Research On Stability Characteristics Of Deep Foundation Pit Support Of Underground Diaphragm Wall Anchored In Water-rich Rock Formation

The development of long-span suspension bridges has an increasing demand for large anchorage foundations.As the size of the anchorage deep foundation pit increases,the construction process has a more significant impact on the surrounding environment.The circular underground diaphragm wall supporting system utilizes the circular arch effect,which can convert the lateral pressure of soil and water of the supporting structure into the internal axial pressure and reduce the shear force and bending moment,so that the compressive properties of concrete can be brought into full play.Due to its obvious advantages in space force characteristics,it has been gradually applied to the anchorage engineering of extra-large suspension bridges.However,at present,there are not many research cases on the deformation behavior and influencing factors of the deep foundation pit of the underground diaphragm wall in the water-rich rock formation.In particular,it is necessary to consider the influence of seepage on the deformation of the foundation pit enclosure and surrounding strata,and it is necessary to carry out targeted research on it.In this paper,taking the deep foundation pit of the underground diaphragm wall of the Longtan Yangtze River Bridge as an engineering example,the numerical simulation of the foundation pit seepage and the pumping test are carried out to explore the influence of the waterrich rock layer on the stability of the foundation pit.The whole process of foundation pit excavation construction is analyzed and simulated;based on the field test of the bearing capacity of the weakly cemented gravel-bearing sandstone foundation and the interface friction mechanics parameters,the depth correction coefficient of the bearing capacity of the weakly cemented gravel-bearing sandstone foundation is studied.The main work and results are as follows :(1)The whole process monitoring of anchorage foundation pit excavation was carried out.The main monitoring contents include the monitoring of water level outside the pit,the observation of soil settlement around the foundation pit,the observation of the settlement of buildings around the foundation pit,the observation of the deep horizontal displacement of the diaphragm wall,the observation of the vertical and horizontal displacement of the top of the diaphragm wall,the observation of the supporting structure(including the underground Diaphragm wall and lining)internal force monitoring.Combined with the finite element numerical simulation results and the actual situation in the field,the structural deformation characteristics of the circular underground diaphragm wall enclosure with weakly cemented gravel-bearing sandstone as the base bearing layer and the influence of foundation pit excavation on the surrounding environment are systematically analyzed.(2)Numerical simulation of pumping and on-site pumping tests were carried out to determine the support permeability of underground diaphragm walls in water-rich rock formations.Three hypothetical seepage locations are expected,and the commercial software Seep/w is used to numerically simulate the possible groundwater seepage,and build corresponding seepage models for analysis.Through the on-site pumping experiment,the underground diaphragm wall is divided into 8 sections and monitored at the same time,and the water-stop effect at the bottom of the underground diaphragm wall is tested one by one.Combined with the analysis results of the groundwater seepage numerical model,it is found that the water-stopping effect at the bottom of the underground diaphragm wall is poor,and the water-stopping effect at the joints of the underground diaphragm wall is relatively good.(3)The numerical simulation analysis of the deep foundation pit excavation of the circular underground diaphragm wall of the anchorage was carried out.Using Plaxis3 D finite element software,a finite element numerical simulation model was established,and combined with the actual construction conditions,the excavation of the deep foundation pit of the circular underground diaphragm wall of the anchorage was analyzed.Combined with the field monitoring results,the results show that the maximum horizontal displacement of the deep underground diaphragm wall and its location,and the maximum settlement of surrounding soil calculated by the Plaxis3 D finite element numerical simulation analysis are in good agreement with the monitoring data.The parameter sensitivity analysis was carried out according to the grey relational degree theory.Among the single influencing factors,the internal friction angle is more sensitive to the influence of the surface settlement around the foundation pit.(4)The in-situ load plate test and direct shear test of the anchorage base were carried out.The mechanical properties of the weakly cemented gravel-bearing sandstone bedrock surface were studied by in-situ in-situ tests,including the in-situ direct shear test of the basement and the shallow load plate test,and the foundation bearing capacity and friction coefficient of the weakly cemented gravel-bearing sandstone were obtained.In addition,according to the comparison between the deep load plate test results before excavation of the foundation pit and the shallow load plate test results after excavation,the depth correction coefficient of the bearing capacity of the weakly cemented gravel-bearing sandstone foundation is obtained.