Study On Differential Settlement Characteristics And Control Standards Of Subgrade Widening Of Yan'an North Transit Line

The 13 th Five-Year Development Plan outline puts forward the requirements of promoting the upgrading of national provincial highway quality and the construction of bottleneck sections.On the basis of the old roads,widening and renovation are good engineering measures to alleviate traffic pressure,save construction investment and shorten construction cycle.The experience of existing road reconstruction and expansion projects shows that improper treatment of the combination of new and old roadbed and unreasonable treatment of widening foundation will lead to excessive differential settlement between new and old roadbed,resulting in longitudinal cracks of pavement,seriously affecting the normal use of highway.The reason is that the analysis of differential settlement characteristics of roadbed widening and the problem of control standards have not been reasonably solved,resulting in foundation.There is no basis for the way and degree of treatment.In view of the above problems,relying on the reconstruction project of Yan'an North Transit Line,this paper uses the combination of theoretical analysis,numerical simulation and field test section monitoring to analyze the differential settlement characteristics of roadbed widening,differential settlement control standards of roadbed widening and the rationality of wet and soft foundation treatment methods.Firstly,the site monitoring scheme of monitoring section relying on engineering is introduced in detail,and the laboratory test of shear strength of widened roadbed filling is carried out.Then,based on the practice of Yan'an North Transit Line Widening Project,the FLAC 3D calculation model of unilateral widened roadbed is established.The influence of roadbed widening height,roadbed widening width,excavation dimension of old roadbed slope,setting of roadbed retaining wall on differential settlement between new and old roadbed is discussed.The new foundation modulus,new roadbed modulus and new roadbed heaviness are analyzed by means of orthogonal test design.The influence degree of foundation differential settlement and horizontal displacement.Thirdly,the calculation model of old and new subgrade,new and old subgrade and pavement structure as a whole is established.By changing the mechanical parameters of the subgrade,different differential settlement values are obtained on the top surface of the new and old subgrade to simulate the post-construction settlement of the new and old subgrade in the operation process,and then the additional stress response of each structural layer of the pavement is analyzed.Starting from the structural requirements of pavement,when only considering the ultimate tensile stress of pavement materials,the maximum allowable differential settlement of widened side pavement is 46.1 mm and the corresponding allowable slope change rate is 0.44%.When only considering the fatigue effect of pavement materials,the maximum allowable differential settlement of widened side pavement is 16.3 mm and the corresponding allowable slope change rate is 0.15%.The ultimate tensile stress and fatigue of pavement materials are comprehensively considered.When the effects are combined,the maximum allowable differential settlement of widened side pavement is 28 mm and the corresponding allowable slope change rate is 0.266%.Finally,the control standard of differential settlement is divided into four grades.Finally,monitoring and data analysis of cement-soil mixing pile and riprap silt squeezing treatment for wet and soft foundation widening section for one year are carried out,and the engineering effects of these two foundation treatment methods are analyzed and evaluated.Based on the differential settlement control standard of widened side pavement,the optimum design of cement-soil mixing pile treating wet and soft foundation is carried out.