Summary of research on frost heave for subgrade in seasonal frozen ground

Shuang Jia ,BoWen Tai ,ShouChen Qi ,Lei Li ,Tao Chen

1.School of Hydraulic and Electric Power,Heilongjiang University,Heilongjiang,Harbin,Heilongjiang 150086,China

2.State Key Laboratory of Frozen Soil Engineering,Northwest Institute of Eco-Environment and Resources,Chinese Academy of Sciences,Lanzhou,Gansu 730000,China

3.The Fourth Engineering Co.,Ltd.,China Railway 14th Bureau,Jinan,Shandong 250000,China

ABSTRACT The building of railways on seasonally frozen ground is inevitable as China pursues economic development and the improvement of its citizens' living standards.However,railway construction in seasonally frozen soil areas is often faced with frost heave,leading to uneven subgrades which seriously threaten traffic safety.This article summarizes extant research results on frost heave mechanism,frost heave factors,and anti-frost measures of railway subgrades in seasonally frozen soil areas.

Keywords:seasonal frozen ground;railway subgrade;frost heave;anti-frost measures

1 Introduction

Frozen ground contains ice that is below 0 °C(Xuet al.,2001).Frozen ground can be divided into permafrost and seasonally frozen ground according to the length of its freezing time.Frozen soil classifi‐cation standards are presented in Table 1 (Xuet al.,2001).Frozen ground in China accounts for about 75% of the total area of the country,and seasonally frozen ground accounts for 53.5% of the total area of the country (Cao,2017).The seasonal frozen soil is mainly distributed to the north of 30°N,in north‐west,north,northeast and other regions of China.With the rapid development of China's economy and railway industry,more railways now pass over sea‐sonally frozen areas,such as the Ha-Da,Ha-Qi and Lan-Xin railways.When the temperature drops be‐low 0 °C,a series of problems caused by frost heave and deformation will seriously affect the construc‐tion,operation,and maintenance of railways,as well as railway traffic safety.For high-speed railways in particular,railway track stability has higher require‐ments,and a greater need to strictly control embank‐ment frost heave.

Table 1 Classification basis of frozen soil

Early frost heave studies believed that frost heave deformation was physical deformation.Water migra‐tion is one of the causes of frost heave in the soil.Taber's (1929) research found that during the process of water migration,water condenses into ice under the action of crystallization force,which causes frost heave.Later,Everett (1961) put forward the first frost heaving theory (capillary theory) and elaborated the relationship between the interface of ice water under pore pressure,but his theory was limited as it did not consider other relevant factors like temperature.Mill‐er (1972) then proposed a second frost heave theory and the concept of the frozen edge.Through experi‐ments,Loch and Miller (1975) believed that the exis‐tence of the frozen edge was the cause of freezing.Other relevant theories include O'Neill's (1985) pro‐posal of a rigid ice model based on the second freez‐ing theory,Beskow and Fil (1947) proposal of the ad‐sorption film theory,and Takagi's (1980) subsequent explanation of the frost heaving mechanism using the adsorption film theory.Harlan (1973) proposed a hy‐drothermal coupling model to illustrate the hydrother‐mal migration of frozen soil.Young RNet al.(1985)showed through tests that,according to the soil's own conditions,there are three types of possible strength changes which occur in soil after freezing-thawing:in‐crease,decrease,no change.

Researchers in China have only been studying frozen ground engineering since the construction of the Qinghai-Tibet Railway beginning in the 1960s.Here,a few relevant studies will be briefly dis‐cussed.Wu (1982) classified frozen soil engineering by taking into account differences in engineering properties.Maet al.(1994) studied the mechanical properties of frozen soil through triaxial shear strength tests and triaxial creep strength tests.By an‐alyzing the relationship between soil water potential and water content,Xuet al.(1985)obtained an expo‐nential equation for soil water potential and water content,unfrozen water content and temperature.Liet al.(2000) summarized the characteristic parame‐ters of the frozen margin (composed of the unfrozen water content,the moisture conductivity coefficient,and ice and the water phase pressure) and the frost heave model.Wang and Jiao (2010) analyzed the re‐lationship between frost heave and thawing precipi‐tation and temperature change from the perspective of microstructure.Zhuet al.(2010) built a threefield coupling model for water,heat,and force using the finite element program from the perspective of the microscopic mechanics mechanism,and ana‐lyzed the distribution law of water,as well as heat and force fields in permafrost subgrade.Zhu (2008)analyzed the stress and strain of subgrade during frost heaving using the finite element program.Based on the moisture,temperature and load of fro‐zen soil,Liet al.(1998) proposed a systematic fore‐casting model suitable for open saturated water to predict the frost heave of soil.Finally,Duet al.(2016) established a microscopic model of frost heave through physical analogy.

Through the deeper understanding of frost heav‐ing mechanisms afforded by these aforementioned studies,Chinese researchers have been able to better explore the factors influencing frost heave from the standpoint of practical engineering (Figure 1).Experi‐mental studies have shown that particle size,mineral composition,and compactness of soil all effects frost heave within soil (Duet al.,2006).In particular,Wang and Yue (2013) explored the influence of sub‐grade soil particle size on frost heaving sensitivity through laboratory frost heaving tests.Similarly,Wang (1980) studied the influence of powder con‐tent on the frost heave performance of fine sand through laboratory experiments.Findings demon‐strated that the higher the powder content,the great‐er the frost heave coefficient.Surface water infiltra‐tion,or excessive groundwater level,can also easily cause subgrade frost heave (Zhaoet al.,2011).Zhanget al.(2000) analyzed the influence of precipitation,evaporation,surface water,and other moisture condi‐tions on subgrade frost heave.Subgrade moisture ac‐cumulates as a result of water migration during freez‐ing.Frost heave thus becomes more serious in the context of high moisture content.Zhanget al.'s(2007) study of the frost heaving properties of coarse grained soil through a frost heaving test in a closed system supports this finding.Results of the study showed that water content was the main factor affect‐ing soil frost heave.

Figure 1 Frost heave mechanisms and influencing factors

Temperature is yet another factor affecting water migration in soil.Maoet al.(2014)found that the sub‐grade temperature in seasonally frozen soil areas changes with the seasons and found the embankment to have been greatly affected by the temperature after conducting on-site monitoring of subgrade sections.Mao(2014)reported that when the temperature is neg‐ative,the temperature field is different due to the dif‐ference in positions of the sunny-shady slopes of the subgrade.Building on this research,Jiang and Cheng(1990) analyzed the influence of seasonal loads in fro‐zen soil areas on the frost heave of the foundation through field tests—providing load and foundation frost heave curves.Wang (2014) later studied the rela‐tionship between train load and subgrade frost heave deformation in the deep-season frozen soil area of Northeast China.They found that when a railway is put into operation,the vibration load of the trains will affect subgrade stability.It also bears mention that salt can affect frost heave within soil by affecting the os‐motic pressure and freezing temperature of the soil.Notably,Liet al.(2009) conducted a freezing-thawing cycle study on saline soil and found that the salt con‐tent had a significant influence on the frost heaving and salt heaving of subgrade.The results of these stud‐ies,practical applications,and test analyses suggest that the main factors affecting the frost heave of sub‐grade are:soil quality,moisture condition,tempera‐ture,load,and topographic and geological conditions.

Research on the frost heave mechanisms of sub‐grade and the factors affecting frost heave can pro‐vide a basis for research on subgrade antifreeze mea‐sures.At present,commonly used anti-freezing mea‐sures used in engineering projects include:the re‐placement method,the lifting method,insulation,arti‐ficial salinization of subgrade (Wang,2014),and wa‐ter-proofing drainage (Li,2010).Still,the efficacy of these measures requires research and testing.In terms of insulation,for instance,different types of insula‐tion material,thickness,and depth of burial result in different antifreeze effects (Sun,2003).Yanet al.(2015)used XPS as thermal insulation material in lab‐oratory and field tests to analyze the effectiveness of thermal insulation material on the anti-freezing capac‐ity of subgrade.Test monitoring data was necessary to prove that the material could significantly reduce freezing damage.To provide another practical exam‐ple,in winter,after the subgrade is frozen water moves upward under the effect of the temperature gra‐dient and capillarity.When subgrade is relatively wet or when adoption of the lifting method is found to be unsuitable,the water barrier layer can be set to pre‐vent the rise of capillarity and reduce the water con‐tent in the soil.Researchers proposed a rectification plan for frost heave problem along a high-speed rail line:"draining+sealing+drainage+heat preserva‐tion" (Linghu,2008).By draining the subgrade infil‐tration and capillary water,Linghu was able to set up drainage blind ditches to rectify frost heave.Through comparative testing it was shown that the installation of a soil and water barrier film plus crushed stone cushion subgrade was effective at preventing frost heaving.The Lan-Xin Railway was able to set up blind drainage ditches and drain holes to drain water from the subgrade and achieved good results (Guan,2013).To name a few other measures in place,the Ho‐hhot Railway Bureau uses large-scale road mainte‐nance machinery to improve the road lifting damage(Wen,2006),while the Shenyang Railway Bureau dis‐solves salt into the frost-damaged subgrade soil to re‐duce the freezing point of the water in the subgrade,thereby reducing subgrade frost heave.Xuet al.(2011) carried out a frost heave test on subgrade fill‐ings based on the Shen-Ha Railway and analyzed the influence of silt clay content on frost heave.The frost heave coefficient was found to increase with the in‐crease of silt clay content.He (2017) analyzed the suppression effect of the replacement method on the frost heave of railway subgrade and proposed a frost heave calculation model.

2 Characteristics of frost heave in railway subgrade

China's railway lines are long and extensive.As these lines cover a variety of climatic and geological conditions,their respective seasonal permafrost sub‐grade frost heave characteristics are also different.Subgrade frost heave generally includes four stages:initial fluctuation,rapid development,stable develop‐ment,and melting down (Shi,2019).The period dur‐ing which frost heave generally occurs is from early November to mid-March of the following year.In terms of the level of frost heave under normal circum‐stances the following hierarchy can be observed:road cutting >embankment >transition section.Northeast China has a long freezing period,large diurnal temper‐ature difference and large freezing index,meaning that the region's frost heave is greater than that found in North and Central China.Uneven frost heave can easily occur and affects the overall safety of subgrade operation.Shi(2019)tested the frost heave of the Har‐bin-Dalian Railway subgrade and found that the maxi‐mum frost depth of subgrade was 3 m,and the maxi‐mum frost heave amount was 23.3 mm.The frost heave deformation mainly occurred on the surface of the foundation bed,and the frost heave of the surface of the foundation bed accounted for about 50% of the total frost heave.40% of the frost heaving over 4 mm across the whole line occurred in the transition sec‐tion,and 35%occurred in the cut,which was more se‐rious at the joint of the cut and the base plate.Tian(2019) monitored the Gan-Qing section of the Lan-Xin high-speed railway and found that the maximum freezing depth was 3.62 m.In the past,frost heave mainly occurred on the surface layer and upper layer of the basic bed,accounting for about 80%of the total frost heave.The most severe frost heave on the low embankment section is about 6 −25 mm.Due to the different geological and moisture conditions of the railway lines across the region,uneven frost heave is prone to occur.

3 Main influencing factors of frost heave

Different frost heave mechanisms have different factors affecting frost heave.The main influencing factors of frost heave are presented in Figure 2.

Figure 2 Main influencing factors of frost heave

3.1 Soil

3.1.1 Fine grain content

The size of soil particles is a key factor affecting the frost heave of subgrade.The size of soil particles directly affects interactions between the soil and wa‐ter.Coarse-grained soil is not prone to frost heave be‐cause of the large pores between the particles and the soil framework's weak binding ability with water.The results showed that the soil did not experience frost heaving when the soil particle weight was less than or equal to 6% of the total soil particle weight,and that soil did experience frost heaving when the soil particle weight was greater than 6% of the total soil weight.Frost heave-sensitive soil is prone to frost heave,and the greater the proportion of fine particles in the soil,the greater the frost heave sensi‐tivity of soil.In general,the frost heaving property of coarse-grained soil is small,but with an increase of fine grain content in coarse-grained soil,water transfer ability is enhanced.As the frost heaving sen‐sitivity of coarse-grained soil increases,the overall frost heaving property of the soil also increases.Changet al.(1999) conducted frost heaving tests on fly ash filler,and the results showed fly ash to be a frost heaving sensitive material,with its amount of frost heave inversely proportional to its density.Ex‐periments showed that the frost heaving rate of the soil increased with an increase of the proportion of fine particles in the soil (Tianet al.,2014).The maxi‐mum frost heaving rate was to be found between 0.05 mm and 0.002 mm in particle size.When the soil par‐ticle is less than 0.002 mm,the specific surface area gradually increases with the decrease of particle size,the binding ability of soil particle and unfrozen water is enhanced,the surface adsorption water film is in‐creased,the water migration channel is reduced,the water migration is prevented,the permeability is re‐duced,and thus the frost heave gradually decreases.China's soil particle classification standards are pre‐sented in Table 2.

Table 2 China's soil particle classification standards

3.1.2 Mineral composition

The mineral composition and nature of soil is dif‐ferent from region to region.The influence of the min‐eral composition on the frost heave of fine-grained soil is greater than that of coarse-grained soil.The main components of fine-grained soil are sand,pow‐der,and clay.Clay is generally prone to frost heave due to its strong water absorption capacity and poor permeability.Common clay minerals include illite,ka‐olinite and montmorillonite(Li,2008).When the min‐eral composition of cohesive soil is mainly kaolinite,the frost heave is noticeably greater than with other compositions.Maet al.(2013) conducted frost heav‐ing tests on soils containing different levels of silt clay.Under conditions of equal moisture content,the higher the content of silt clay,the higher the frost heaving rate.Sandy soil is non-frost heaving sensitive soil,and therefore does not easily produce frost heav‐ing.For coarse-grained soil,the mineral composition has little effect on its frost heave.The main influenc‐ing factors are thus the proportion of fine-grained soil in the coarse-grained soil and the mineral composition of the fine-grained soil.

3.1.3 Density

The amount of frost heave which occurs within soil is also related to soil density.When density is small,there is a larger swellable space between soil particles.Conversely,the smaller the displacement be‐tween the particles,the smaller the amount of frost heave.Thus,the amount of frost heave gradually in‐creases with the increase of density.When density reaches a certain value,the amount of frost heave will reach its maximum.Then as the density increases,the pores between soil particles become smaller and smaller,restricting the migration of water and the amount of frost heave slowing shrinks.Wuet al.(1981) conducted experiments demonstrating that when the dry density of soil reached a certain value,the particles reached their optimal agglomeration con‐ditions.The density at the time the frost heave of soil reached its maximum was found to be the most condu‐cive to water migration.When the frost heave test was performed on pebble soil samples,it was found that as the dry density increased,the frost heave rate of the soil first increased and then decreased (Wang and Yue,2013).When the soil density reaches its most unfavorable dry density,the amount of frost heave is at its largest,making this period of time the most unfavorable in terms of subgrade stability.

3.2 Water content

Water migration greatly influences heat transfer and freezing front formation.With the decrease of temperature in winter in seasonally frozen soil areas,when the temperature of soil on the subgrade surface drops below 0 °C,moisture in the soil will gradually freeze and form ice crystals,which will cause dis‐placement between soil particles and the expansion of the soil volume.At the same time,under the effect of thermodynamics,unfrozen water in the soil migrates to the freezing front.With the decrease of tempera‐ture,the freezing front moves downward,and the freezing depth increases until it reaches its maximum depth.According to the mechanism of frost heave,soil frost heave can be divided into in-situ frost heave and partial segregation frost heave.In-situ frost heave occurs when condensed water forms segregated ice on the freezing front and the volume expands by 9%.the volume may increase by up to 109% (Li and Zhu,2002).The frost heave of soil in a closed system mainly depends on the water content of the soil.In an open system,the frost heave rate is much higher than that of a closed system due to the replenishment of ex‐ternal water to the soil.In an experimental study on the characteristics of frozen soil on the Lanzhou-Xin‐jiang Railway subgrade,it was found that in a closed system,when the soil's moisture content is very small,the soil may not undergo frost heave,and soil shrink‐age may even occur when the temperature decreases(Maet al.,2010).When water in the soil freezes into ice,the volume of the ice is too small to fill the pores of the soil,so there will be no frost heaving.Frost heaving occurs when the water content of soil exceeds the initial frost heaving water content,and frost heav‐ing therefore increases with the increase of water con‐tent (Li and Lai,2013).When the density is constant,the relationship between the frost heave rate and wa‐ter content increases linearly.Frost heave tests of silty clay have shown that the moisture content above the freezing front of a soil sample increased,while the moisture content below the freezing front decreased(Lenget al.,2006).The main factor affecting soil frost heaving was water migration.The frost heaving rates of different kinds of coarse-grained soil increased with increases in water content in closed indoor systems(Zhanget al.,2007).Compared with saturation,com‐pactness,and other factors,water content has the great‐est influence on soil frost heaving and is its main cause.

The frost heave of subgrade is not only affected by the initial water content of subgrade soil,but also related to water migration.Frost heaving in the sub‐grade of a closed system mainly occurs in the surface layer,resulting in in-situ freezing.The process is fur‐ther affected by rain,snow,and other external factors like groundwater.When the local water level is lower than the critical value,subgrade frost heave is less af‐fected by groundwater infiltration.Research on frost heave along the Harbin-Dalian Railway subgrade shows that shallow groundwater has a greater impact on the frost depth of the subgrade (Wang and Li,2014).The frost heaving is related to the water con‐tent of soil,and the frost heaving rate is proportional to the water content.Thus,when the groundwater lev‐el is higher than the critical value,the groundwater constantly replenishes water to the soil through capil‐lary action.During the freezing period,the ice lens in‐creases continuously.The higher the groundwater lev‐el,the greater its impact on frost heave.Different types of soil have different capillary effects.The capil‐lary rise height of different soil types is presented in Table 3(Xuet al.,2001).

Table 3 The capillary rise height of different soil types

3.3 Temperature

Temperature is a direct influencer of soil frost heave as it affects the freezing index and freezing rate.There is always some unfrozen water in frozen soil.Under the action of temperature gradation,the unfrozen water migrates to areas with lower tempera‐tures,forming partial ice and causing frost heave(Xuet al.,1997).Through analysis of the soil tem‐perature field,it can be found that as the freezing time increases,the freezing front develops rapidly before gradually becoming flat (Zenget al.,2015).The temperature gradient is proportional to the freez‐ing rate.During the freezing process,temperature changes at different positions within the soil are dif‐ferent.The temperature gradient on the upper sur‐face of the soil changes the most,and it gradually de‐creases from top to bottom.The relationship be‐tween the temperature gradient and frost heave in open systems has been thoroughly explored.Water migration within soil is also affected by temperature gradient.The higher the temperature gradient,the smaller the frost heave (Zhanget al.,2005).Frost heave tests have been carried out on soils with differ‐ent cold end temperatures (Wang and Li,2015).When cold end temperatures are different but the de‐velopment trend of frost heave is the same,the amount of frost heave increases rapidly after the first cold shrinkage of the soil.As time increases,the growth rate gradually decreases until the amount of frost heave remains within a certain stable range.Analysis of the interior of the soil under such condi‐tions has revealed that the freezing front continuous‐ly moves from the cold end to the warm end over time.Moisture migrates from bottom to top.The higher the cold end temperature,the shorter the time required.That is,the lower the temperature,the fast‐er the freezing speed.Studies of high Harbin-Dalian railway base frost heave show that when tempera‐tures began to reduce,the subgrade surface will dis‐play repeated freezing and thawing.When the temper‐ature drops below freezing,the long-term frozen depth and frost heave quantity will increase rapidly.When the temperature drops down to −20 °C and be‐low,the frost heave growth slows until the frost heave rate reaches a steady state of dynamic balance.When the temperature starts to rise gradually,the maximum freezing temperature is reached before it reaches melting temperature(Shiet al.,2014).There‐fore,the minimum temperature is not the maximum freezing depth.When the temperature exceeds 0 °C,the surface of the subgrade gradually melts down‐wards,and the freezing line starts to move upwards,melting in both directions until the subgrade is com‐pletely melted.The freezing depth increases with the increase of daily accumulated temperature and nega‐tive temperature.If the maximum freezing depth is reached in a short time,both water migration and the frost heave will be less pronounced.If the soil is re‐peatedly freezing and thawing,especially in rainy and snowy weather when capillarity water migrates upward and water supply is sufficient,then frost heave will be relatively large.

3.4 Load

Under the weight of additional loads,soil densi‐ty can increase and affect the moisture transfer rate of soil.Load thus has an inhibitory effect on soil's frost heave.Zhouet al.(1996) conducted outdoor experiments in Inner Mongolia to study the relation‐ship between load and frost heave in subgrade foun‐dation soil.The curve of load and frost heave showed an exponential relationship.The larger the load,the smaller the frost heave.Frost heaving tests of the subgrade soil along the Bao-Lan line have al‐so been carried out to simulate the frost heaving de‐formation of the subgrade under train loads (Tian,2008).The frost heaving rate of soil without load is greater than that under load,and the frost heaving rate decreases with an increase of load.When the soil is silty,the frost heaving rate is not greatly af‐fected by presence of additional load.Under dynam‐ic loads,the temperature of frozen soil tends to rise(Zhanget al.,2006).Freezing tests of the soil under different loads have shown frost heave changes to be similar although their loads are different in open sys‐tems.The soil first appears to experience cold shrink‐age,then quickly enters a state of frost heave before decelerating until the frost heave reaches a stable state.From this point on,the amount of frost heave decreases with an increase of additional load (Wang,2014).Through numerical simulation analysis,it has been found that under the action of traffic loads,the soil underlying frozen soil subgrade produces verti‐cal acceleration (Liet al.,2008).The principal stress,pore water pressure,and displacement of the subgrade will be affected.In actual engineering,the hazards of the dynamic responses to train load should be considered.

4 Protective measures

The main factors affecting soil frost heave are soil quality,temperature and moisture.Corresponding pro‐tection measures should be taken according to the main factors affecting soil frost heaving.The main protection measures are as follows:Insulation meth‐od,replacement method,drainage and water isolation measures,artificial salinized subgrade,and elevation method.Protective measures are presented in Figure 3.

Figure 3 Protective measures

4.1 Insulation method

Ice crystals are produced when the temperature reaches freezing.The installation of an insulation lay‐er in the subgrade can attenuate the heat exchange be‐tween the subgrade and the outside to a certain extent.Insulation layer can control the transfer of negative temperature to the subgrade,reduce the freezing depth,and thereby reduce the hazards of frost heave.In recent years,the thermal insulation method has been widely used in the prevention and treatment of subgrade frost heave.The thermal insulation board mainly use the low thermal conductivity of thermal in‐sulation materials to reduce the heat in the permafrost roadbed.The smaller the thermal conductivity of the insulation material,the better the effect.Currently,the most commonly used thermal insulation material is polyethylene foam material,which can be divided in‐to EPS board and XPS board according to different production modes.When analyzing the temperature field distribution of subgrade with XPS insulation board and comparing it to ordinary subgrade through finite element calculation,the soil temperature with‐out insulation board has been found to be lower.When the line of frozen soil is low,the temperature of the insulation board is subject to sudden change,and the line of frozen soil will be higher than that of ordi‐nary subgrade.Results have shown that thermal insu‐lation board has a good insulation effect (Wang,2014).Lü Fei (2016) subjected subgrade AB fillers under different thicknesses of insulation boards to multiple freeze-thaw cycles through laboratory model tests.The results show that XPS insulation boards to be effective in terms of temperature insulation.The greater the thickness of the board,the better the ef‐fect.Taking into account factors such as project cost and construction requirements,the thickness of the in‐sulation layer is generally 10 cm.The negative base temperature value of the insulation board is greater than the negative base temperature value under the in‐sulation board.In theory,insulation board should be set as close to the surface of the subgrade as possible,but the influence of the train load on the insulation board needs to be considered (Zhao,2009).Through numerical analysis,Xuet al.(2010) showed that after the laying of insulation materials,the freezing depth line rose,and the subgrade uplift displayed an obvi‐ous difference.The installation of an insulation layer could effectively reduce freeze-heaving deformation within the embankment,but there was still a large amount of freeze-heaving deformation at the foot of subgrade slope.Partial thermal insulation and full sec‐tion thermal insulation tests carried out on Harbin Railway showed that partial thermal insulation could only suppress embankment frost heaving,meaning that full section thermal insulation would have an overall better effect than partial thermal insulation(Yan,2019).

4.2 Replacement method

In seasonally frozen soil areas,subgrade deforma‐tion must meet certain engineering specifications.This requirement is complicated by the fact that the properties of subgrade soil in different regions are dif‐ferent.In some areas,subgrade soil is mainly silty clay,which is prone to frost heave.Replacing frost heave sensitive soil with non (weak) soil has proven to improve frost resistance.Common filling materials used to decrease soil sensitivity include gravel and coarse sand,but some natural fillers cannot meet engi‐neering requirements (Yeet al.,2007).Pure coarse soil is not prone to frost heave,making it an effective subgrade filling material for preventing frost heave.The content of fine particles can be controlled through laboratory screening tests to obtain a filler gradation which will meet the requirements of antifrost heave subgrade (Yu,2014).Through field inves‐tigations and laboratory tests,the optimal thickness and other anti-frost heave subgrade can be obtained(Zhuet al.,2020).When the content of fine-grained soil is greater than 6%,it is considered to be frost heave sensitive soil.High-speed railway foundation bed construction preferentially uses A and B anti‐freeze fillers or improved soil materials with a fine particle content of less than 5%.Silty soil is sensitive to frost heave.Xuet al.(2011) studied the A and B group fillers of the Ha-Da Railway.The study's in‐door experiment showed that when the content of powder and clay particles in the subgrade filler in‐creased,the capillary action between soil particles was enhanced,and capillary water migrated to the freezing front during freezing.By increasing the freez‐ing coefficient and replacing the A and B fillers to make them free of powder and clay,the results in terms of subgrade frost heave were significantly re‐duced.By studying the frost heave law of coarsegrained fillers with different silt and cement content,experiments have shown that the frost-heave rate can be controlled within a certain range by adding cement to coarse-grained fillers(Jinet al.,2016).

4.3 Drainage and water isolation measures

Huanget al.(2015) monitored the frost heave de‐formation of subgrade and found that frost heave was affected by drainage problems.In particular,frost heave at the subgrade bottom was caused by high wa‐ter content within the subgrade soil and a high ground‐water level.Frost heave can be rectified by reducing soil moisture content and external water supply.In or‐der to improve water conditions and the amount of frost heave occurring within the foundation soil,the dynamic compaction method can be used.Wanget al.(2002) found through indoor tests that the properties of foundation soil change significantly following the application of dynamic compaction.The water con‐tent and permeability of the foundation are obviously reduced,while the bearing capacity is significantly improved,and frost heave is reduced.Waterproofing measures,drainage measures and water isolation mea‐sures can be taken to deal with precipitation,snow‐fall,groundwater recharge and other related issues.Waterproofing measures mainly prevent water on the subgrade surface from penetrating downward.A fiber concrete waterproof layer was adopted on the sub‐grade surface of the Harbin-Dalian Railway,and the joints of the subgrade and the track floor are closed(Si,2008).Protection roads were set up at the corners of the subgrade to prevent frost heave via the accumu‐lation of water at the corners from penetrating the sur‐face of the subgrade.

When the groundwater level is high,groundwater replenishment causes severe frost heave within sub‐grade.The installation of drainage facilities can re‐duce the groundwater level,control the water content of the subgrade,and reduce moisture concentration in frozen areas,thereby effectively controlling frost heave.Common drainage measures include drainage ditches,drainage side ditches,seepage pipes,blind ditches,and drainage channel.The use of a poorly hy‐drophilic composite geomembrane as a drainage layer can not only quickly drain water but also can block capillary action and prevent upward water migration(Huoet al.,2002).When the groundwater is shallow,a water barrier can be set to prevent it from entering the foundation.The water-permeable layer is divided into a water-permeable,water-resistant layer,and wa‐ter-impermeable,water-repellent layer.The water-per‐meable layer is generally made of gravel or coarse sand with a thickness generally between 0.1 −0.2 m.Sand and gravel with large pores can prevent upward capillary water migration.The impermeable water bar‐rier is divided into closed and unclosed.Commonly used materials include asphalt and plastic film.The thickness is generally 2.5−3.0 dm.The installation of a water barrier weakens water migration,reduces the water content of the subgrade soil,and achieves the goal of preventing frost damage.A well-known sec‐tion of railway in Xinjiang has successfully adopted engineering drainage measures to reduce moisture within subgrade soil and to reduce the frost heave(Ma,2009).Xiaet al.(2011) adopted a soft perme‐able pipe in Lan-Xin Railway,which has demonstrat‐ed effective water insulation and drainage functions.It can discharge excess water and block the upward migration of water,and thus has a strong capacity for inhibiting frost heaving.

4.4 Artificial salinized subgrade

After injecting salt solution into a body of soil,the surface force and capillarity of its particles can be weakened,and water migration rate inhibited (Pei and Lu,1982).Therefore,the freezing temperature of soil can be reduced and the unfrozen water content can be increased.When the temperature is lowered,part of the salt precipitates out of the solution and crystalliz‐es.Unidirectional freezing tests conducted under open systems have proven that soil deformation is dominat‐ed by frost heave while salt heave is relatively small(Binget al.,2006).Therefore,simple artificial sali‐nized subgrade is a convenient frost heave prevention measure.The Shenyang Railway Bureau has adopted the salt injection method in Zhangdang,Liaoyang,Dandong and a number of other railway sections to ef‐fectively control freezing damage.It must be noted that,although the artificial salt injection method can prevent some level of frost heave hazard for subgrade,frost heave cannot be completely eradicated,and the compressive strength of the soil will decrease as the salt content increases (Yanget al.,2019).Salt con‐sumption should therefore be controlled according to the specific engineering requirements of each area of application.

4.5 Elevation method

In areas with relatively abundant surface water,when the groundwater level is high and the subgrade is close to the groundwater,the water supply is suffi‐cient and the water content of the subgrade is high,which is easier to produce frost heave.The subgrade is raised by filling the soil on the surface of the sub‐grade,and the ballast is increased at the same time.In‐crease the height of the track,increase the distance from the groundwater,and reduce the water supply of the groundwater to the subgrade can control the amount of frost heave (Daiet al.,1994).The Daqing section increased the height of the track bed to rectify the frost damage through the method of raising the road.Through measurement,the study found that the method of raising the road has an effective effect.

5 Conclusions

China's frozen soil accounts for about 75% of the country's total area.The construction of railways in seasonally frozen soil areas is an inevitable require‐ment for China's continued economic development.Frost heave is an inevitable problem in the process of railway construction in cold regions,and one which is not conducive to the safe operation of high-speed ve‐hicles.Chinese researchers have accumulated experi‐ence in engineering practice and have achieved fruit‐ful results in analyzing subgrade frost heave mecha‐nisms,influencing factors,and prevention measures.Soil quality,water,and temperature are the main fac‐tors affecting subgrade frost heave.In engineering ap‐plications on the Harbin-Dalian and Lan-Xin rail‐ways,frost heave of the subgrade has been controlled and reduced by methods such as improving subgrade fillers,controlling moisture conditions,installing insu‐lation layers,and artificially salting subgrades.Subse‐quent railway projects in cold regions must pay more attention to the influence of soil quality,moisture,and temperature on frost heave.To further assist the con‐tinued quality of these projects,researches much strive to continue summarizing frost heave laws in or‐der to strengthen frost heave monitoring systems,and provide solid theoretical support and practical sugges‐tions for railway construction in cold regions.

Acknowledgments:

This research was supported by the Foundation for Excellent Youth Scholars of "Northwest Institute of Eco-Environment and Resources",CAS (grant num‐ber:FEYS2019002);the Research Project of State Key Laboratory of Frozen Soil Engineering (grant number:SKLFSE-ZQ-52);and the Open Project of State Key Laboratory of Mechanical Behavior and Sys‐tem Safety of Traffic Engineering Structures,Shijia‐zhuang Tiedao University(grant number:KF2020-02).