Research On Control Technology For Uneven Settlement Of Collapsible Loess Subgrade Under Complex Conditions

Based on the constructing Hebei section of Yanqing-Chongli Expressway,this paper focuses on control technology for uneven settlement of collapsible loess subgrade under complex conditions in the ZT5 test section.Through investigation and analysis of the causes of loess subgrade disease and collapsible loess engineering characteristics,indoor geotechnical experimental research was carried out,and the comprehensive treatment techniques are put forward,such as the reinforcement of foundation compaction,the compaction of embankment and the reinforcement of geogrid at the joint of filling and digging in order to control the uneven settlement of roadbed,based on the indoor geotechnical test,a finite element model was established to simulate and analyze the control effects of the uneven settlement of the embankment horizontally and vertically,and the control technology of uneven settlement of collapsible loess subgrade is further optimized.The main research contents and results obtained are as follows:(1)Through laboratory tests,the particle gradation,compaction characteristics,and limit moisture content of the loess filler and the foundation loess are analyzed to obtain physical indicators such as the curvature coefficient,uneven coefficient,optimal moisture content,maximum dry density,and liquid-plastic limit.The first is to determine whether the loess filler meets the requirements of the expressway for the roadbed filler.The second is to determine mechanical parameters such as cohesion,internal friction angle,and compressive modulus of the loess filler and foundation loess through direct shear tests and consolidation tests,and provide accurate parameters for finite element numerical simulation.(2)Using the finite element software PLAXIS to numerically simulate the uneven settlement of a high-filled embankment under the influence of different factors,we get: The uneven settlement of the cross section of the embankment increases due to the decrease in the compression modulus of the foundation material;c.When the slope ratio of the lateral slope of the foundation is 1: 4 to 1: 7,the unevensettlement of the cross section of the high-filled embankment follows the slope of the foundation The ratio decreases and increases.(3)For the reinforcement of the "V" gully slope foundation,dynamic compaction is used for disposal.From the theoretical and experimental aspects,the purpose is to determine the effective reinforcement depth,the layout and spacing of the compaction points,the number of compaction points and the The number of passes;for the uneven settlement control of the filling-excavation joint,the geogrid reinforcement technology is used to improve the performance of the embankment soil by using the friction and locking between the geogrid and the soil to control the uneven settlement.At the same time,the finite element software PLAXIS is used to simulate the uneven settlement of the embankment when the geogrid is laid at different layers under the same strength,so as to determine the optimal laying scheme.(4)Through the comparison of the field structural performance test data,the deflection values in the test section are less than the design deflection value of 120(0.01 mm),and the performance of the loess subgrade structure is good.At the same time,two types of deflection meters,PFWD and Beckman beam,were measured.The obtained graphs of the dynamic rebound modulus and the static rebound modulus are basically the same.At the same time,it can be proved that these two instruments can complete the work of subgrade rebound modulus detection.(5)Long-term monitoring of the uneven settlement of the embankment(after the completion of the filling)was found from the monitoring data: a.During the winter break,the settlement rate of the embankment was less than 0.7 mm/d;b.During the pavement construction,the embankment settlement rate was less than 0.188 mm/d;c.After the pavement construction is completed,the settlement rate of the embankment is less than 0.14 mm/d.This indicates that the pavement construction has entered the stabilization period of embankment settlement.Monitoring of the cross section and longitudinal section of the roadbed(after completion of the pavement construction)for 48 days revealed that the cumulative settlements at each point were less than 6mm.