Reuse of Excavated Clayey Silt in Cement-Fly Ash-Bentonite Hybrid Back-fill Grouting during Shield Tunneling

Excavated soils from tunnel construction need high treatment cost and pollute the environment. To investigate the feasibility of excavated clayey silt reused in back-fill grout, the flowability, stability and strength were taken as measurement indexes of grouting performance. The clayey silt was tested to be reused as substitutes for fly ash, bentonite and sand, respectively. The experimental results indicated that the clayey silt reused as a substitute for fly ash decreased the flowability and strength of grout mixes, and the clayey silt reused as a substitute for bentonite decreased the stability of grout mixes, and neither of them was feasible. The clayey silt reused as a substitute for sand decreased the flowability, but the grouting performance could be improved by adjusting the mix proportion to meet all grouting requirements. After adding the proportion of water to improve the flowability and increasing the cement:fly ash ratio to improve the strength, a scheme of clayey silt reutilization was suggested, which was cement:fly ash:bentonite:clayey silt:water 280:230:100:680:660. At the end of this paper, the pore structure feature tests, X-ray diffraction (XRD) tests and scanning electron microscope (SEM) tests were performed to analyze the different morphology, microstructure and mineralogy characteristics before and after the clayey silt was reused as a total substitute for sand in grout mixes.