Wet shotcrete support is the main support method for underground engineering construction at present. The rebound rate is one of the indicators to evaluate the support effect, and a lower rebound rate will improve the construction effect of wet shotcrete. The concrete aggregates available in different regions of China have different particle sizes. To achieve better supporting effect, the discrete element method is used to simulate the spraying process of wet shotcrete. By constructing the nozzle structure and simplifying the concrete structure, the paper establishes the discrete element model of wet shotcrete. Using the above model, the paper analyzes the effects of injection distance and shrinkage angle on the injection rebound rate of concrete with three aggregate sizes of 5∼11 mm, 11∼17 mm, and 17∼25 mm. The optimal construction parameters of concrete with different aggregate particle size ratios were obtained through simulation experiments. The simulation results are highly consistent with the field experimental structure. It is feasible to simulate wet shotcrete spraying process by the discrete element method.
In order to use image recognition for the three-dimensional reconstruction of target objects faster and more conveniently, with the help of various machine vision technologies such as mathematical algorithms and machine learning algorithms, it aims to help engineers complete the three-dimensional reconstruction of cone-like aggregate piles. First, under the method based on a genetic algorithm, the proposed method can be used to identify the most common contour segments to deal with the contour recognition of aggregate piles and complete the work of region segmentation. The common fragment illustrates the particular logic contained in the outline. Then, this paper shows that the explicit representation of shape contour contributes to shaping representation and object recognition. Multiple two-dimensional virtual slices are used to divide the target object in the field of view of the binocular camera into multiple cross-sectional areas, so that an appropriate ellipse of the material pile contour curve similar to a cone is obtained to approximately express these cross-sectional areas. Finally, the three-dimensional reconstruction of the surface of the three-dimensional target object is completed by many two-dimensional elliptical slices. Experiments show that the method of three-dimensional reconstruction of images in a rapid and straightforward ability to prove its feasibility of the method.
It is known that, long-distance concrete pipeline transportation often causes pipe blocking accidents. For the problem of pipeline blockage, we have invented a swirl speed-increasing device. In this study, the discrete element method was used to calibrate the parameters of the contact model through the slump bucket experiment, and a discrete element model of horizontal pumping concrete was established. Based on the particle unit, the flow-promoting mechanism of the swirl speed-increasing device was studied, and the effects of the pumping speed, the volume fraction of coarse aggregate and the wind speed output of the device on the movement state of the particles and the flow-promoting distance were analyzed. The results show that the maximum velocity and flow-promoting distance of the particle unit increase with the increase of pumping speed and wind speed, and decrease with the increase of the coarse aggregate volume fraction. When the pumping speed and wind speed are large, the installation distance of the flow promoting device should be increased. When the volume fraction of concrete aggregate is large, the installation distance of the flow promoting device should be shortened. Our swirl speed-increasing device can solve the problem of pumping concrete pipeline blockage. The discrete element model of pumping concrete established in this study can well reflect the motion state of the concrete unit in the pipeline, which verifies the applicability of the discrete element model. The numerical simulation conclusion in this paper can avoid system energy waste and pipeline blockage caused by improper selection of device installation spacing.
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