Stress concentration in partially caved goaf is the main cause of dynamic pressure accident in the lower seam mining. Aimed at the characteristics of caving in shallow partially caved goaf (PCG) and the specificity of interior load‐bearing structures, a classification criterion of goaf caving was proposed. The characteristics of cooperative load‐bearing in pillar group and gob pile were revealed by numerical calculation, physical simulation, and theoretical analysis. Taking intermittent mining as an example, combined with the deformation behaviors of the main roof and the loading characteristics of waster rock mass, the characteristics of the load‐bearing of gob piles were described in the form of a piecewise function. A FLAC3D model of intermittent mining is modeled to evaluate the stability of pillars, the long‐term stability and distribution laws of overlying loads were revealed through a refined model. The results show that the width of the caved zone bearing the load in the intermittent goaf is about 20 m, and the maximum load‐bearing capacity is 1.055 MPa. The maximum depth of the plastic region in one side of the pillar is 1.48 m, and the load on elastic zone is about 11.33 MPa. This study provides a method for the study of the caving characteristics and load uncertainty in PCG, and the results provide important theoretical values for the safe mining of lower coal seams.
In this study, the transfer and dissipation of strain energy in the surrounding rock of a deep roadway were analyzed, considering the objective strain softening and dilatancy behaviors. The strain energy increment was decomposed, and its variation was analyzed based on the incremental plastic flow theory; then, a numerical simulation was conducted for verification and further analysis. The results were verified by the field monitoring data of a coal mine gateway. The results show that in the elastic stage, the volumetric elastic strain energy density Uev decreases, while the shear elastic strain energy density Ues increases. In the plastic stage, both Uev and Ues decrease. The volumetric plastic strain energy density Upv is negative, and its absolute value increases, leading to strain energy accumulation. In contrast, the shear plastic strain energy density Ups is positive and increases, leading to strain energy dissipation. Considering strain softening, the elastic strain energy decreases, the plastic strain energy increases, and the region of strain energy dissipation expands. Considering dilatancy, the plastic strain energy varies more significantly, and the effect of strain softening is amplified. The strain energy is transferred from the deep part to the shallow part of the elastic zone and then to the plastic zone. The preexisting and input strain energies in the plastic zone are transformed into considerable amounts of plastic strain energy and then dissipated. Thereafter, a significant plastic strain appears, leading to the large deformation of the surrounding rock.
In many previous tunnel analyses, the axial in-situ stress was ignored. In this work, its effect on the deformation and failure of the surrounding rock of a deep tunnel was revealed, considering the objective strain softening and dilatancy behavior of the surrounding rock. Analysis based on the incremental plastic flow theory was conducted, and C++ was used to write a constitutive model for numerical simulation to verify and further analyze this effect. Then, the results were validated by the field monitoring data of a coal mine gateway. Results show that the effect of the axial in-situ stress σa0 is more significant when strain softening is considered, compared with the results of a perfectly elastoplastic model. When the axial stress σa is σ1 or σ3 at the initial yield, an increase or decrease in σa0 intensifies the deformation and failure of the surrounding rock. When σa is σ2 at the initial yield, 3D plastic flow partly controlled by σa may occur, and an increase in σa0 intensifies the deformation and failure of the surrounding rock. The effect of σa0 will be amplified by considering dilatancy. Considering both strain softening and dilatancy, when σa0 is close to the tangential in-situ stress σt0 or significantly greater than σt0 (1.5 times), σa will be σ2 or σ1 at the initial yield, and then 3D plastic flow will occur. In the deformation prediction and support design of a deep tunnel, σa0 should not be ignored, and the strain softening and dilatancy behavior of the surrounding rock should be accurately considered.
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