Study On Creep Characteristics Of Soft Clay And Methods For Reducing Post-construction Settlement Of Expressway

With the progression of the "Belt and Road" strategy,the development of China’s transportation industry has entered a new pattern.With the rapid development of expressway,vehicle bumping at bridge head has not been effectively solved.A large amount of deep soft clay is distributed in Zhejiang province.These soft clay usually has the characteristics of high water content,high compressibility,low permeability,low shear strength,and obvious creep characteristics,which is one of the main reasons for bridge head jump.The method of drainage consolidation and light embankment can change the soil from normal consolidation state to over-consolidation state and greatly reduce the creep coefficient of the soil,so the secondary consolidation settlement after construction can be reduced by using this method.Based on the project "Secondary consolidation characteristics of Soft clay in Zhejiang and Reduction of post-consolidation settlement" issued by Zhejiang Transportation Planning and Design Institute,the creep characteristics of soft clay,practical calculation method of post-consolidation settlement and replacement design of light embankment in Zhejiang province are systematically studied in this paper.The main research work and results are as follows:(1)One-dimensional secondary consolidation(creep)and conventional geotechnical laboratory tests were carried out on normally consolidation soft clays in Zones I1,I2,I3,I4 and I5 of Zhejiang Province.Soft clays from Wenzhou,Shaoxing,Huzhou,Taizhou and Ningbo were selected for study.The correlations between the secondary consolidation(creep)coefficient of normally consolidated clay and soil sample type,consolidation pressure,depth,water content,liquid index,plastic index,compression index and other factors is analyzed,and the value range of secondary consolidation(creep)coefficient of soft clay in each area is given,which provides a certain reference for engineering design.(2)The creep test of over-consolidation soil was carried out.Based on onedimensional Yin-Graham EVP theory,a method to determine the creep coefficient of over-consolidation soil was proposed.The influence of over-consolidation ratio of weakly over-consolidation soil on creep coefficient of soft clay and time effect of creep coefficient are studied.It is considered that the creep coefficient of weakly overconsolidation soil decreases with the increase of over-consolidation ratio.The relationship between creep coefficient and over-consolidation ratio can be fitted by exponential function.The creep coefficient of over-consolidation soil decreases first and then increases with the development of time.Therefore,it is suggested to use the creep coefficient of 7 days in the laboratory to calculate the post-construction consolidation settlement in practical engineering.(3)The macro and micro tests of soil deformation under one-dimensional dynamic load were carried out.SEM and NMR were used to explain the deformation mechanism.The deformation of soil under dynamic load decreases with the increase of overconsolidation ratio.The structure of soft clay will be destroyed under dynamic load.Compared with the creep deformation under static load,the deformation under dynamic load can not be ignored for the shallow overconsolidated soil of subgrade.Microscopically,after the action of dynamic load,the large pores are compressed and compacted,and the structure becomes more dense.The fragmentation of large particles and the aggregation of small particles occur simultaneously,and the aggregate structure of soil particles is flake Mosaic.The larger the over-consolidation ratio is,the more ordered the pores are.(4)According to the EVP model and the actual situation of the project,a practical method was proposed to calculate the post-construction consolidation settlement by using the creep coefficient of the over-consolidation soil determined by the test and the engineering monitoring data.The feasibility and accuracy of the method was verified by comparing with the actual engineering data.(5)According to the practical calculation method of post-construction settlement and creep characteristics of over-consolidation soil,the specific design process of replacement thickness of lightweight embankment is put forward,and the determination method of replacement time is put forward.