Microparameter calibration is an important problem that must be solved in the discrete element method. The Gaussian process (GP) response surface methodology was proposed to calibrate the microparameters based on the Bayesian principle in machine-learning methods, which addresses the problems of uncertainty, blindness, and repeatability in microparameter calibration methods. Using the particle flow code (PFC) as an example, the effects of the microparameters on the macroparameters were evaluated using the control-variable method, and the range of the microparameters was determined based on the macroparameters. The uniform design (UD) method and numerical calculation were used to obtain training samples, and a GP response surface methodology suitable for multifactor, multilevel, and nonlinear processes was used to establish the response surface relationships for macro–micro parameters of rock-like materials in discrete element method. According to the macroparameters obtained from the uniaxial experiments conducted on rock specimens, the microparameters were calibrated using the GP response surfaces. Numerical calculations of uniaxial compression and Brazilian splitting were performed using microparameters, and the results were compared with laboratory experiments for verification. The results showed that the relative errors of the GP response surface and laboratory test values were 5.3% for the modulus of elasticity, −7.8% for compressive strength, and −2.6% for tensile strength. The nonlinear GP response surface considered the characteristics of multiple interacting factors, and the established nonlinear response surface relationship between the microparameters and macroparameters can be used for the calibration of microparameters. The accuracy of the microparameters was verified according to the stress–strain curve and failure morphology of the rock specimens. The method of using the GP response surface to establish the macro–micro parameter relationship in the discrete element method can also be extended to other numerical simulation methods and can provide a basis for accurately analysing the microdamage mechanism of rock materials under complex loading conditions.

This research explored the effect of aging and blending duration on the degree of blending (DoB) and performance of recycled HMAs, as well as the actual contribution of DoBs to the performance of recycled HMA. Nine groups were designed, including one group designed as “full blending group”; one group designed as “no blending group”; and seven experimental groups designed as “partial blending groups” under the different combinations of RAP materials and blending durations. The volumetric indicators, DoB, moisture susceptibility, high- and low-temperature performance of each group were assessed. It was found that prolonging the blending duration can effectively restore the performance of recycled HMAs containing RAP with serious aging. With the extension of blending duration by 180s, more RAP binder was activated, and the DoB of blended mixes enhanced by around 5%~30%, resulting in about 2%~16%, 8%~21%, and 7%~15% improved in the tensile strength ratio, shear strength, and fracture energy of recycled HMAs. In addition, the result also showed that about 10% increase in DoBs, corresponding to RAP binder activated by about 0.15% in this research, can contribute to around 1%~25%, 5%~16%, and 3%~24% improve in the water stability, high- and low-temperature performance of recycled HMAs, respectively.

Based on practical engineering, considering the characteristics of unsymmetrical loading, shallow burying, and weak surrounding rock of the tunnel, MIDAS finite element software is adopted to analyze the influence effect and deformation characteristics of a temporary steel support when the tunnel is excavated by a two-step center diaphragm method (CDM). The simulation results are compared with the field monitoring results. It can be seen that: (1) Affected by the unsymmetrical loading, the settlement of the right spandrel of the tunnel is obvious. The existence of a temporary steel support reduces the settlement of the surrounding rock at the spandrel greatly, making the distribution of principal stress at the spandrel more reasonable. (2) The deformation of the temporary steel support at the upper bench undergoes four stages: convergence, expansion, convergence, and stabilization; and the deformation at the lower bench undergoes five stages: convergence, expansion, convergence, expansion, and stabilization. (3) There is an obvious “bench-type” phenomenon in the principal stress change of the temporary steel support. The analysis results provide a scientific basis and technical guidance for the construction optimization of unsymmetrical loading tunnels using the same support technology.

Joined fractures are an important factor affecting natural rock masses’ mechanical and deformation properties. In this paper, indoor uniaxial compression experiments reproduce prefabricated cracks’ generation, extension, and coalescence in rock-like specimens. For the fractured specimens, a single crack with an inclination of $\alpha$ = 45° was placed on the left and right sides, and a third crack with an angle of $\beta$ = 30°, 45°, 60°, and 90° to the single crack on the right side was placed in groups III–VI, respectively. All cracks extended in the thickness direction. Vertical pressure was applied at a constant loading rate of $v$ = 0.1 mm/min until the stress dropped dramatically. In addition, numerical calculations were performed on the rock specimens using PFC2D, a sub-module of the Discrete Element Method (DEM). The experimental results agree with the numerical simulations in that the strength of the specimens containing a conjugate crack is significantly reduced, and the mechanical and deformation properties of the specimens are related to the internal angle of the conjugate crack, with the lowest peak strength and lowest percentage energy dissipation at $\beta$ = 45°.