This paper aims a review of the literature related to soil reinforcements to achieve lower soil thermal conductivity （2）. The use of various natural and synthetic fibers, polymers, geosynthetics, agricultural waste/materials, and nanoclays is discussed and existing prediction models that have been thought to affect low thermal conductivity are presented.
Crushed rock subgrade, as one of the roadbed-cooling methods, has been widely used in the Qinghai-Tibet Railway. Much attention has been paid on the cooling effect of crushed rock; however, the mechanical properties of crushed rock are somehow neglected. Based on the discrete element method, biaxial compression test condition for crushed rock is com- piled in FISH language in PFC2D, and the natural shape of crushed rock is simulated with super particle ＂cluster＂. The ef- fect of particle size, crushed rock strength and confining pressure level on overall mechanical properties of the crushed rock aggregate are respectively analyzed. Results show that crushed rock of large particle size plays an essential frame- work role, which is mainly responsible for the deformation of crushed rock aggregate. The strength of gravel has a great influence on overall mechanical properties which means that strength attenuation caused by the freeze thaw cycles cannot be ignored. The stress-strain curves can be divided into two stages including shear contraction and shear expansion at different confining pressures.
The stress produced by repeated train loads decreases with increasing railway subgrade bed depth, and slightly weathered coarse particles of subgrade bed fillings can be broken at different levels under continuous load. Thus, the mass of fine soil, with a diameter of not more than 0.075 mm, is different at different depths. Fine soil is also sensitive to frost heave and thaw settlement. In order to study the effects of non-uniformly distributed fine soil on the mechanical properties of coarse-grained soil of the Shenyang-Dandong Railway, triaxial tests were conducted with three types of specimens, un- dergoing six freeze-thaw cycle numbers （0, 1, 3, 7, 9, 12） and three confining pressures （100, 200, 300 kPa）. The freezing temperature is -5 ~C and the thawing temperature is ＋15 ~C. The stress-strain behavior, static strength, resilient modulus, cohesive force and the angle of internal friction were measured for different tested specimens. As a result, the law of static strength and resilient modulus of different specimens following the increase of freeze-thaw cycles under three confining pressures is obtained. The changing law of cohesive force and friction angle of three specimens following the increase of freeze-thaw cycles is also calculated, and the different results of different specimens are also compared.
QianMi YuJianKun LiuJingYu LiuDingJun LvTengFei Wang
The temperature distributions of different parts of a subgrade were analyzed based on the results of three years of moni- toring data from the Harbin-Qiqihaer Passenger Dedicated Line, a high-speed railway, including the slope toes, shoulders, and natural ground. The temperature variation with time and the maximum frozen depths showed that an obvious sun- ny-shady effect exists in the railway subgrade, which spans a seasonal frozen region. Development of frost heave is af- fected by the asymmetric temperature distribution. The temperature field and the maximum frozen depths 50 years after the subgrade was built were simulated with a mathematical model of the unsteady phase transition of the geothermal field.
In the numerical simulation of long-term subgrade temperature fields, the daily variation of soil temperature at a certain depth h is negligible. Such phenomenon is called the ＂boundary layer theory.＂ Depth h is defined as the boundary layer thickness and the soil temperature at h is approximately equal to a temperature increment plus the average atmosphere temperature. In the past, the boundary layer thickness and temperature increment were usually extracted from monitored data in the field. In this paper, a method is proposed to determinate the boundary layer thickness and temperature incre- ment. Based on the typical designs of highway or railway, the theoretical solution of boundary layer thickness is inferred and listed. Further, the empirical equation and design chart for determining the temperature increment are given in which the following factors are addressed, including solar radiation, equivalent thermal diffusivity and convective heat-transfer coefficient. Using these equations or design charts, the boundary layer thickness and temperature increment can be easily determined and used in the simulation of long-term subgrade temperature fields. Finally, an example is conducted and used to verify the method. The result shows that the proposed method for determining the upper thermal boundary of subgrade is accurate and practical.
为了探究多年冻土区地表层的土体在冻融循环过程中力学性质的变化,通过对不同环境冷却温度、压实度、冻融循环次数、含水率和围压下的含砂粉土试样进行室内不固结不排水三轴试验,研究冻融循环后土体应力-应变关系曲线、破坏强度、弹性模量、抗剪强度参数的变化规律,然后借助正交试验的显著性分析理论,研究以上因素以及各因素间交互作用对破坏强度和弹性模量的影响程度.结果表明:含砂粉土的破坏强度和弹性模量随冻融循环次数增加,在围压300 k Pa或冷却温度-5℃的情况下呈先增加后减小最后稳定的趋势,且在冻融循环3次后达到最大值;随冻融循环次数的增加,内摩擦角在冷却温度为-5℃的情况下逐渐减小并趋于稳定,在冷却温度为-10℃和-15℃的情况下先增加后减小最后趋于稳定;围压、含水率、压实度、冻融循环次数对破坏强度和弹性模量影响显著,冷却温度的影响较弱,压实度和含水率、冻融循环次数和围压的交互作用对破坏强度影响显著.综上分析得出,在研究冻融循环对土体力学性质的影响时应综合考虑各因素及其之间的交互作用.
Freeze-thaw damage is the most common disease of semi-rigid bases in cold regions, which may greatly affect the dura- bility of roadways. In this study, the compressive strength and frost resistance of four different types of semi-rigid bases （lime-fly ash-stabilized sand, cement-stabilized sand, lime-fly ash-stabilized gravel, and cement-stabilized gravel） are assessed by varying the materials content. Based on freeze-thaw and compressive strength tests, this paper presents the performance of the different materials, each having different physical properties, and the optimal amounts of materials contents are proposed.
