Characteristics of Penetration Strength and Fracture Features under Nonuniform Stress Field around Borehole Surrounding Rock.

In the process of mining coal energy, the excavation of roadways and drilling causes the formation of a nonuniform stress field around the penetration holes, which will lead to a significant concentration of stress around the penetration hole. It even leads to the destruction of the surrounding rock of the penetration hole, affecting the integrity of the surrounding rock of the penetration hole. It has an effect on the strength of the rock obtained by the borehole penetration methods. Based on Abaqus software, the numerical model of borehole penetration was constructed by embedding cohesion elements between solid elements. After analyzing the simulation results obtained under different stress boundaries and penetration directions, the following findings are obtained. (1) Using the occurrence of cracks in the borehole surrounding rock as the criterion, the rock is categorized into either an elastic stress state or a plastic stress state after applying different stress boundary conditions. (2) When the borehole surrounding rock is in the elastic (plastic) stress state, the penetration strength increases (decreases) with the increase of lateral pressure coefficient. (3) In the elastic stress state, borehole surrounding rock's fracture area and crack penetration depth increase (decrease) with the increase of lateral pressure coefficient when the penetration direction is parallel (perpendicular) to the maximum principal stress. In the plastic stress state, the fracture area increases, while crack penetration depth decreases with higher lateral pressure coefficient.

Failure evolution and instability prediction of fiber-reinforced polymer-confined cement mortar specimens under axial compression.

In geotechnical engineering, a large number of pillars are often left in underground space to support the overlying strata and protect the surface environment. To enhance pillar stability and prevent instability, this study proposes an innovative technology for pillar reinforcement. Specifically, local confinement of the pillar is achieved through fiber-reinforced polymer (FRP) strips, resulting in the formation of a more stable composite structure. In order to validate the effectiveness of this structural approach, acoustic emission characteristics and surface strain field characteristics were monitored during failure processes, while mathematical models were employed to predict specimen instability. The test results revealed that increasing FRP strip confinement width led to heightened activity in acoustic emission events during failure processes, accompanied by a decrease in shear cracks but an increase in tensile cracks. Moreover, ductility was improved and deformation resistance capacity was enhanced within specimens. Notably, initial crack generation occurred within unconfined regions of specimens during failures; however, both length and width as well as overall numbers of cracks significantly decreased due to implementation of FRP strips. Consequently, specimen failure speed was slowed down accordingly. Finally, the instability of the partial FRP-confined cement mortar could be more accurately predicted based on the model of FRP-confined concrete. It was verified by the test results.