Monitoring And Analysis On Soldier Pile And Steel-reinforced Concrete Inner Bracing Support System

Soldier pile and steel-reinforced concrete inner bracing support system is widely used in the subway station foundation pit engineering with high requirements for deformation control.But the design theory of this support system is still not perfect.The force and deformation laws of this system during the whole construction process need to be further studied.In this paper,the combined methods of field monitoring,theoretical analysis and numerical calculation are used to analyze the force and deformation of soldier pile and steel-reinforced concrete inner bracing support system during the whole construction process,and the working behavior of soldier pile and steel-reinforced concrete inner bracing is studied.The bearing capacity of support structure is calculated,and the numerical analysis model is established by FLAC^3D Software.The single factor analysis and optimization analysis of support system are carried out.The main research contents and results are as follows.?1?In the whole process of construction,the axial force of inner bracing,the bending moment and horizontal displacement of soldier pile are monitored.The development laws of the force and deformation of support structure with the excavation of the foundation pit and the construction of main structure are analyzed.?2?The evolution process of the bending moment and horizontal displacement of soldier pile is analyzed.The results show that the inflection point of the pile body during the excavation of the foundation pit is located at the excavation surface of the foundation pit.The maximum bending moment value of soldier pile above the excavation surface is about twice than the embedded section.The horizontal displacement curve of soldier pile is approximately P-shaped.The maximum horizontal displacement occurs at about 1/2 of the excavation depth of the foundation pit.The force and deformation of soldier pile reach the maximum value in the whole process after the first inner bracing is removed,indicating that the inner bracing demolition stage may also be the control working condition in the design.It is recommended that the soldier pile and inner bracing support system should be closely analyzed for inner bracing dismantling and changing conditions.At the same time,the construction stage of main structure should maintain the same monitoring frequency as the foundation pit excavation stage.?3?The earth pressure acting on the support structure is monitored during the whole construction process.The results show that during the foundation pit excavation stage,the active earth pressure outside the pit shows a decreasing trend.In the construction stage of main structure,the active earth pressure outside the pit remains basically stable.Based on the comparison results of the static,active and monitored earth pressures,the simplified calculation formula of earth pressure coefficients under different excavation depths are summarized.The calculation results of the formula are compared with the monitored results to verify the feasibility of the calculation formula and provide reference for engineering with similar geological conditions.?4?The calculation model of the engineering example is established by LIZHENG Software.The incremental method calculation results of the force and deformation of soldier pile are compared with the monitored results.The results show that the measured position of the inflection point is about 3m higher than the incremental method.But the measured bending moment and horizontal displacement are basically enveloped in the calculated value.When using the incremental method to analyze soldier pile and steel-reinforced concrete inner bracing support system,the calculation results of the force and deformation of soldier pile have certain safety reserve.?5?The numerical model of the engineering example is established by FLAC^3D Software to simulate the force and deformation of support structure in the whole construction process,and the simulation results are compared with the monitored results.The applicability of the modeling method and parameter selection is verified,and conditions for subsequent optimization analysis are provided.?6?The FLAC^3D Software is used to analyze the influence of design parameters such as bracing form,pile diameter and horizontal spacing of steel inner bracing on the stress state of support system,and optimization design is based on this.The results show that with the pile diameter increases,the horizontal displacement of the middle part of the soldier pile decreases obviously,but the horizontal displacement of top part of soldier pile has almost no change.The change of the horizontal spacing of the steel inner bracing will influence the soldier pile near the bracing position and have less impact on other locations.Reasonable combinations of design parameters can be based on field conditions and design goals.