Research On Frost Heave In Subgrade Of High-speed Railway In Cold Regions And Its Engineering Countermeasures

The Harbin-Dalian High-speed Railway(HDHR), acrossing the Northeast China, started construction in July28th,2007, and opened for operation in December1st,2012. It is the first new-built and operating high-speed railway in cold regions around the world. In order to ensure safety and smooth running of high speed multiple units, it is the key and urgent technical problems that the stringent using reasonable measures to effectively limit subgrade frost heave within allowable range.Although there have solved the problem of subgrade frost prevention by various methods in HDHR, but because of the special geographical environment, the deformations of subgrade frost heave are not completely eliminated. Under the action of impact load at high speed and seasonal freeze-thaw process, the frost heave deformation as the main performance of subgrade deformation become the major problems that affect line stability and driving safety. Therefore, through researching on subgrade frost heave deformation and engineering countermeasures, there would provide the basis and technical support in HDHR to ensure the safe and reliable operation, to improve engineering measures of subgrade frost heave, and to maintenance and repair track, as well as provide guidance for similar engineering design and construction. The dissertation, combining with phenomenon of subgrade frost heave and improvement measures before and after the HDHR opening in2012-2013year, by employing the methods of investigation and analysis, laboratory test, set field-test section, theoretical analysis and calculation, will make an analysis of the surrounding engineering and permafrost environment character in cold regions,anti-freezing measures having been taken during construction period which affect subgrade frost heav of HDHR, research on the process, characteristics and laws of seasonal frost heave on unballasted track subgrade through analysising data of subgrade frost heave deformation include artificial observation, long-term monitoring system, typical test section and track inspection railcar of operations department, etc. explore the mechanism and main factors of subgrade frost heave of high-speed railway,experiment the frost heaving characteristics of subgrade filling of high-speed railway in cold regions,and examine the suppressing anti-freezing measures and engineering countermeasures of seasonal frost heave of subgrade and make an evaluation about engineering effect. The main innovative result are listed as follows:(1) The seasonal frost heave of high-speed railway subgrade in cold regions has the particularity and widespread. The specially subgrade structure with ballastless track result in the upper of subgrade bed to form nearly closed frost heaving space. The enclosed space is easy to result in moisture gathering over water loss and it is easy to form the uneven frost in the freezing process.(2)The data during the first freeze-thaw period (2012-2013years) from5000artificial and42long-term automatic monitoring sections along the whole line were analyzed after the railway line was opened. The results show that the development of the frost heave includes the frost heaving occurring volatility, the rapid growing stage, the stable frost heaving stage and the thawing settlement stage. Althrough frost heaving deformation occurred along the whole line, the frost heaving values are under controlling. The frost heaving deformation main exists in upper of subgrade bed and deformation level related to the subgrade structures. As a whole, the frost heave in the transition section is slight and the embankments part is middle level and it is most serious in the cutting sections and base-plate jointing parts. The maximum amount of frost heave is fluctuated along the line.(3) The seasonally frozen depth of subgrade is increased along the direction of the line from the southern to the northern.The maximum frozen depth of field observation is1.2～1.6times of the statistics frozen depth of meteorological department and it is1.43times of average value, so we should be determined comprehensively the maximum frozen depth according to the specific worksite situation in the design. The shoulder cable groove increases channels of the ground surface water infiltration to the road basic body and result in changes the heat convection of subgrade body road so it is not necessary that track cable groove is arranged on the shoulder of ballastless track line.(4) Throught the indoor experimental research, field investigation and observation data analysis, it shows that water content is the dominant factor influencing subgrade frost heaving ratio. After graded crushed stone mixing cement by more than5%, hardening of the rubble basically eliminated the fine grained soil frost heave sensitivity.In cold regions, it is feasible to change the subgrade surface into without frost heave monolithic structure both design and subgrade frost heave control of high-speed railway.There are given two new anti-heaving subgrade structure.(5) Because of the universality of seasonal frost heave, non frost heaving soil layer in the seasonal frozen depth and arranging the fiber concrete sealing layer on the subgrade surface and setting two cloth a membrane isolate faults and water filling frost heaving protects measures are all set up. In addition, the system of monitoring data analysis shows that the measuring point of the frost heave less than12mm in the whole line accounted for96.9%, there had effect on eliminating and reducing the deformation of seasonal frost heave.(6) Because of the particularity of seasonal frost heave, the main way is to control the frost heaving deformation of subgrade surface layer for the rectification work. It put forward main countermeasures to solve the operating period of the seasonal frost heave of subgrade and that is to improve bed filler properties, to cut off the subgrade surface moisture infiltration, to reduce the underground water level, to change frost heave condition with partial insulation way. Observation data indicates that the frost heave of the bedding surface layer was mixed graded crushed stone with cement are essential below4mm, the uneven frost heave of setted sewer sections reduce21.1%, measures to prevent and treat seasonal frost heave obtained obvious effect.(7) Numerical analysis of main engineering countermeasures simulation conclude illustrates that under dynamic loads the modulus of elastic of frost heave is significantly improved in reinforcing subgrade section at frost heaving condition. In melt season because the water was not instantaneous discharged, modulus of elasticity of the subgrade upper part is reduced and it may lead to larger settlement deformation of subgrade, therefore we should pay more attention on it. By means of comprehensive analysis, the basic thought and method of assessment about subgrade status are put forward on high speed railway in cold regions.The research results of the project have been applied in design and construction of HDHR subgrade as well as renovation and maintenance of frost heaving. At present, high-speed multiple units are smooth running respectively in300km/h in summer condition and200km/h in winter condition. It proved effective and reliable of prevention measures for solving the issue of high-speed railway subgrade frost heave in cold regions and it can ensure mormal running of multiple units both regular speed and safety.