Roadbase performance and modification mechanisms of TBM-excavated gneiss muck utilizing a fiber–geopolymer synergistic stabilization system

The disposal of TBM muck has become a pressing issue in tunnel construction. This study addresses the utilization of 295,000 m 3 of gneiss muck excavated from the DXL Tunnel in western China by modifying it with low‑calcium fly ash (FA), ground granulated blast‑furnace slag (GGBS), carbide slag (CCS), and basalt fiber (BF) to produce a roadbase material. The optimal mix, designated X‑FGC (FA:GGBS:CCS = 9:4:1 with 0.3% 12‑mm BF), was determined via response surface methodology. Mechanical and durability properties were assessed through unconfined compressive strength (UCS), splitting tensile strength (STS), wet‑dry cycling, and sulfate attack tests; the modification mechanisms were investigated using FTIR, XRD, and SEM. The results show that, after 7 days of curing, the modified muck achieves a UCS of 7.22 MPa and an STS of 0.764 MPa. After 20 wet‑dry cycles and 40 days of sulfate exposure, the UCS loss rates are 11.3% and 4.53%, respectively. Performance enhancement is attributed to the formation of a multi‑gel system (N‑A‑S‑H, C‑A‑S‑H, and C‑S‑H), which cements particles, fills pores, and promotes fiber bridging. However, although increasing the X‑FGC content generates more gels and improves mechanical strength and wet‑dry resistance, it also retards polymerization, reduces the degree of gelation, and impairs sulfate resistance. Overall, X‑FGC exhibits promising mechanical performance and durability as a roadbase material, while incorporating a high proportion of TBM‑excavated gneiss muck (approximately 79% of the total wet‑dry mass in the optimal mixture).