Development and Engineering Application Validation of Alkali-Activated Full-Component Shield Tunnel Spoil Regenerated Solid-Waste Grouting Material

Shield tunneling generates a large volume of shield tunnel spoil (STS). Conventional disposal practices are associated with high costs, low resource recovery efficiency, and considerable environmental burdens. Meanwhile, synchronous backfill grouting materials used during shield advancement are critical for controlling ground settlement and maintaining lining stability. To address these issues, this study proposes an Alkali-Activated Full-Component Shield Tunnel Spoil Regenerated Solid-Waste Grouting Material (AFS-RSWGM). The material is formulated through the synergistic utilization of cement, fly ash, shield-sieved sand, and spoil soil, and incorporates alkali activation together with the pozzolanic effect of fly ash to enable full-component resource utilization of STS. This approach aims to reduce spoil handling costs, improve recycling efficiency, and enhance the performance of grouting materials. Key findings include:

(1) Based on cement, fly ash, shield-sieved sand, and shield tunnel spoil soil, and coupled with NaOH solution alkali activation and the fly ash pozzolanic effect, AFS-RSWGM was developed. Response Surface Methodology (RSM) was employed to optimize three key factors, namely the water–binder ratio (A), binder–sand ratio (B), and shield tunnel spoil soil (STSS)–water ratio (C). The construction adaptability and mechanical performance were evaluated using macroscopic indices such as compressive strength, fluidity, and bleeding rate. The results indicate that, after mix optimization, the 28-day compressive strength increased by approximately 32% compared with traditional cement-based grouting materials (TCGM), while fluidity increased and bleeding decreased, demonstrating superior overall adaptability and mechanical performance.

(2) The micro-mechanism underlying the performance enhancement of the optimized mix was investigated through multi-scale characterization, including rheological tests, hydration heat analysis, and SEM/XRD/FTIR. The results show that the optimized slurry exhibits a reduced yield stress and pronounced shear-thinning behavior. The hydration heat evolution displays a bimodal exothermic profile, with a significantly intensified second exothermic peak. Microstructural analyses reveal that alkali activation promotes the dissolution of aluminosilicate components in the spoil, producing abundant amorphous C-A-S-H gel and AFt. In addition, fly ash continuously supplies reactive SiO₂ and Al₂O₃, refining the interfacial transition zone (ITZ) and reducing porosity. Accordingly, a coherent evidence chain of "performance enhancement–structural evolution–reaction mechanism" is established.

(3) In the Jinan Metro Line 4 project, the left and right lines adopted TCGM and AFS-RSWGM, respectively, for synchronous grouting. A comparative analysis of settlement monitoring data verified the engineering effectiveness of the proposed material. The results demonstrate that AFS-RSWGM limited the maximum surface settlement to approximately 12.1 mm, representing a 68.6% reduction relative to conventional slurry, and achieved settlement stabilization 65 days earlier. Moreover, no issues such as segment dislocation, cracking, or grout leakage were observed during right-line construction, indicating a marked improvement in lining integrity. Based on the above engineering verification outcomes, construction risk identification and control were carried out, highlighting the transition from material optimization to intelligent construction management and control.