Analyses of deformation behaviors of subway excavations and frequent through-wall leaking hazards in water-rich sandy strata

To explore the deformation behaviors of subway excavations in thick water-rich sandy strata and the characteristics of frequent through-wall leaking hazards, the databases of excavation deformation and through-wall leaking events were first built to carry out comprehensive statistical analyses; hence, the deformation laws of retaining walls and ground surface behind the walls were obtained, and the key influencing factors were identified. Then, a novel method for classifying through-wall leakages during deep excavation in water-rich sand was proposed; afterwards, their primary triggering factors were revealed, and the effective measures were also summarized. The statistical analyses show that: (1) The maximum lateral wall deflection, δ_hm, varied from 0.03% H (H, excavation depth) to 0.48% H; the magnitudes of δ_hm were affected by the spatial-corner effect, dewatering mode, burial condition of wall toes, and wall exposure length. (2) The profile of ground settlements was a typical concave shape, and the maximum settlement, δ_vm, was 0.26%H_e (H_e, final excavation depth), which occurred at about 0.5H_e behind the walls. (3) Through-wall leaking caused sudden increments of δ_hm, mainly attributed to the rapid increment of total lateral earth pressure induced by seepage force and/or the inappropriate grouting pressure. (4) Most of the leakage-induced settlement increments, ∆δ_vl, were less than 0.5%H_l, and the primary influence zone could be extended to about 2H_l outside the pit, in which H_l denoted the burial depth of leaky spots on the wall; besides, the shape of strata disturbed behind the walls showed a distribution of “inverted cone”. (5) Leaking hazards can be classified into three levels: minor seepage without soil loss, massive leakage with slight soil loss, and massive inrush of water and soils. (6) Through-wall leakages can be effectively controlled by following the perspective of "disaster prevention before accidents and emergency response after accidents".