Research On Deformation Characteristics And Retaining System Performances Of Underwater Excavation For Subway Deep Foundation

The actual needs of Beijing and the the shortage of urban land have caused the gradual increase in the buried depth of subway stations,and some of the main stations have entered the confined water stratum.Controlling groundwater effectivelyhas become an urgent problem in deep excavations of subway stations.Underwater excavation,with the advantages of high construction efficiency and wide application range,is one of the common construction methods in the construction of deep water pier foundations for hydraulic buildings,bridges,ect.However,there are few researches on this excavation technology in the field of domestic and foreign geotechnical engineering,which limits its application andpopularization to a great extent..In the Yongding Men Station project of Beijing Line 8,the underwater excavation method was firstly introduced to the deep foundation pit project,and the combined structure of excavation support and water stop suitable for the underwater excavation method were developed.Due to the lack of research results on the underwater excavation,this paper studied the deformation characteristics of the foundation pit during the underwater excavation process,the working behavior and optimization design of the retaining structure,structural strength of key nodes and other issues based on the proposed excavation project.The specific content includes:1.Based on the field pumping test,the feasibility of underwater excavation technology and the "hanging curtain-concrete bottom sealing" water-stop structure was demonstrated.The field measurement results of the proposed excavation were calculated,the rules of surface settlement and retaining deformation under underwater excavation conditions were summarized.The good control effect of the underwater excavation method on the excavation was verified.Based on the measured data of the lateral wall deflection,the static balance method was used to inversely obtain the horizontal resistance coefficient proportional coefficient value of each soil layer.2.A three-dimensional numerical model for underwater excavation based on the H-S(hardened soil)constitutive was established,and the general characteristics of deep excavation deformation of underwater excavation method were obtained.Meanwhile,the influence of the key design parameters on the lateral wall deflection in the process of underwater excavation was studied,and the optimal critical values of the water level recharge height,the embedded depth of the diaphragm wall and the cross wall,the interval and the arrangement of the cross wall were obtained.3.A two-dimensional finite element model of modified elastic beam(MEFB model)was proposed for deflection calculation of diaphragm wall with cross wall.The model was used to modify the stiffness coefficient k_h of passive earth spring in Winkler elastic foundation model,and quantify the restraining effect of the cross wall on the lateral wall deflection from the mechanical relationship.4.Based on the basic principle of the finite element strength reduction method,the critical reinforcement thickness of the excavation under the plastic zone penetration criterion and the displacement mutation criterion were calculated respectively,and the fitting relationship between the reinforcement thickness and the safety factor of the excavation was obtained,which was mutually verified with the existing back cover thickness calculation formula.Under the same safety factor,the deflection and mechanical characteristics of three anti-floating structures were compared and analyzed,including the weighted bottom plate,the bottom plate with anti-floating pile,and the bottom plate with cross wall.5.The experimental study on shear behaviour of concrete rectangular key-joints were carried out.The crack development,the shear capacity and failure pattern of different specimens were obtained.According to the constitutive relation of concrete damage(CDP),several numerical models based on the test specimens were established,and the effects of various calculation parameters on the shear strength characteristics of concrete rectangular key-joints were studied.Based on the failure pattern of the test specimen and the sensitivity parameters of numerical calculation,referring to the recommended formula of shear bearing capacity of existing concrete joints,the shear capacity calculation formula of concrete rectangular key-joints was established by introducing the coefficient of multi-key coworkly and the stress gradient influence.