Experimental Study and Engineering Application of Polypropylene Fiber Shotcrete for Bored Tunnels in Water-Rich Strata

During the construction of water-rich bored tunnels, it is important to improve the impermeability of the primary support shotcrete to ensure construction safety. This work considers the underground excavation of water-rich strata in the section between the Baisheng Shaft and Xiba River Station of the Beijing Metro Line 12. A polypropylene fiber-reinforced shotcrete (PFRS) was designed using an orthogonal experimental method. The fiber size and content, cement content, sand rate, and water–cement ratio were the key factors in the PFRS performance. Based on the analysis of the experimental data along with numerical simulations and other research methods, a new mix proportion of PFRS is proposed and successfully applied to engineering sites. It is shown that compared with ordinary shotcrete, the impermeability of PFRS is greatly improved as the maximum can be increased by 42.5%. After the application of PFRS on-site, the problems of water seepage and excessive deformation of the tunnel supporting structure in the water-rich strata were alleviated. This fully ensures the overall stability of the tunnel and provides a guarantee for a safe construction site.