Experiment Study Of Frost Heave Characteristics Of Graded Crushed Rock Used As The Filler Of High-speed Railway Subgrade

The railway is the core element of the national economy and plays a vital role in the socialist modernization drive.By the end of 2018,China has opened more than 29,000 kilometers of high-speed railway,which is more than two-thirds of the international high-speed railway mileage.With the rapid development of high-speed railway construction,more and more high-speed railway will be driven in cold areas dominated by seasonal frozen soil.In this area,high-speed railways are constructed and operated,and the subgrade often produces Over-limit frost heave deformation,which greatly affects the smoothness of track structure and the safety of train operation.Graded crushed rock is the most commonly used coarse-grained filler in roadbed.Graded crushed rock is generally considered to be a frost heaving insensitive material,but the on-site monitoring results show that the frost heave of high-speed railway subgrade is mainly caused by the freeze of well-graded gravels.In order to effectively solve the problem of frost heave of subgrade,it is necessary to master the influence of different factors on the frost heave characteristics of graded crushed rock.A series of frost heave tests and theoretical analysis have been carried out for this purpose.The main work and conclusions are as follows:(1)Selecting the moisture content,fines content,dry density and cold boundary temperature as the influencing factors,the one-way frost heave orthogonal test under the closed system was carried out,and the multifactor progressive regression analysis method was adopted to comprehensively consider the single factor and interaction,the significant influence term was selected by the elimination-reward-regression cycle mechanism,and the regression equation of frost heave rate was established.The results show that the primary and secondary factors affecting the frost heave rate are moisture content >dry density>fines content>cold boundary temperature;among them,moisture content,fines content and frost heaving rate are positively correlated,while dry density is related to frost heave rate.The influence depends on the content of fines and cold boundary temperature.The positive and negative effect of cold boundary temperature depends on the dry density,the interaction is significant.Within the factor level,the combination of maximum average heaving ratio was calculated based on regression equation,and the predicted value was in good agreement with validation test data.By controlling the moisture content of well-graded gravels less than 4%,the prevention and control requirement of average heaving ratio less than 1% can be achieved.(2)On the premise of equal moisture content,fine grain content,dry density and temperature gradient,the cold and warm boundary were raised the same temperature,and the one-way frost heaving contrast test of three groups of closed systems was carried out.The results show that with the increase of the average temperature of the graded gravel sample,the freezing rate of the sample decreases,the freezing depth becomes shallower,the frost heaving deformation and the frost heaving rate increase significantly.When the average temperature of the sample is high,the freezing front is slow to move downward,and the depth of freezing is relatively shallow,so most of the sampl is still in an unfrozen state,and the pore connectivity in the soil is relatively good,and a large amount of water can more smoothly migrate upward from the lower part of the sample and freezes into ice under the cold temperature,so it can produce large frost heave deformation and frost heaving rate.(3)The water holding capacity of the aggregate sample with 5% montmorillonite content is 10.2%,while the aggregate sample with 5% Stone powder is 6.8%.In order to simulate the condition that the surface layer filler of subgrade has experienced water saturation to free drainage stability after the continuous rainfall infiltration,freezing test was conducted under the condition of Moisture content ration=1(moisture content = water holdup).The results show that the graded gravel sample mixed with montmorillonite has weak water migration ability due to strong surface activity of fine particles in the initial stage of freezing.The rate of frost heaving deformation is slow and the deformation is small.In the middle and late stages,since the total moisture content is larger,a lot of water in unfrozen areas can slowly and stably migrate to the freezing zone and freeze,so the frost heaving deformation can be stably and continuously increased,while the main moisture migration and frost heave occurred of the graded crushed stone test with the mixed stone powder in the early stage,and the deformation has entered a relatively fast decay in the middle and late stages.Therefore,the final frost heave amount and the frost heave rate are larger than the latter.When analyzing the frost heaving sensitivity of the graded crushed rock with different fine particles,It should not only be evaluated according to the activity of fine particles,water ratio limit,etc,but also should be combined with the water holding capacity of the soil to consider the most unfavorable conditions for comprehensive evaluation.