Spatial Analysis Of Key Nodes For Shallow Underground Excavation

Pile-Beam-Arch method for shallow underground excavation,with the advantages of strong adaptability,such as little impact on the ground,and no need for a large number of demolition,has been widely used in the construction of subway stations in China.The settlement law of the earth surface,the mechanical effect of the side pile and the key node are the keys in the research of the cave-pile technique.Therefore,based on an engineering case,in this paper,two models of plane and space was firstly established through numerical simulation,to conclude the settlement law of surface and the mechanical characteristics of side piles during excavation;then the surface settlement data was analyzed by means of mathematical statistics,to study the tunnel subsidence characteristics for setting up the settlement prediction model suitable for the research area;finally,with the data from the finite element software,the key nodes lack of whole integrity in the structure were analyzed to find out the most unfavorable construction procedure.The main research results of this paper are as follows:(1)The conventional load-structure method was used to analyze the subway station to calculate axial force,shearing force and bending moment;furthermore,it was further optimized aiming at problems in load-structure method,establishing stratum-load plane model to calculate the ground surface settlement.The results showed that in the period of excavating pilot tunnel,the ground settlement was the largest,accounting for 70%of the total settlement.(2)The spatial model was established to conclude the influence of longitudinal excavation on subway station,and the influence of pilot tunnel excavation in different excavation orders on the surface was studied.The results showed that the influence of ground surface settlement was minimal if the upper pilot tunnel was excavated firstly.(3)The feasibility of Peck regression prediction method was assured by using the regression analysis method to fit the measured ground surface settlement data.Through mathematical statistics,settlement correction factor was introduced to obtain the value range of settlement correction factor,to improve prediction accuracy of the corrected Peck prediction model for ground surface settlement.The results showed that when the maximum surface settlement correction coefficient(?)of the tunnel center line was within the range of 0.6?0.9,and the correction coefficient of the surface settlement trough width was within the range of 1?1.19,the revised data was more accurate.(4)For the special joint support system formed in the process of the construction with the cave-pile technique,the numerical simulation method was used to analyze the pile and arch synergy and force conversion,and also the safety of the retaining pile was verified.The results showed that the load borne by the retaining pile was greater than that by the arch structure and mainly bore the axial force;with the progress of construction,the peak axial force moved downward with the increasing depth of excavated soil.(5)The stress value of the connection point between the top arch and the top beam and the horizontal displacement of the connection point between the middle column and the top longitudinal beam under different construction orders were analyzed,to find out the most unfavorable construction order in the construction process,and also the change rules of stress and displacement under different construction orders were analyzed.The results showed that the internal force generated with the excavation method of "side excavation first and center excavation then" was smallest;the maximum stress variation occurred at the stage when the primary branch of the pilot hole was broken and the Buckle arch construction was started;for the horizontal displacement of connection point between the middle column and the top longitudinal beam,middle column was shifted to one side with the construction stage,but it became smaller obviously in the edge construction.The above research results can provide theoretical support for the design and construction with cave-pile technique.