Water-bearing joints within rock engineering in cold areas are often subjected to frost heaving force in cold season due to water–ice phase transition. To evaluate the damage and stability of rock mass in cold regions, a 3D model that considers moisture migration loss during freezing and thawing was established to study the characteristics of frost heaving force within joints. Then, the numerical simulation of cyclic freeze-thawing of water-bearing joints was carried out through equivalent expansion coefficient and particle flow calculation methods. The distribution of frost heaving force in and around the joints was obtained. According to the results of the numerical tests and theoretical calculations, the frost heaving force in joints is basically stable, the tensile stress concentration area appears at the joint tip, and the frost heaving force decreases gradually away from the jointed rock mass area. The frost heaving force decreases considerably with increasing cycle number and moisture migration loss but it increases with increasing mechanical strength and joint geometric size of rock and ice. The comparison between the numerical solution of the equivalent expansion coefficient method and the theoretical solution shows that the force size and distribution law of frost heaving for the two methods are consistent.

Analytical and numerical analysis for frost heaving stress distribution within rock joints under freezing and thawing cycles

The macro-micro mining response of the surrounding rock and overlying strata towards the transformation from open-pit to underground mining is examined in the present study, based on the engineering background of the Jinning phosphate mine (Yunnan Phosphate Chemical Group Co., Ltd.) via simulations involving similar materials, digital photographic measurement technology, and numerical simulation. The mining deformation of the surrounding rock underground, and of the overlying strata, is shown to develop in three stages, namely: (1) small and local deformation, (2) continuous linear increase, and (3) the violent nonlinear collapse of the entire system. The internal distribution of stress in the surrounding rock and adjacent overlying strata of the inclined mined-out area is complicated. The degrees of pressure increase and pressure relief have an important relationship with the size of the mining space. The pressure relief is more complete close to the mined area, and the stress reduction decreases with increasing distance. The cracks propagate in arc shapes and have a tendency to penetrate into the upper and lower ends of the stope. The size of the excavation space plays a key role in the generation, propagation, and penetration of the cracks. Due to the disturbance of the first mining level and the increase in excavation depth, the rate of damage to the surrounding and overlying rock increases in the second mining level. This process generates more cracks, which accelerate the instability of the surrounding rock and overlying strata.

/pdf/macro-micro-response-characteristics-of-surrounding-rock-and-2vlo0o5vgp.pdf

Macro-Micro Response Characteristics of Surrounding Rock and Overlying Strata towards the Transition from Open-Pit to Underground Mining

From the perspective of macroscopic scale, the majority of natural rock mass should be categorized as non-penetrating jointed rock mass. The existing researches in the field of scale effect of joint properties were mainly implemented on penetrating joints, which contradicts engineering practice, and is of high possibility to make the strength estimation of large natural jointed rock mass inaccurate, leading to serious loss of life and property. In response to such case, a series of numerical calculations of direct shear test on non-penetrating horizontal rock-like joints with different scales were carried out by PFC in this paper, to investigate the scale effect of shear mechanical properties of non-penetrating horizontal rock-like joints. First, the model microparameters were calibrated by three physical experiments to guarantee the precise reproduction of the mechanical performances of target rock and joint. Next, the particle parameters (average particle size dave and radius ratio μ) of model were changed, the effect of particle size on joint strength was studied by direct shear calculation, and the determining method for the values of dave and μ was suggested. Then, based on two distribution forms of non-penetrating horizontal rock-like joint (type I and type II joints), the numerical shear tests were conducted on jointed rock models with different persistence rations and model scales, and the variations of shear stress displacement curve and strength characteristics were analyzed. The results indicate: The lower the persistence ration λ of the joint, the more obvious the negative scale effect of joint shear strength. Besides, the scale effect of shear strength gradually decreases when λ > 0.5 for type I joints while λ > 0.8 for type II joints and the scale effects of joint strength parameters only emerge in the case of λ < 0.2.

Scale effect of shear mechanical properties of non-penetrating horizontal rock-like joints

A full-stage creep model for rocks based on the variable-order fractional calculus

Countermeasures against large deformation of deep-buried soft rock tunnels in areas with high geostress: A case study

Shear strength is an important mechanical index of rock. The adjustment and reorganization of rock structure can be reflected by the variation of shear strength. During rock rheology, the shear strength decreases with time due to rock damage. Therefore, from the point of difference of shear strength, the mechanical properties of rock can be effectively studied. In this paper, for the state of direct shear test, the Kachanov creep damage law was adopted to describe the time characteristics of the rock shear strength during the accelerated creep stage. A nonlinear viscoplastic element on the basis of time-dependent shear strength was established by connecting the plastic element representing shear strength with the viscous element in parallel. After introducing the nonlinear viscoplastic element into the classic Burgers model, the rationality of the model was verified by the shear creep test results of rock discontinuity. Results showed that the modified Burgers model can reflect the mechanical properties of rock in three creep stages. In addition, the model parameter δ can also reflect the evolution of internal cracks in rock during creep.

Xing Zhang

Papers

Improved nonlinear Burgers shear creep model based on the time-dependent shear strength for rock