Rheological characteristics and model applicability of shield tunnel backfilling grouts with supplementary cementitious materials

Rheological properties are essential in determining how effective grouting is during the shield tunneling process. Additionally, they form the basis for developing penetration and diffusion models for grout in the surrounding layers. Despite extensive research on the rheological properties of traditional cement-based grouts informing engineering practices, the specific influence patterns and mechanisms related to the rheological properties of grout mixtures containing three industrial solid wastes-fly ash (FA), silica fume (SF), and ground granulated blast furnace slag (GGBS)-as supplementary cementitious materials (SCMs) remain poorly understood. Therefore, this study conducted rheological tests on fresh grout by a new advanced rheometer MCR302e, varying the content of FA, SF, and GGBS. Results show that the modified Bingham and Herschel-Bulkley models proved more effective than the Bingham model in characterizing the rheological behavior of fresh grout. The modified Bingham model demonstrates superior applicability among these models, while the Herschel-Bulkley model presents better stability. The yield stress and plastic viscosity initially increase and then decrease with the addition of FA and GGBS, whereas, with SF, they show a decreasing trend followed by an increase. The thixotropy initially increases and then decreases with the addition of FA and GGBS while it continuously increases with the addition of SF. Furthermore, experimental results demonstrate that including 60% FA content alone results in the most significant improvement in the rheological properties of fresh grout. Compared to using just 60 % FA, the mix of FA and SF mostly increases the yield stress, while the mix of FA and GGBS mainly increases the plastic viscosity. For shield tunnel construction in Nanning, it is recommended to use more than 60% FA, over 2% SF, and no more than 20% GGBS.