Research On The Application Of Combined Support Structure Of Row Pile And Inclined Brace In Foundation Pit

As we all know,with the continuous development of China’s construction industry and population growth,people’s current demand is not only for high-rise buildings on the ground,but also for deeper and deeper underground projects.This means that the design and construction difficulty of deep foundation pit project is increasing,and the traditional pile-anchor support system can no longer meet today’s demand,while the combined support structure of row pile and diagonal brace is more and more popular among engineers because of its flexible arrangement,not restricted by the surrounding buildings and underground municipal pipelines,and does not disturb the surrounding soil during construction,and also has the advantages of facilitating earth excavation and increasing construction space.It is more and more favored by the engineers.In the construction of combined pile and diagonal bracing structure,the diagonal bracing is generally supported on the foundation slab or a certain depth of bracing pile is driven into the bottom of the foundation pit as a support point,and the embedment depth of the bracing pile,the spacing of the diagonal bracing and the inclination angle of the diagonal bracing have great influence on the deformation of the foundation pit,especially the embedment depth of the bracing pile has a greater influence.However,there is no accurate reference for the design specification of the embedment depth of the support pile,which leads to the previous design examples of this type of support structure are taken according to experience,and when the design scheme of this type of support structure is verified in the software of Rizheng and Tongji Qixing,the support pile in the support structure is regarded as immovable pile,while in the actual project,the support pile will be generated with the excavation of the foundation pit and Therefore,the design calculation of the working condition will not match with the actual working condition.This paper takes the above problem as the starting point,combines with engineering examples,and takes the combined support structure of row pile and inclined support as the main object of research,and carries out a series of analysis and research on it,and the work and results carried out in this paper are as follows:(1)Based on the elastic foundation beam theory,the synergistic effect between"row pile+diagonal brace+support pile"is considered,i.e.,the displacement of row pile and support pile and the compression deformation of diagonal brace are considered,and the theoretical solution of the combined support structure of single row pile and diagonal brace is derived with the corresponding boundary conditions,and then the theoretical solution is carried out for two cases based on the theoretical solution.The theoretical solution is then used to calculate two cases.The first example is to analyze the effect of whether the support pile is regarded as movable pile or not on the displacement of the row pile through theoretical calculation,and the maximum displacement of the row pile when the support pile is regarded as movable pile is about 30%larger than that when the support pile is regarded as immovable pile.The second calculation example is to calculate the displacement of the row pile when the support pile is regarded as immovable pile by Qixing software,and the displacement of the row pile when the support pile is regarded as movable pile by theoretical calculation,and the maximum displacement of the row pile when the support pile is regarded as movable pile is about 44.1%larger than that when the support pile is regarded as immovable pile after comparison.(2)Through the finite element software Midas GTS NX,the numerical simulation analysis of the combined support structure of single-row pile and diagonal brace and double-row pile and diagonal brace in the actual project was carried out,and after the study of the law of the displacement results of the row pile,it was concluded that the displacement results of the row pile calculated in two dimensions would be large,and the maximum deviation of which reached about 36%of the three-dimensional results,and it was also learned that the establishment of three-dimensional At the same time,it is learned that it is very necessary to establish a three-dimensional model to simulate the analysis,and the results will be more reasonable than the two-dimensional simulation results.(3)Taking the combined support structure of single-row piles and diagonal bracing as the main object of analysis,Midas GTS NX software was used to study the change of different factors on the deformation of the foundation pit and the force law of diagonal bracing.Under the condition that the soil parameters and other parameters are not changed,the embedded depth of support pile(10m,12m,14m),inclination angle of diagonal brace(20~o,30~o,40~o)and spacing of diagonal brace(6m,8m,10m)are changed respectively,and the results of numerical simulation and analysis show that:When designing the combined support structure of row pile and diagonal brace in foundation pit project,the embedded depth of support pile can be appropriately increased,and the monitoring range also needs to be appropriately expanded outward.At the same time,according to analogy,when the embedded depth of the support pile increases to about twice the excavation depth of the foundation pit,the maximum displacement of the row pile will no longer change,and it is known whether the support pile is embedded in the weathered rock and soil layer,there is no obvious inhibiting effect on the maximum displacement of the row pile.When designing this type of support structure in the foundation pit,a small attempt should be made to increase the inclination angle of the inclined brace,otherwise the deformation of the foundation pit will increase abruptly.The spacing of diagonal bracing has a great influence on the deformation of the pit and the force of diagonal bracing,and from the safety point of view,the spacing of diagonal bracing in the simulation should not exceed 8 m.At the same time,it is learned that when using the combination of pile and diagonal bracing support structure in the pit project,it is very necessary to design the spacing of diagonal bracing carefully,and it is very critical to choose the appropriate spacing for the stability of the structure,and while increasing the spacing of diagonal bracing,it should be considered to increase the The stiffness of the diagonal brace should be considered.After studying the effect of whether the support piles are considered as movable piles on the bending moment of the row piles,it is found that the absolute value of the maximum bending moment of the row piles will be larger when the support piles are considered as immovable piles than when the support piles are considered as movable piles.The maximum negative bending moment difference value is about 7.6 times of the calculated value when the support pile is considered as immovable pile,while the maximum positive bending moment difference value is about 22.6%of the calculated value when the support pile is considered as immovable pile.(4)Through the finite element software Midas GTS NX,the overall numerical simulation of the deep foundation pit of Nanchang Wanxiangcheng Complex Project was carried out,and the deformation of the foundation pit and the force of the diagonal brace were obtained.In the area of the pit with single-row pile and diagonal bracing,the single-row pile with the largest displacement occurs in the southwest direction of the pit,about 4m away from the intersection with other supporting structures;the construction of the supporting pile and diagonal bracing can effectively inhibit the displacement of the row pile by continuing to tilt.Combining the numerical simulation results,the actual monitoring data and the theoretical solution results calculated in Chapter 2,the corresponding surface settlement and the horizontal displacement of the row pile into the pit were obtained,and after analysis,it was learned that due to the complexity of the construction process and site conditions,the theoretical calculation and the numerical simulation may have deviations from the actual monitoring at the site,but the results of the numerical simulation and the theoretical calculation provide a However,the results of numerical simulation and theoretical calculation provide a very valuable reference role for the construction on site.