Study On Differential Settlement Control Technology Of Widened Clay Roadbed For Expressway Reconstruction And Expansion

The 14th Five-Year Plan puts forward the requirements of improving the quality and upgrading of national and provincial roads and the construction of bottleneck sections.Considering the objective requirements of saving economic investment,reducing the construction period and improving the carrying capacity of highway traffic volume,it is a very practical technical measure to reconstruct and expand the existing roads.According to the previous experience of highway widening,when the original roadbed is widened,the newly filled roadbed will interact with the old roadbed,resulting in differential settlement during the construction period and the post-construction operation stage.The prediction of differential settlement between the new and old roadbed and the optimization of treatment methods are the engineering keys to the problem of highway w idening.In this paper,the differential settlement control technology of new and old roadbed in reconstruction and expansion project is studied by using finite element numerical simulation.The main content and conclusions are as follows:(1)In this paper,by comparing the current settlement standards of major domestic expressways,the basic control standard of differential settlement of the old subgrade of this project is proposed.Based on this standard,the settlement and deformation characteristics of the new and old subgrade and foundation during the construction period and the later operation stage are studied by using numerical analysis method.In the construction stage,as the filling process progresses,the surface settlement of the new subgrade gradually increases,and the side of the old subgrade moves inward due to the load of the new subgrade,but the settlement of the construction period is generally small.In the post-construction operation period,due to the different degree of consolidation of the new and old foundations,the consolidation settlement of the old foundation is small while that of the new foundation is large,and the settlement of the old and new subgrade is different to some extent.The consolidation of the subgrade is basically completed after the post-construction operation for 15 years.(2)The adaptive capacity of the commonly used treatment measures for widening the subgrade splinting zone was studied,and the design and optimization of the excavation step size and exposure time as well as the laying level and number of the reinforcement treatment technology were carried out.The results show that too small or too large step size will increase the settlement,and the exposure time will affect the resilience of the excavation step,so as to affect the final settlement of the subgrade.The treatment effect of reinforcement at the bottom of the road surface or roadbed is better than that of reinforcement at the middle of the road surface.The more layers of reinforcement are laid,the smaller the settlement is.However,the treatment effect of reinforcement at the whole layer is not much better than that of the surface and the upper and lower layers of the road surface.(3)The deformation characteristics and change rules of highway widening project under different soft soil conditions are studied,and the influence changes of unfavorable factors on the roadbed,such as different soft soil types,thickness of soft soil and difference of soil quality between old and new roadbed,are analyzed.The results show that the consolidation rate of the three kinds of soft soil is higher than that of the soft silty clay than that of the silty clay.With the increase of the thickness of the soft soil layer,the settlement of the foundation increases.For the deep soft foundation with the thickness of more than 6m,a single excavation step or subgrade reinforcement treatment is not enough to eliminate the differential settlement of old and new subgrade to a safe level,and composite foundation treatment research is still needed.(4)The influence of treatment pile type,pile length and pile spacing on settlement characteristics of widened subgrade is studied.The differences between the treatment depth of prestressed pipe pile and cement mixing pile are compared and analyzed,and based on the calculation of the maximum treatment thickness of the two kinds of piles in this project,the optimization design parameters of pile length and pile spacing for composite foundation are proposed.The analysis shows that the treatment depth of prestressed pipe pile is higher than that of cement mixing pile.Prestressed pipe pile under the maximum thickness of soft soil treatment 12 m of the optimal parameters for the pile spacing of 3 m of pile,pile length 16 m,the thickness of cement mixing pile in soft soil treatment of its biggest optimal parameters of pile under 9 m to 2.5 m,pile length,pile spacing 21 m,other than the operation condition of the maximum thickness of soft soil treatment in security under the premise of appropriate to relax pile parameters.