In terms of the degradation of mechanical parameters of rock mass in the hydrofluctuation belt of a reservoir bank slope arising from rainfall and the reservoir level fluctuation, the moderately weathered sandstone in a side slope of the Three Gorges Reservoir Region is selected as a research object to carry out “drying-saturation-drying” tests for disks with two thicknesses (h=25 mm,h=50 mm) in different cycles; a spiral CT machine, an ultrasonic velocity meter, and a light Schmidt hammer are utilzed to conduct nondestructive testing on dry-wet cycles; through the Brazilian splitting test, the uniaxial tensile strength of “dry” and “saturated” sandstones under different dry-wet cycles is obtained. The research shows that, with the increase of the dry-wet circles (n), the longitudinal wave velocity and the rebound strength of sandstones are linearly decreased withn; the uniaxial tensile strength of sandstones and the mean CT number of cross sections are logarithmically decreased withn; the fitting equation of macro/micromechanical parameters and dry-wet cycles (n) of sandstones is raised, which is provided as a reference basis for the weathering process of sandstones under dry-wet cycles.
The Contact Bond Model (CBM) of Particle Flow Code (PFC) can effectively simulate the mechanical behaviours of rocks. The physical and mechanical parameters of CBM directly influence the results of the numerical simulation of rocks. To reveal the influence mechanism of the contact normal bond strength of the CBM on Brazilian tensile strength, this study proposes a numerical test model and calculation method for the Brazilian tensile strength. First, the theoretical relationship between the Brazilian tensile strength and the contact normal bond strength of the CBM was derived. Second, 70 groups (350) of PFC Brazilian disk numerical tests were designed, and a numerical model of the Brazilian tensile strength was established. Third, the effects of the contact normal bond strength on the Brazilian tensile strength under various ball radii and ball radius ratios were analyzed according to the PFC numerical simulation results, and the correctness of the theoretical relation was verified. Results demonstrate that the Brazilian tensile strength is significantly and linearly related to the contact normal bond strength and can be expressed as the product of the contact normal bond strength and the scale coefficient. The scale coefficient is affected by the geometric parameters of balls. The scale coefficient generally increases with the minimum particle size, whereas the change in the coefficient becomes irregular. Results reveal the physical significance of the PFC normal bond strength to a certain extent. The proposed method can determine the contact normal bond strength.
Discrete Element Method (DEM) has been successfully utilized to model rock behavior based on Particle Flow Code (PFC), which is extensively employed in solving various problems related to rock engineering and geomechanics. Therefore, a convenient method for selecting appropriate microparameters of PFC for model generation is necessary. The present study aims to develop a novel approach that calculates proper micro-strength parameters for the Contact Bond Model (CBM). Firstly, based on Plackett–Burman (PB) design, qualitative research is conducted and it is found that the main factors that influence the Brazilian tensile strength is microscopic tensile strength. We analyzed the stress conditions of a Brazilian disc's vertical diameter using both continuum models and DEM. From this analysis, we establish a theoretical relationship between rock tensile strength and micro-strength parameters. Subsequently, a large number of numerical Brazilian tests were conducted to obtain the statistical relationship between the geometric parameters of balls, micro-strength parameters and the Brazilian tension strength. The results of the numerical simulation were then used to refine the theoretical equation mentioned above, resulting in a modified equation for rock tensile strength and micro-strength parameters. Finally, after verification, we confirm the feasibility of the method in this paper.
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