Hongyuan Zhou

The in‐service strength test of underground space strata is a hot issue in geotechnical engineering. In this study, we conducted mechanical analyses on the soil and the cutting bit involved in a rotary drilling process, which revealed the key factors influencing drilling. This allowed us to establish theoretical relationships among torque, propulsion, and soil strength parameters (i.e., unconfined compressive strength, shear strength, and cohesion). Moreover, with consideration of the effects of water content and geostatic stresses, we used an independently developed rotary drilling system with a 3D flexible boundary loading device to conduct 75 rotary drilling model indoor tests on silty clay in 15 groups. Based on the test results, we simplified the theoretical relationships and determined model parameters to obtain 15 quantitative relationships with 5 different water contents (4.8%, 9.7%, 14.8%, 19.6%, and 24.9%). Finally, through field application, we verified the quantitative relationships and we discussed their scope of application. The results showed it is feasible to predict the soil in‐service strength based on rotary drilling parameters. Under the premise that water content is considered, the drilling parameters were found correlated linearly (correlation coefficients are all greater than 0.85) with strength parameters. In field application, absolute errors between the prediction and investigation results were less than 5%, satisfying engineering requirements.

The present paper is devoted to investigate the effects of waste polyester fiber fabric blocks on the strength and mechanical behavior of cemented sand. In the investigation, samples were prepared at four different percentages of waste polyester fiber fabric block content (0.0%, 0.5%, 1.0%, and 1.5% by weight of soil) and two different aspect ratios (2 : 1 and 3 : 1), and conventional triaxial compression tests were carried out after the curing period. The test results indicated that the addition of fibers increased peak and residual shear strengths of cemented sand and changed its brittle behavior to a more ductile one. As the fabric block content increased, the brittleness index and initial stiffness decreased, and the peak strain and internal friction angle increased. The optimal combination of the content and aspect ratio was determined to be 0.5% and 3 : 1. The integration of the fabric blocks with the cemented sand matrix was analyzed by using the scanning electron microscopy (SEM). It is found that the reinforcement effect is related to the bond strength and friction at the interface. The micromechanical properties of the fiber/matrix interface were influenced by the undulations between the fabric block components. In summary, this study presented a low-cost and environment-friendly method for reinforcing cement-stabilized sand.

Overburden deformation has a considerable influence on slope stability during underground mining works. A simulation experiment for underground mining is performed in this study. The movement of the overlying strata caused by mining during the mining process observed using a three-dimensional (3D) optical displacement monitoring system. Based on the simulation experiment results, a particle contact-based meshfree method (PCMM) is proposed; The elastoplastic analysis and sliding process simulation of slope are realized By intruducing the softening Mohr-Coulomb model and maximal tensile stress model into continuous media elements. The deformation process and the slope failure are analyzed by using the PCMM numerical simulation method. The results showed that the overburden gradually degraded as the work face continuously pushed forward. Furthermore, the basic roof bore the force generated by the upper rock mass. Hence, a "rock beam" force distribution developed. The farther the distance from the coal seam, the smaller the subsidence. The rock strata subsided and induced the "beam" structure effect when the work face continued to advance past the design stop line, causing a rock mass partial settlement on the slope and a serious horizontal movement on the free face. This behavior significantly affected the slope stability, even inducing cracks and potential slip and sliding surfaces. The numerical simulation results indicated that the deformation process of the overlying strata is "immediate roof starts to collapse -- rock mass of the main roof collapses-- overlying strata generate an abscission layer --fracture of the free surface develops --the overlying strata begins to sink". Meanwhile, the slope deformation and failure process is "overlying rock mass exhibits shear failure-- the overburden is completely sheared-- the overburden collapses slope surface exhibits a tensile crack-- stepped crack is connected-- the slope begins to slide to the outside." The research results provide guidance for underground mining.

In this study, based on the mining of the 13210 working face in the Yima coal mine of the Gengcun village, China, a simplified mechanical model for the analysis of dynamic destabilization of the overlying strata during underground mining was constructed. The numerical simulation was used to analyze the stress patterns in the advanced abutments of the tunnel face and the characteristics of dynamic failures in the overlying strata. Furthermore, similitude experiments were conducted to study the process of stress release and deformation in the overlying strata, and to analyze the effects of overburden destabilization on the ground surface settlement. The theoretical analysis indicated that if the geometric parameters of a working face are fully determined, a stiffness ratio no greater than 1 is required for dynamic destabilization to occur. The numerical simulation results show that the stress in the overlying strata decreases with a decrease in distance from the tunnel face. The stresses in the advanced abutments initially increase with an increase in distance from the tunnel face, followed by a decrease in stress, and an eventual stabilization of the stress levels; this corresponds to the existence of a “stress build‐up zone,” “stress reduction zone,” and “native rock stress zone.” In similitude experiments, it was observed that a “pseudoplastic beam” state arises after the local stresses of the overlying strata have been completely released, and the “trapezoidal” fractures begin to form at stress concentrations. If the excavation of the working face continues to progress, the area of collapse expands upward, thereby increasing the areas of the fracture and densification zones. Owing to the nonuniform settlement of the overlying strata and the continuous development of bed‐separating cracks, secondary fractures will be generated on both sides of the working face, which increase the severity of the ground surface settlement.