56.58% of the designed axial force. In the process of beam section hoisting, the cable clamp bolts should be tensioned in time to ensure construction safety.

China is one of the countries with the most serious disasters of dammed lakes, where the dammed lakes pose a great threat to the safety of people’s lives and properties among the upstream inundated area and the downstream outburst flood routing area. A deep understanding of the scouring and breaching processes of landslide dam can provide important scientific and technological basis for the design of emergency discharge channel and the formulation of downstream emergency measures. According to the event of 2018 Baige landslide-dammed lake in the Jinsha River, the scouring and breaching processes of landslide dam were systematically studied by conducting the laboratory physical modeling tests. Test results showed that the erosion and breaching processes of landslide dam could be divided into four stages, i.e., flow incubation stage, retrogressive scouring stage, dam break development stage, and riverbed rebalancing stage. When the retrogressive erosion scarp was traced back to the upstream slope top and the inlet section of the discharge channel was eroded, the discharge channel would connect to form a ramp with the bottom slope i>0. Then the flow velocity and flow rate increase suddenly, and the dam break developed rapidly. The results also showed that when the excavation width of the chute remained constant and the depth increased, the peak flow would decrease, the peak time would be delayed and the process of the burst flow became smoother; when the depth of the chute was fixed, the peak time would be delayed as the width increased. Finally, according to the testing results, some suggestions were put forward for the optimal design of the discharge channel: The location of the discharge channel should be arranged in the pass with the lowest crest elevation to reduce the peak flow and shorten the breaching time; when excavating the spillway, priority should be given to increasing its depth to minimize the water level of the dammed lake during dam breaching.