EFFECTIVENESS OF FIRST LEVEL STRUT IN REDUCING THE FOUNDATION PIT DEFORMATION INDUCED BY PRE-EXCAVATION PUMPING

Pumping inside an excavation is able to induce enclosure wall deflection towards the foundation pit, further leading to ground settlement outside the excavation. Generally, the enclosure wall deflections induced by pumping are significant before soil excavation, as the strut system inside the foundation pit cannot be fully completed and the enclosure wall is only supported laterally by the earth inside the excavation. In this paper, the effectiveness of the first level strut in reducing the foundation pit deformation induced by pre-excavation pumping was systematically studied. Firstly, the influence of the first level strut on the foundation pit deformation was investigated through pumping test in a strata system with strong hydraulic connectivity among aquifers. Then, a three-dimensional numerical model was established to study the influence of some typical parameters(e.g., the pumped depth) on the deformation control effect of the first level strut. Results show that the restraining effect of the first level strut on wall deflection is substantial only in shallow-buried position(within depth of 11 m in this paper), but not in deep-buried position; what is more, as the pumped depth increases, the deformation-restraining effect gradually weakens, indicating that for the projects with large pumping depth before soil excavation, only adopting the first level strut may not be able to effectively limit the development of the enclosure wall deflection. Additionally, the restraining effect of the first level strut on ground settlement could only work within a certain range behind the excavation(about 10 m in this paper); when there is hydraulic connection between the inside and outside of the excavation, the first level strut would have no restraining effect on the maximum ground settlement owing to the fact that the drop of water level outside the excavation is the main factor causing the ground settlement; on this occasion, the artificial recharge should be adopted to limit the ground settlement.