Application Study On Differential Settlement Control Of Pile-net Composite Foundation In Widening Of Expressway

The "13th Five-Year" development plan clearly proposes to promote the reconstruction and expansion of the national highway with an early construction period and heavy traffic.The problem of differential settlement of new and old roadbeds is inevitable when the expressway is renovated and expanded.How to control this differential settlement is the key issue in the process of design and construction of the expressway renewal and expansion.The composite foundation can play its own characteristics to improve the bearing capacity of the foundation,reduce the differential settlement of the new and old subgrades,and is widely used in the reconstruction and expansion of expressways.However,the differential settlement design and construction methods involved in the reconstruction and expansion of the expressway involved in the current specification are few,and the experience accumulated from the project has not formed a technical system.Therefore,this paper studies the differential settlement control during the highway reconstruction and expansion.Due to the different bearing characteristics of the composite foundation under different pile end bearing layer conditions and the lack of a large amount of test data in the calculation results when performing finite element analysis,this paper uses the self-processed model box to develop the pile end bearing layer as hard The indoor model test of the bearing characteristics of single pile under the condition of soil layer and soft soil layer,the difference between the bearing characteristics of suspension type and end bearing type single pile was compared and analyzed.The research results show that the bearing capacity of the pile is better when the rigidity of the bearing layer is greater or the rigidity of the pile body;The additional radial stress of the soil on the side of the pile is exerted,which causes the difference in the axial force and the frictional resistance on the side of the pile;considering the influence of the soil on the side of the pile and the soil at the pile end,the modified single pile axial force distribution function can be more accurate Reflect the change of single pile axial force.Next,based on the indoor model test of single pile bearing characteristics,FLAC 3D finite element analysis software was used to carry out numerical simulation analysis to verify the accuracy of the calculation model and the rationality of parameter selection.Limited by the test conditions and the selection of test parameters,the indoor model test failed to study the impact of more design parameters on the settlement of the composite foundation.Therefore,the single-factor analysis and orthogonal analysis method were used to analyze the factors influencing the settlement of composite foundation.The results show that pile spacing,pile diameter and pile length have the greatest impact on the settlement of the composite foundation.However,due to the influence of construction machinery and economics in engineering practice,the two factors of pile spacing and pile length should be given priority when designing the composite foundation.Finally,with Reconstruction and Expansion project of the Jiqing Expressway engineering background,based on the orthogonal analysis results of the influencing factors of the settlement of the composite foundation,the optimization scheme of composite foundation design was proposed with the settlement as the control condition.The optimization design scheme was used to analyze the treatment effect of composite foundation under different pile end bearing layer stiffness conditions using finite element software.The research results show that the optimized design scheme proposed with the settlement as the control condition can not only effectively control the settlement of the surface and the roadbed and the differential settlement of the new and old roadbeds,but also meet the maximum slope rate It is the requirement of differential settlement control standard.Through numerical simulation of the expressway reconstruction and expansion project under the condition that the pile end bearing layer is a soft soil layer,it is concluded that when the pile end bearing layer is a soft soil layer,the pile-net composite foundation and other soft soil foundation treatment methods should be adopted(Such as high-pressure rotary spray method,vacuum preloading,plastic plate drainage method and pre-throwing high technology)to deal with the conclusion of deep soft soil.