Wall deflection induced by pre-excavation dewatering in large-scale excavations

Field observation is carried out in a large-scale excavation to obtain the wall deflection induced by pre-excavation multiple well dewatering, and the maximum deflection of 15 mm, reaching 47.6% of the allowable wall deflection for the project, is observed. As for the contiguous pile wall, making dewatering wells installed at the retained side of the wall can reduce the wall deflection during dewatering. Then, a three-dimensional soil-fluid coupled finite element model is established, and 35 numerical calculations are carried out to study the effect of excavation width on wall deflection. The results indicate that the wider excavations exhibit larger wall deflections under the same dewatering time and dewatering depth. As to the metro excavation with the excavation width of 20~40 m, the dewatering-induced maximum wall deflection is sensitive to the variation of excavation width. However, with the increasing excavation width, being greater than 40 m, the maximum wall deflection has less difference among excavations with different widths. The overlying effect of soil deflection emerging at both sides of excavation axis can account for the above-mentioned wall deflection difference among different excavations. The narrower excavation will exhibit more apparent overlying effect, and then give rise to smaller wall deflection.