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• Identified transition condition between permeation grouting and fracturing grouting in soft rock. • Vertical fractures are more prevalent than horizontal ones during fracturing grouting. • Grouting breakdown pressure significantly exceeds theoretical prediction due to filtration. • Clarified the mechanism by which injection rate and water/cement ratio affect breakdown pressure. Fracturing grouting is an effective method for reinforcing surrounding rock in deep soft rock roadways. However, the mechanism of grouting-induced fracturing remains not fully understood. In this study, a series of laboratory-scale experiments are conducted to investigate the effects of grout injection rate and water/cement ratio on the grouting behavior of soft rocks. The results indicate that, for a given water/cement ratio, fracturing grouting occurs only when the injection rate exceeds a threshold. As the rate increases, the breakdown pressure gradually decreases and eventually reaches a stable value. When the injection rate is fixed, a similar trend is observed with increasing water/cement ratio. Vertical fractures are more frequent than horizontal ones, primarily because they require lower breakdown pressure to initiate. Furthermore, the observed breakdown pressure significantly exceeds theoretical predictions. This derivation is attributed to slurry filtration during injection, leading to filter cake formation that prevents pressure transmission. Lower injection rates and water/cement ratios intensify filtration and increase breakdown pressure, while higher values suppress filtration and facilitate earlier fracturing. These findings improve the understanding of fracturing grouting mechanisms in soft rocks and provide guidance for optimizing injection strategies in deep soft rock reinforcement.
Wei et al. (Mon,) studied this question.