Multi-Case Analysis and Prediction Formulas for Deformation of Underlying Tunnels Caused by Foundation Excavation

Based on the statistics on 32 cases of actual underground engineering projects in China where the foundation was excavated above existing tunnels, according to the composition of site soils, they could be divided into two types of strata: fine-grained soil containing sand and fine-grained soil containing gravel. Sorting and summarizing the relevant information including the plane sizes of excavated areas, the relative position and conventional control measures, etc., the main factors induced the maximum uplift displacement of tunnels were analyzed. On the basis, the prediction formula for the maximum uplift displacement of tunnels caused by deep excavation in the strata of fine-grained soil containing sand and fine-grained soil containing gravel were proposed, which were compared and analyzed with other prediction formula and actual engineering cases for verification. The results indicated that the larger the excavated area <italic>A</italic>, unloading ratio <italic>R</italic> and the shape factor of excavated areas <italic>α</italic>, and the longer the longth of tunnels passing under excavated areas <italic>L</italic>_t, the larger the maximum uplift displacement of tunnels caused by deep excavation <italic>w</italic>_max was, but the correlation between a single influencing factor and the maximum uplift displacement of tunnels was not strong. Thus, it was necessary to comprehensively consider the main influencing factors to scientifically predict tunnel deformation caused by foundation excavation. Eventually, <italic>αR</italic>(lg <italic>A</italic>+ln <italic>L</italic>_t) was used as the overall influence factor to characterize the maximum uplift displacement of tunnels induced by deep excavation of foundation, and it was found that wmax showed a good linear relation with <italic>αR</italic>(lg <italic>A</italic>+ln <italic>L</italic>_t).