The Research On Mechanical Characteristics Of Rock-socketed Retaining Structure And Construction Technology For Anchor Pit

Anchor pit appears during the constrution of span bridge the depth and plan sizeof which exceed that of ordinary pits significantly. There are some problems in thedesign of rock-socketed retaining and construction safety for anchor pit.Load-bearing and deformation character of rock-socketed diaphragm isgeneralized from in-situ data of typical pits. It is different from conventionaldiaphragm that rock-socketed diaphragm resist negative moment on bedrock whichcontrols the design because of the restraint function of bedrock. The influence ofdifferent thickness and rock-socketed length on load-bearing character is given. Theresults show that thickening diaphragm can decrease deformation but increaseinternal force greatly, especially the negative moment. Proper rock-socketed length isbeneficial to stabilization of retaining structure and decrease deformation.According to mechanical character of rock-socketed diaphragm, new designmethod of normal section is presented in which the gravity of support system andrestraint function of bedrock are considered. Compared with traditional method, therebar ratio of diaphragm decreases obviously and plastic rotation of diaphragmsection near bedrock is guaranteed. This new design method is confirmed byexperiment and carried out in north anchor pit of Runyang bridge. Based on theappropriate material constitutive equation, failure criterion and model of cracking, thenumerical analysis is used to simulate experiment. The results justify the measureddata of experiment and compare the influent of elastic modulus of different bedrock,so proper rock-socketed length is presented.Elastic foundation beam method is widely used in the design of retainingstructure for foundation pit. Traditionally, the "m" of soil is determined by empiricalequation which does not consider the dynamic factors during excavation and archaction of round pit. Grounded on optimum method, the back analysis is used tocalculate the "m" of soils, which then are applied to predict the displacement of theretaining structure in next step and describe the variation of "m" during excavation. This method was carded out during the construction of north anchor of Runyangbridge; Based on the bending theory of shells, differential equation of the elasticsubgrade beam is derived which consider the arc action of retaining structure.Because of the influence of joint between diaphragm panels, the reduction factor ofring stiffness is presented through a typical round anchor pit.The excavation process of the south anchor pit of Huangpu bridge is simulatedby using spatial nonlinear FEM in which the interaction between soil and structure isconsidered. The stress and deformation of the support system during constructionprocess are performed and justified by measured value. The results show that thecylindrical retaining structure subjected mainly to the axial compressive anddeformation rule is different from that of strip one. Maximum lateral displacementdepth grows with excavation, and stops until it reaches a certain depth. So thetraditional design method which strength section of brace is not suited for therock-socketed cylindrical retaining structure and the optimal design is needed.Through investigation in several typical anchor pits, the packaged technology ofrock-socketed diaphragm is generalized, including panel construction, cut off water,observation system and joint between panels. Combined with practical engineering,the optimal method during construction is given.Lastly, author summarizes work of the paper and presents the further researchdirections.