Macro-micro coupled analysis of synchronous grouting in coarse-grained strata: CFD–DEM insights into ground settlement control and fabric evolution

This study investigates the macro- and micro-mechanism of synchronous tail grouting in ground surface settlement (GSS) control of coarse-grained strata through a computational fluid dynamics-discrete element method (CFD–DEM) coupled model, focusing on two grouting port configurations (Type I: upper-lower symmetrical layout; Type II: lateral staggered distribution) in six-grouting-port shield machines. Analyzing particle contact characteristics and force chain evolution patterns, the connection between particle-scale behavior and macroscale GSS during grouting is elucidated. Key findings demonstrate that grouting duration is the primary factor for GSS development. Grout injection disrupted the particle contacts and force-chain networks above the tunnel center, weakening natural arching and redistributing interparticle forces. The central axis of the soil-arching effect exhibited a strong linear correlation with the spatial position of the uppermost grouting port. The soil located 0.25 D –0.5 D (where D represents the tunnel diameter) from the tunnel centerline horizontally, exhibited heightened sensitivity to grouting, with a low fabric anisotropy ( α ) and oscillating principal direction ( β ) sharply between 0° and 180°, reflecting pronounced displacement and isotropy. This study provides theoretical support for intelligent shield machine selection and grouting strategy optimization in geotechnical engineering, with significant implications for soil displacement control in complex strata.