Performance and grouting parameters of new advanced reinforcement materials for underground tunnels with dense fine sand layers

In order to mitigate the impact of advanced grouting reinforcement on the underground environment, it's necessary to employ low-pressure slow injection for grouting. The dense fine sand layer exhibits a tight structure and high clay content. Consequently, under low grouting pressure, the diffusion radius of the existing grouting materials is limited, leading to ineffective reinforcement of the tunnel face. Based on the principles of component activation and particle size optimization, the novel grouting material is developed using the ground granulated blast-furnace slag (GGBS), ultrafine fly ash (UFFA) and ultrafine Portland cement (UFPC) as the primary components, with the addition of an appropriate quantity of composite activating. Through the laboratory experiments, the characteristics of slurry fluidity, setting time and unconfined compressive strength of the novel grouting materials under the influences of varying mix ratios and contents of composite activator are investigated. The reinforcement effects of the Portland cement (OPC), UFPC and new grouting materials are compared and analyzed using a self-developed small pipe grouting full-scale test device. The diffusion characteristics and the change in the final grouting pressure under the influences of overburden load and water content are elucidated. The experimental results show that at a water-solid ratio of 1:1, a mass ratio of 4:2:4 for the GGBS, UFFA and UFPC, and a composite activator content of 4%, the new grouting material exhibits optimal slurry fluidity, controllable setting time and high stone body strength. When using the advanced small pipe grouting method for reinforcement, the OPC slurry diffuses in dense fine sand layer through compaction process, the UFPC slurry through compaction-splitting process and the new grouting material through permeation-splitting processes. The new grouting material exhibits superior diffusion and reinforcement properties, surpassing the UFPC, while the OPC demonstrates the least favorable characteristics in comparison. Furthermore, the new materials should maintain low-pressure grouting during the initial stage of the process to mitigate the risk of excessive grout-causing localized sand splitting and subsequent reinforcement failure. The final grouting pressure linearly increases with the increase of the overlying load and water content, with an increase of 50 kPa in the overlying load resulting in an approximate 50 kPa increase in the grouting pressure. A 2% increase in the water content leads to an increase of around 40 kPa in the grouting pressure. The research findings have certain guiding significance for the theoretical researches and engineering applications of grouting.