Research on an evaluation method for the strength of broken coal mass reinforced by cement slurry based on digital drilling test technology

Fault fracture zones, high stress, extremely soft rock, and other complex conditions of underground engineering cause the surrounding rocks to become friable and fragmented after chamber excavation. It is common to observe phenomena such as bolt body slips, fractures, severe support buckling and breaking, resulting in safety issues. The broken rock mass is often reinforced by cement slurry, and the relative strength increase in the surrounding rock is an important index used to evaluate the reinforcement effect. However, it is currently challenging to quantify the strength of surrounding rock after reinforcement by cement slurry. A digital drilling test technique provides an effective method for quantitative evaluation of the strength of a broken coal mass reinforced by cement slurry. The key is to establish a quantitative relation between the drilling parameters and the strength of the rock mass after reinforcement. Therefore, this study used the 1000-km-deep Zhaolou coal mine in China as an engineering background. Based on a multi-function rock drilling test system developed by the authors, digital drilling tests and uniaxial compression tests of broken coal mass reinforced by cement slurry were performed, and the variation law of drilling parameters with uniaxial compressive strength was analyzed. Additionally, a quantitative relational model between the uniaxial compressive strength and drilling parameters was built by means of an energy analysis method. A method to quantitatively evaluate the reinforcement strength of a broken coal mass based on a digital drilling test technique was developed. A study on the impact laws of different water:cement ratios and coal particle sizes with respect to the strength of broken coal masses after reinforcement was also conducted. The study results provide an effective method for the quantitative evaluation of the reinforcement effect of broken surrounding rocks and optimization of the cement slurry parameter.