Study On Frost Heave Mechanism Of Cement Graded Gravel Used In High-speed Railway Subgrade In Severe Cold Area

High-speed railway has developed rapidly in recent years,especially,the construction of"four vertical and four horizontal lines" as the main framework of high-speed railway is accelerating since the 18th CPC National Congress.The largest and highest running speed of high-speed railway network is gradually forming in China.More and more high-speed rail lines will pass through the cold areas with frozen soil distributed widely.The frost heave problem is very serious in these areas,leading to roadbed deformation and track uneven easily,which seriously affected the high-speed rail operation safety.Subgrade is the main source of frost heave deformation in rail structure,so it is of great significance to study the frost heaving performance and anti-frost heaving of cement stabilized graded gravel used in high-speed railway subgrade.This paper has researched the composition design,performance optimization and mechanism research of cement stabilized graded gravel used as high-speed railway subgrade material in the background of Harbin-Dalian and Harbin-Qiqihar railway lines.Firstly,based on the macroscopic mechanical properties and particle gradation theory,the gravel gradation of roadbed surface layer in high-speed railway had been scientific designed and experimental verified.Secondly,the basic mechanical properties,shrinkage performance and frost resistance property of cement stabilized graded gravel were characterized by ultrasonic in-situ detection,fiber Bragg grating(FBG),X-ray computed tomography(X-CT)and other technical methods.Lastly,the durability evaluation method and life prediction model of cement stabilized graded gravel used as subgrade material are established by the indicator of ultrasonic attenuation.The main conclusions of this research are listed as follow:(1)Based on the combination of CBR test and gradation theory calculation method,the gradation of roadbed gravel was designed accurately.The gradation of coarse aggregate(>4.75 mm)was determined by CBR test through stage filling method,while the gradation of fine aggregate(<4.75 mm)was obtained by theoretical calculation of maximum density curve.The design gradation divided the boundaries of the permeability and non-permeable graded gravel.The method of numerical simulation was used to verify the skeleton structure of the design gradation with good skeleton and high density are obtained(2)The microstructural evolution of cement stabilized graded gravel was studied based on the ultrasonic in-situ test.The early microstructure development of the roadbed material could be divided into three stages--dormant stage,acceleration stage and deceleration stage.The studies had shown that cement content,water-cement ratio,curing temperature and gravel gradation could affect the formation and development of microstructure in cement stabilized graded gravel.In particularly,controlling a reasonable cement content and keeping low water-cement ratio,high curing temperature or good material composition in construction process can improve structural stability and work performance of roadbed materials.(3)The mechanism of cement in graded gravel was researched by comparison with fly ash in ultrasonic test.It shown that filling effect was mainly happened at early stage while the cementation effect was mainly worked at later stage.The hydrated products of cement made the gravel bond to each other and filled the voids among gravel.The structure formation and development of subgrade material was the result of cement filling effect and cementation effect.(4)With the addition of cement in roadbed material,the strength,pore structure and anti-frost heave performance has been improved significantly,while the drying shrinkage constantly increased thus affecting the stability of the roadbed.The compressive strength was about 10 MPa which completely satisfied the requirement of the strength in subgrade when the cement content was 5%.The frost heave rate was less than 0.1%with 5%cement mixed in gravel,so the frost resistance performance has improved obviously.Based on the mechanical properties,drying shrinkage,frost heaving resistance and economic practicality,it was suggested that the cement content should be controlled about 5%in high-speed railway subgrade,which guiding the engineering applications scientifically.(5)The frost heaving process and influencing factors of cement stabilized graded gravel were studied by in-situ test of FBG.It shown that cement and water content had a great influence on the frost heaving resistance of roadbed material,for instance,lower water content was controlled could improve its durability.The X-CT test provided a microscopic point of view to analyzes the mechanism of cement for frost resistance improvement of subgrade material,revealing the reasons for the macro-frost heave of the roadbed.(6)The durability evaluation and life prediction model of cement stabilized graded gravel was established by the freeze-thaw cycle test.Ultrasonic non-destructive test could get the attenuation characteristic of ultrasonic pulse velocity(UPV)during freezing and thawing process.The quantitative relationship between UPV and the number of freeze-thaw cycles could be calculated by a large amount of test data.Life prediction model of railway roadbed material in cold areas was proposed and corrected by research of this paper,which had a higher accuracy and better applicability.