Optimizing backfill grout to minimize shield tunnel segments uplift by considering time-dependent grout buoyancy

Large grout buoyancy can induce shield tunnel segment uplift during backfill grouting, resulting in segment misalignment, leakage, and damage. The conventional anti-flotation design methodology employs setting time as the optimization index, overlooking the time-dependent characteristics of buoyancy before the grout hardens. Considering the shape of the shield tail gap, this study presents the design of a testing apparatus capable of simulating grout buoyancy on the segment. Orthogonal experiments were conducted to examine the effects of grout mix proportions on buoyancy. To account for the time-varying behavior of buoyancy, the integrated buoyant force over time was calculated to evaluate its impact on the tunnel segments. The test results indicate that the factors influencing grout buoyancy, in order of significance, are bentonite–water, water–binder, binder–sand, and cement–fly ash ratios. The interaction mechanism of two factors on buoyancy is revealed. The fluidity and consistency of the backfill grout are more appropriate for characterizing the magnitude of buoyancy, rather than the setting time. This study enhances the understanding of the time-dependent characteristics of grout buoyancy and guides optimizing anti-float grout in practical grouting applications.