| High-speed railway has become the first choice for public transportation in contemporary resource-saving and environment-friendly society because of its outstanding features,such as high vehicle speed,low energy consumption,and low environmental pollution.With the rapid development of high-speed railway,China is gradually forming the world’s largest and fastest high-speed rail network.According to the national "long-term railway network planning" and "the Belt and Road" strategy,by 2020,the scale of domestic high-speed railway will reach 30,000 kilometers,and the Asia-Europe high-speed railway will reach 10,000 kilometers as well.The high-speed railway network will inevitably cover a large area of seasonal frozen region,including the northeast,northwest,and north China.Compared with other countries at the same latitude,the construction and maintenance environment of high-speed railways is quite harsh in China.In cold regions of China,temperatures are extremely low and durations are extremely long,which can easily result in frost heave deformation of the subgrade.It would cause the track irregularity and affect the operational safety of high-speed railway severely,which poses a threat to the railway construction technology.The effective solution to the problem of frost heave is the important foundation for the safety and efficiency of high-speed railway,however,it is noteworthy that the key to this problem is to understand the frost heave mechanism of the filler used in high-speed railway systematically and comprehensively.This paper has researched the frost heave problems respectively from three aspects of frost heave performance,microscopic characteristics and frost heave mechanism simulation.In addition,a multi-scale study has been carried out on fine-grained soil,fine-grained filler,cementitious fine-grained filler and subgrade in the background of Harbin-Dalian and HarbinQiqihar passenger dedicated line.Firstly,the particle microstructure of fine-grained soils was analyzed by scanning electron microscope and other techniques to study the influence of mineral components and particle size on the sensibility of frost heave.The finite volume method was utilized to verify the factors of frost heave and to explore the mechanism of heat and water transfer in soil during freezing.Secondly,X-ray computed tomography was adopted to observe the mesostructure of finegrained filler to study the influence of fine-grained clusters and pore structure on frost heave.The discrete element method was used to verify the effect of fine-grained content and frost heave ratio of soil on fine-grained fillers and to study the interaction between the confinement of coarse-grained skeleton and the expansion of fine-grained soil during freezing.At the same time,the cement solidification method of fine-grained filler was evaluated based on the frost heave performance and the micro-pore structure,and the solidification mechanism of cement were analyzed.Finally,based on the engineering monitoring data and the finite element method,the frost heave of high-speed railway subgrade was simulated and predicted,and the influence of fine-grained filler on the overall deformation of subgrade was investigated.The main conclusions based on experimental and theoretical studies are listed as follows:(1)The difference in the frost heave properties of kaolin and montmorillonite during frost heaving process of pure fine-grained soil is due to the difference in their microstructure.Montmorillonite has more bound water and less free water due to its layered structure.The excellent water holding capacity of montmorillonite means a lower frost heaving rate,on the contrary,kaolin has a higher frost heaving rate.Based on the finite volume method for simulating the frost heaving process of pure fine-grained soil,by coupling the rigid ice model and the modified Kozeny-Carman equation,the relationship between frost heaving rate and soil types and fineness was established.The results showed that the soil types in increasing order of frost heave are sand,silt and clay,and the frost heave rate increases with the specific surface area.A ternary map based on the frost heave properties of three soil types was proposed,and it was suggested that the clay content in fine-grained soil with micro frost ratio should not exceed 30%.(2)Based on X-CT digital image processing,a quantitative method for measuring the cluster ratio of fine-grained soil in fine-grained filler was proposed.The difference in frost heaving rate of filler was mainly due to the internal fine-grained soil cluster structure,which is formed by multi-step clustering process.Fine-grained soil clusters appeared at the amount of 5% fine-grained soil,while the cluster ratio could reach 74% at the amount of 30% fine grained soil.According to the analysis of the connectivity and fractal dimension of cluster,the finegrained soil content should be strictly controlled below 10%.Based on the discrete element method for simulating the frost heaving process of fine-grained filler,the change of contact force of the coarse and fine particles before and after frost heaving was quantitatively characterized,and the interaction between the confinement of coarse-grained skeleton and the expansion of fine-grained soil was analyzed.The filler presents as skeleton pore structure at low fine grain content,which is dominated by the confinement of coarse-grained skeleton,while the filler presents as suspension dense structure at high fine grain content,which is dominated by the expansion of fine-grained soil.When the amount of fine-grained soil is 8%,the optimum skeleton dense structure is achieved because of the balance between the confinement and expansion.(3)A full-range pore structure testing method for fine-grained filler based on X-CT and MIP was proposed.The pore size range was 1nm to 10 mm,including the macropores(>100μm),mesopores(1~100μm),and micropores(<1μm).The pores between skeleton are approximately more than 500μm;the pores between skeleton and soil particles are approximately 100~500μm;and the pores between soil particles are approximately 0.1~1μm.Fine-grained filler and cementitious fine-grained fillers both have the best pore structure when the fine grain content is 8%,however,the porosity of latter obtained by X-CT is only about 10% of that of the former,mainly due to the solidification of cement.In fine-grained filler,cement plays a dual role both in physical filling and chemical gelling.The physical filling takes effects mainly in macropores around the coarse particles,while the chemical gelling takes effects mainly in micropores around the fine particles.(4)The surface of the roadbed is gradually cooled due to the heat conduction and heat exchange between the subgrade and air.The temperature drop of the shoulder is the fastest,followed by the center,and the slope is the slowest.Therefore,the frozen depth of corresponding regions has the same law with the temperature.Longitudinal displacement and effective stress are concentrated on shoulders and slopes initially,then concentrated on the center.Since the small deformation does not affect the temperature distribution of the subgrade structure,the frost heave amount of subgrade only depends on the expansion rate of materials under the same conditions. |
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