Study On Differential Settlement Mechanism And Connection Control Technology Of New And Old Subgrades In Soft Soil Area

With the growth of national transportation demand,there are more and more old highway reconstruction and expansion projects.The differential settlement of the old and new roadbeds is easy to cause pavement damage and roadbed collapse,resulting in differential settlement deformation disaster control.However,the differential settlement mechanism of new and old roadbed in soft soil area is not clear,and the connection control technology of new and old roadbed is still not perfect.Based on this,this paper adopts the research methods of theoretical analysis,numerical test and engineering practice to conduct in-depth study on the differential settlement characteristics of the new and old subgrades in the soft soil area of the reconstruction and expansion of the first-class highway G347 in Anjiu II,summarizes and expounds the differential settlement mechanism and calculation method of the new and old subgrades,and verifies the rationality of the numerical prediction model combined with the engineering case.It analyzes the influence of subgrade widening mode,width,height,filler performance,soft soil distribution and other factors on the differential settlement of the new and old subgrades,and finally discusses the improvement effect of bench excavation,geogrid laying and foundation treatment cohesion control technology on the differential settlement.The superiority of foundation treatment method under different soft soil distribution is explored.The main work of this paper is as follows :(1)The differential settlement mechanism of the old and new subgrades is clarified.Based on Boussinesq basic solution,the calculation formula of additional stress at any point in the foundation under the new subgrade load is derived.Three kinds of subgrade post-construction settlement prediction models,including hyperbolic model,exponential model and Logistics model,are summarized,and their calculation methods are introduced.(2)Through the comparative analysis of the measured data,theoretical calculation and prediction model results,the study shows that the prediction results of the numerical model are highly consistent with the field settlement curve,which verifies that the numerical model can better predict the post-construction settlement trend of the subgrade.(3)The research on the effect of different parameters on differential settlement by numerical method shows that bilateral widening can effectively reduce the differential settlement and lateral displacement of old and new subgrades.Increasing the width and height of widening will increase differential settlement and shear stress.Excessive width is prone to overall sliding instability failure,and excessive height is easy to cut into the foundation soil layer.Increasing the elastic modulus of the filler and reducing the filler weight can reduce the differential settlement of the subgrade and reduce the shear stress of the joint,and the influence of the weight on the settlement is more obvious.The shallower the soft soil depth is,the greater the thickness is,the greater the differential settlement and shear stress is.(4)Research on the improvement effect of cohesion control technology on differential settlement of subgrade shows that increasing step width and reducing step height can reduce differential settlement of subgrade and reduce internal shear stress of subgrade.Geogrid laying has little effect on the differential settlement of new and old subgrade,but it can improve the overall stability of subgrade.When there is no hard soil layer in the foundation,when the thickness of silt soft soil is less than 3 m,excavation and replacement should be selected.When the thickness is more than 3 m,pipe pile should be selected for soft soil foundation treatment.When there is a hard soil layer in the foundation,when the thickness of the hard soil layer is less than 10 m,the pipe pile treatment is the most suitable.When the thickness is greater than 10 m,the equal load preloading treatment can meet the needs.