Calculation of the control effect of portal anti-floating frame on the uplift of tunnel caused by pit excavation

Purpose The excavation of subterranean structures, such as foundation pits, has the potential to induce an upward displacement or “uplift” of adjacent shield tunnels. To mitigate this uplift, the implementation of the portal anti-floating frame (PAF) has demonstrated efficacy, yet the theoretical underpinnings of its operation remain underexplored in current academic discourse. Design/methodology/approach Initially, an image source methodology is employed to ascertain the supplementary stress imposed upon the tunnel due to the pit’s bottom uplift, engendered by the foundational excavation. This stress is then compounded with the additional stress exerted on the tunnel, which emanates from the interactive dynamics between the anti-floating slab and the surrounding soil, as well as the interaction between the slab and the uplift piles. Subsequently, a rotational and dislocation coordination model is applied to quantify the resultant deformation of the tunnel. In calculating the lateral frictional resistance offered by the uplift piles, the Mindlin solution is invoked to derive the deformation of the tunnel attributable to this resistance. Findings The theoretical values procured from the proposed method are subjected to comparative analysis with empirical measurements, simulation outputs and other extant computational approaches. The congruence between the calculated and measured values substantiates the validity of the proposed analytical method. Originality/value The findings elucidate that the anti-floating slab is the primary determinant in mitigating tunnel uplift, whereas the uplift piles serve a supplementary function. Moreover, the study reveals a decline in the control efficacy of the PAF concomitant with increased depths of the foundation pit. Therefore, for deep foundation pits, it is recommended to enhance the effectiveness of the anti-floating frame by augmenting the thickness of the anti-floating slab or by extending the length of the uplift piles.