Abstract: A repeated fracture grouting method is proposed to address the limited number and reinforcing effect of main fracture grouted veins in dense and weak strata. This approach enhances the effectiveness of fracture grouting technology by improving the distribution and reinforcement of grouted veins. The method enhances the reinforcement effect in dense weak strata by controlling the timing of multi-stage fracture grouting, increasing the number of main grouted veins within the reinforcement target area, and establishing a network of fracture grout veins. To investigate the impact of repeated fracturing grouting interval time on the diffusion characteristics of different fracturing sequences, physical experiments were conducted on repeated fracturing grouting in typical dense weak media, based on the Xianglushan Tunnel of the Central Yunnan Water Diversion Project. A diffusion mechanical model for the repeated fracturing slurry was established. This paper summarizes the diffusion patterns and morphological characteristics of fracturing slurry under varying fracturing sequences and interval times. Based on the theory of circular hole expansion, a diffusion mechanical model for repeated fracturing grouting is developed, accounting for the influence of intermediate principal stress, along with a corresponding formula for fracturing pressure. The mechanism underlying the directional changes in multi-stage fracture diffusion is clarified. The research reveals that in dense weak strata, an extrusion-diffusion “O-type” vein package initially forms. As the internal pressure of the slurry increases, an “I-type” fractured vein develops along the dominant rupture plane, oriented in the direction of the vertical minor principal stress. As the preceding slurry approaches initial setting, the solidified grouted veins at the fracture tip alter the local stress state, causing the subsequent fracturing channel to change direction non-homogeneously and form a “Y-type” fractured veins by exploiting a new weak plane. A dimensionless parameter, the “volume-pressure ratio”, is introduced based on the analysis of the ratio between grouted volume and fracturing pressure increase. The findings indicate that performing 2−3 repeated fracturing grouting with intervals of 50%−75% of the slurry’s initial setting time facilitates the formation of extensive branching grouted veins, significantly enhancing the grouting effect in dense weak strata.

Key words: dense weak strata, fracture grouting, repeated fracture grouting, multi-sequence fracture grouting, diffusion mechanism, grouting theoretical analysis