Study On Design Strategy Of Support Forms In Foundation Pit Of Metro Station

This paper is based on three stations in Zhengzhou piedmont alluvial inclined plain area,which are Xuesong Road Station on Line 1,Building West Road Station on Line 5 and Longhai West Road Station on Line 5.Through the comparative study of incremental method calculation and on-site measurement,the internal force and deformation development characteristics of different internal support type corresponding support systems are summarized.Then,based on the field test results,the numerical analysis model and parameters are verified.Taking Longhai West Road Station as an example,this paper carries out numerical simulation analysis(using different support combinations)to analyze the deformation law of foundation pit excavation and the influence of different support combinations on foundation pit excavation.In this paper,the type A refers to the inner support of the steel pipe,and the type B refers to the inner support of the reinforced concrete.The main work and conclusions are as follows:(1)Combining the engineering geological conditions of Xuesong Road Station,Building West Road Station and Longhai West Road Station,the article views monitoring data in foundation pit excavation and enclosure structure calculation result by LiZheng Software.It is concluded that the maximum horizontal displacement,the maximum axial force of the support and the maximum settlement of the ground surface are proportional to the depth of the foundation pit;The horizontal displacement of the piles of the A-A-A support type and the B-A-A-A support type is “D” type,the maximum horizontal displacement of the B-A-A-A support type is obviously shifted downward,and the horizontal displacement of the B-A-B-A-A pile is multi-waveform.In the A-A-A system,the axial force of the first steel support is the most severe,and the peak value is about three times the valley value.The fall prevention measures should be paid attention to during the construction process.The axial force of the second steel support and the third steel support is generally stable,and the third steel support axial force is the smallest;the BABAA support type second steel inner support has the strongest axial force fluctuation and the greatest risk of falling during construction process.The third steel support axial force develops fastest,and the final axial force is the largest,which is roughly 5-8 times of the inner reinforced concrete inner support;The depth of the maximum horizontal displacement of the pile and the maximum axial force of the support increases gradually with the depth of the foundation pit,but the depth of the maximum displacement of the pile is 2~4m below the depth of the maximum axial force.The deeper the foundation pit is excavated,the bigger of influence of the surface settlement occur.After each steel support is installed,the axial force of the support above it will be significantly attenuated,and the attenuation is about 9%~17% of the original axial force.(2)Taking the Longhai West Road Station of Zhengzhou Metro Line 5 as an example,and using MIDAS-NX to Simulate construction excavation and support,the test comparative analyses monitoring data,LiZheng design values and numerical simulation data.The research shows that the horizontal displacement deformation laws of the piles are relatively close.In the third support erection position,the horizontal displacement of the piles is significantly reduced,indicating that the concrete supports and the retaining piles have better integrity.The horizontal displacement of the foundation pit has a greater effect;the variation law of the support axial force is relatively close,and the axial force of the first reinforced concrete support has been relatively stable during the excavation of the foundation pit,and the internal force value is much smaller than that of the third reinforced concreter support.The third steel support axial force developed fastest,and the final axial force reached the maximum..(3)On the basis of the above research,the bracing combination forms are reduced to B-A-A-A-A type internal bracing system and A-A-A-A-A type internal bracing system respectively.The design strategy of the support form is studied by comparing with the original support combination form for Longhai West Road Station,The research shows that in the B-A-B-A-A support scheme of Longhai West Road Station,the third concrete support can effectively reduce the horizontal displacement of the foundation pit and control the deformation of the foundation pit.Even if it is replaced by a steel support for significantly shortening the construction period and saving the cost,a larger diameter 800,16 mm steel pipe support should be used to meet the engineering safety needs.(4)Through the analysis of three different support combination stations in Zhengzhou piedmont alluvial inclined plain area,the design strategy for the support system of metro station in Zhengzhou B area is preliminarily obtained.