Seasonally frozen soil effects on the dynamic behavior of highway bridges

Frozen ground is significantly stiffer than unfrozen ground. For bridges supported on deep foundations, bridge stiffness is also measurably higher in winter months. Significant changes due to seasonal freezing in bridge pier boundary conditions require additional detailing in order to ensure a ductile performance of the bridge during a design earthquake event. This paper reports the latest results obtained from a project that systematically investigated the effects of seasonally frozen soil on the seismic behavior of highway bridges in cold regions. A bridge was chosen and was monitored to study its seismic performance and assess the impact of seasonally frozen soil on its dynamic properties. A Finite Element (FE) model was created for this bridge to analyze the impact of seasonal frost. It was found that when frost depth reaches 1.2 m, the first transverse modal frequency increases about 200% when compared with the no-frost case. The results show that seasonal frost has a significant impact on the overall dynamic behavior of bridges supported by pile foundations in cold regions, and that these effects should be accounted for in seismic design.     

Experimental investigation on static and dynamic resilient moduli of compacted fine soil

To investigate the static and dynamic resilient modulus of fine soil, and adapting to the new design code and maintenance system of highway subgrade in China, a series of static and dynamic tests were carried out according to the standard laboratory test methods (JTG E40-2007 and JTG D30-2015, respectively). The effects of initial water content, compactness and freeze-thaw cycles on the static and dynamic resilient moduli of fine soil were investigated and analyzed. Experimental test results show that with increasing water content, dry density and freeze-thaw cycles, the static moduli reduces about 10.2%~40.0%, 14.4%~45.5%, and 24.0%~50.3%, and dynamic moduli reduces about 10.9%~90.8%, 2.5%~38.4%, and 0.0%~46.0%, respectively. Then, the empirical mathematical relationship between static and dynamic resilient moduli was established under different water content, dry density and freeze-thaw cycles. The investigation results can be used to determine the dynamic modulus of fine soil by widely used static modulus, which could meet the requirement of adopting dynamic modulus index in new specification.     

Discussion on applying an analytical method to optimize the anti-freeze design parameters for underground water pipelines in seasonally frozen areas

Adopting the quasi-three-dimensional (Quasi-3D) numerical method to optimize the anti-freeze design parameters of an underground pipeline usually involves heavy numerical calculations. Here, the fitting formulae between the safe conveyance distance (SCD) of a water pipeline and six influencing factors are established based on the lowest water temperature (LWT) along the pipeline axis direction. With reference to the current widely used anti-freeze design approaches for underground pipelines in seasonally frozen areas, this paper first analyzes the feasibility of applying the maximum frozen penetration (MFP) instead of the mean annual ground surface temperature (MAGST) and soil water content (SWC) to calculate the SCD. The results show that the SCD depends on the buried depth if the MFP is fixed and the variation of the MAGST and SWC combination does not significantly change the SCD. A comprehensive formula for the SCD is established based on the relationships between the SCD and several primary influencing factors and the interaction among them. This formula involves five easy-to-access parameters: the MFP, buried depth, pipeline diameter, flow velocity, and inlet water temperature. A comparison between the analytical method and the numerical results based on the Quasi-3D method indicates that the two methods are in good agreement overall. The analytic method can be used to optimize the anti-freeze design parameters of underground water pipelines in seasonally frozen areas under the condition of a 1.5 safety coefficient.     

Field monitoring of railroad embankment vibration responses in seasonally frozen regions

To investigate the vibration characteristics of a railway subgrade in different seasons, three field experiments were carried out in the seasonally frozen Daqing area of China during spring, summer, and winter. The vibration characteristics and attenuation rates of the subgrade induced by passing trains were investigated, and the influences of the season, train speed, train type, train load, and number of train compartments are described in this paper. The results show that: (1) near the rail track the vibration in the vertical direction was more significant than in the lateral and longitudinal directions, and as the distance from the railway track increased, the acceleration amplitudes and the attenuation rates all decreased in all three directions; (2) the acceleration amplitudes and attenuation rates decreased in the three different study seasons as the distance from the railway track increased, and the attenuation rates in the freezing period were the largest; and (3) the acceleration amplitude induced by a freight train was greater than that by a passenger train, and the subgrade vibration increased with increasing passenger train speeds when the number of train compartments was similar. These results have great significance for enhanced understanding of the characteristics of train-induced vibration embankment response in seasonally frozen regions, and provide essential field monitoring data on train-induced vibrations in order to improve the performance criteria of railroading in seasonally frozen regions.     

东北黑土区土壤剖面地温和水分变化规律

东北黑土区土壤侵蚀的结果使土壤在坡面上发生再分配,土壤腐殖质层厚度的空间变异增大。腐殖质层厚度的变化又引起地温和土壤水分等土壤物理性质的变化,地温和水分是影响和反映冻融侵蚀作用的重要因子,也是影响地表和土壤剖面物质运移的重要因素。本文通过实测不同厚度腐殖质层剖面的地温和土壤水分,分析了地温和水分随时间和土壤剖面深度的变化规律。结果显示腐殖质层厚度对土壤温度和含水量有显著影响,腐殖质层较厚的剖面解冻速度比薄层黑土区要慢,不同深度土层温度到达0℃的日期也不相同,腐殖质层较厚的剖面冻结时间要滞后1周左右。同时,腐殖质层较厚的黑土区土壤含水量明显大于薄层黑土区,土壤水分运移的深度范围也大。     

Experimental study on DX pile performance in frozen soils under lateral loading

Experiments about working mechanism and mechanical characteristics of the DX model pile foundation under lateral dynamic and static loading were conducted by using a model system of the dynamic frozen ...     

Experimental study of the effects of non-uniformly distributed fine soil on mechanical properties of Shenyang–Dandong Railway coarse-grained soil

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, undergoing six freeze-thaw cycle numbers(0, 1, 3, 7, 9, 12) and three confining pressures(100, 200, 300 k Pa). 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.     

An experimental study on the relationship between acoustic parameters and mechanical properties of frozen silty clay

To study the influence of temperature and water content on ultrasonic wave velocity and to establish the relationship between ultrasonic wave velocity and frozen silty clay strength, ultrasonic tests were conducted to frozen silty clay by using RSM-SY5(T) nonmetal supersonic test meter, and the tensile strength and compressive strength of silty clay were measured under various negative temperatures. Test and analysis results indicate that, ultrasonic wave velocity rapidly changes in the temperature range of 1 °C to 5 °C. Ultrasonic wave velocity increased with an increase of water content until the water content reached the critical water content, while decreased with an increase of water content after the water content exceeded the critical water content. This study showed that there was strong positive correlation between the ultrasonic wave velocity and the frozen soil strength. As ultrasonic wave velocity increased, either tensile strength or compressive strength increased. Based on the experimental data, the relationship between ultrasonic wave velocity and frozen silty clay strength was obtained through regression analysis. It was found that the ultrasonic test technique can be used to test frozen soils and lay the foundation for the determination of frozen soil strength.     

古尔班通古特沙漠季节性冻土入渗特性试验研究

以古尔班通古特沙漠南缘融雪期冻结土壤入渗试验为依据,分析了沙漠地区季节性冻土水分入渗特性及主要影响因素.结果表明:沙漠地区季节性冻土具有较高的入渗能力,沙漠冻结风沙土的稳渗率为0.26～0.30 mm/min,是田间冻结壤土的10～20倍,可以保证融雪水及时入渗进入土壤,为融雪水的高效储存创造了有利条件.入渗能力随着土壤含水率的升高而减小.沙地土壤初始的低含水率、土壤大孔隙结构特征是沙地冻结土壤具有较高入渗能力的主要原因.研究结果为进一步研究春季融雪水在沙地的再分配过程奠定了基础.     

Impacts of snow cover and frozen soil in the Tibetan Plateau on summer precipitation in China

This paper presents an analysis of the mechanisms and impacts of snow cover and frozen soil in the Tibetan Plateau on the summer precipitation in China, using RegCM3 version 3.1 model simulations. Comparisons of simulations vs. observations show that RegCM3 well captures these impacts. Results indicate that in a more-snow year with deep frozen soil there will be more precipitation in the Yangtze River Basin and central Northwest China, western Inner Mongolia, and Xinjiang, but less precipitation in Northeast China, North China, South China, and most of Southwest China. In a less-snow year with deep frozen soil, however, there will be more precipitation in Northeast China, North China, and southern South China, but less precipitation in the Yangtze River Basin and in northern South China. Such differences may be attributed to different combination patterns of melting snow and thawing frozen soil on the Plateau, which may change soil moisture as well as cause differences in energy absorption in the phase change processes of snow cover and frozen soil. These factors may produce more surface sensible heat in more-snow years when the frozen soil is deep than when the frozen soil is shallow. The higher surface sensible heat may lead to a stronger updraft over the Plateau, eventually contributing to a stronger South Asia High and West Pacific Subtropical High. Due to different values of the wind fields at 850 hPa, a convergence zone will form over the Yangtze River Basin, which may produce more summer precipitation in the basin area but less precipitation in North China and South China. However, because soil moisture depends on ice content, in less-snow years with deep frozen soil, the soil moisture will be higher. The combination of higher frozen soil moisture with latent heat absorption in the phase change process may generate less surface sensible heat and consequently a weaker updraft motion over the Plateau. As a result, both the South Asia High and the West Pacific Subtropical High will be weaker, hence causing more summer precipitation in northern China but less in southern China.     

Anomaly feature of seasonal frozen soil variations on the Qinghai-Tibet Plateau

The seasonal frozen soil on the Qinghai-Tibet Plateau has strong response to climate change, and its freezing-thawing process also affects East Asia climate. In this paper, the freezing soil maximum depth of 46 stations covering 1961-1999 on the plateau is analyzed by rotated experience orthogonal function (REOF). The results show that there are four main frozen anomaly regions on the plateau, i.e., the northeastern, southeastern and southern parts of the plateau and Qaidam Basin. The freezing soil depths of the annual anomaly regions in the above representative stations show that there are different changing trends. The main trend, except for the Qaidam Basin, has been decreasing since the 1980s, a sign of the climate warming. Compared with the 1980s, on the average, the maximum soil depth decreased by about 0.02 m, 0.05 m and 0.14 m in the northeastern, southeastern and southern parts of the plateau, but increased by about 0.57 m in the Qaidam Basin during the 1990s. It means there are different responses to climate system in the above areas. The spectrum analysis reveals different change cycles: in higher frequency there is an about 2-year long cycle in Qaidam Basin and southern part of the plateau in the four representative areas whereas in lower frequency there is an about 14-year long cycle in all the four representative areas due to the combined influence of different soil textures and solutes in four areas.     

Anomaly feature of seasonal frozen soil variations on the Qinghai-Tibet Plateau

The seasonal frozen soil on the Qinghai-Tibet Plateau has strong response to climate change, and its freezing-thawing process also affects East Asia climate. In this paper, the freezing soil maximum depth of 46 stations covering 1961–1999 on the plateau is analyzed by rotated experience orthogonal function (REOF). The results show that there are four main frozen anomaly regions on the plateau, i.e., the northeastern, southeastern and southern parts of the plateau and Qaidam Basin. The freezing soil depths of the annual anomaly regions in the above representative stations show that there are different changing trends. The main trend, except for the Qaidam Basin, has been decreasing since the 1980s, a sign of the climate warming. Compared with the 1980s, on the average, the maximum soil depth decreased by about 0.02 m, 0.05 m and 0.14 m in the northeastern, southeastern and southern parts of the plateau, but increased by about 0.57 m in the Qaidam Basin during the 1990s. It means there are different responses to climate system in the above areas. The spectrum analysis reveals different change cycles: in higher frequency there is an about 2-year long cycle in Qaidam Basin and southern part of the plateau in the four representative areas whereas in lower frequency there is an about 14-year long cycle in all the four representative areas due to the combined influence of different soil textures and solutes in four areas.     

Study on potential evapotranspiration and wet-dry condition in the seasonal frozen soil region of northern Tibetan Plateau

This study was based on the CEOP/CAMP-Tibet observed data at AWS (Automatic Weather Station) of MS3478 in the seasonal frozen soil region of northern Tibetan Plateau from March 2007 to February 2008. The variation characteristics of PE (potential evapotranspiration) were analyzed based on the Penman-Monteith method recommended by FAO (the Food and Agriculture Organization of the United Nations). The contributions of dynamic, thermal and water factors to PE were discussed, and the wet-dry condition of the plateau region was further studied. The results indicated that daily PE was between 0.52 mm and 6.46 mm for the whole year. Monthly PE was over 107 mm from May to September, but decreased to less than 41 mm from November to February. Annual PE was 1,037.8 mm. In the summer, thermal PE was significantly more than dynamic PE, but conversely in the winter. Annual variation of thermal PE was of sine wave pattern. In addition, drought and semi-drought climate lasted for a long time while semi-humid climate was short. The effect of water and dynamic factors on PE varied considerably with the seasons. Annual variation of thermal PE was of sine wave pattern.     

干旱区盐渍地遥感动态监测及其驱动力研究——以渭干河-库车河三角洲绿洲为例

土壤盐渍化作为主要的土地退化形式之一,已成为一个全球性问题.盐渍地的时空动态变化特征分析研究,是发展研究盐渍地动态监测与预报技术的重要基础工作之一,对于及时掌握盐渍化程度与分布,合理制定土地利用政策与生态改良措施,实现区域可持续发展等方面具有重要作用.针对目前塔里木盆地北缘地区广泛存在的土壤盐渍化问题,基于GIS和遥感变化检测方法,对渭-库绿洲1989、2001、2007年三个时期的盐渍地时空动态变化特征及其驱动力进行了分析.研究结果表明:1989-2007年间重度盐渍地的变化程度剧烈,呈剧烈增加态势;中轻度盐渍地面积减少,主要向重度盐渍地和非盐渍地转化;非盐渍地有所减少,主要向中轻度盐渍地和重度盐渍地转化;绿洲内部盐渍化程度有所降低,而绿洲外围盐渍化程度加剧;盐渍地重心具有向绿洲外围东南方向转移的趋势;通过分析盐渍地时空动态变化特征,表明盐渍地时空变化是一个极其复杂的过程,同时受到自然和人文两大系统的驱动,人为不合理的灌溉是促使土壤盐渍化的主要驱动力.研究为盐渍地时空动态分析与评价提供了普适性较强的实现方法,为构建盐渍地定量监测模型提供了一定科学依据.     

1960-2015年新疆塔什库尔干河谷季节性冻土对气候变化的响应

利用1960-2015 年新疆塔什库尔干河谷季节性冻土的冻结始日、冻结终日、年冻结日数、年累积冻土厚度、最大冻土深度等特征指标资料,采用气候倾向率、气候突变、气候变化趋势的持续性等方法,分析近56 a该地区季节性冻土的年际、年代际变化特征。研究发现：（1）在全球变暖的背景下,1960-2015 年新疆塔什库尔干河谷气温变化亦呈上升趋势,升温趋势的持续性较强,升温幅度0.03 ℃·a^-1、0.29 ℃·（10 a）^-1、0.74 ℃·（30 a）^-1。（2）在1960-2015年期间,该地区季节性冻土呈退化趋势,具体表现为;冻结始日推迟,冻结终日提前,年冻结日数减少,年累积冻土厚度减小,最大冻土深度减小。（3）在1960-2015年期间,该地区季节性冻土持续退化趋势持续性强。（4）1960-2015 年新疆塔什库尔干河谷季节性冻土对气温变暖的具体响应呈现为退化状态。（5）按气候升温率Gt;0.034~0.046 ℃·a^-1 计算,在气候变暖背景下,该地区季节性冻土到2050 年（较2000 年）的冻结始日将推迟12~15 d、年冻结日数将减少21~27 d、年累积冻土厚度将减少36.3%~46.7%。     

Test on dynamic characteristics of subgrade of heavy-haul railway in cold regions

Dynamic characteristics of heavy-haul railway subgrade under vibratory loading in cold regions are investigated via low-temperature dynamic triaxial tests with multi-stage cyclic loading process. The relationship between dynamic shear stress and dynamic shear strain of frozen soil of subgrade under train loading and the influence of freezing temperatures on dynamic constitutive relation, dynamic shear modulus and damping ratio are observed in this study. Test results show that the dynamic constitutive relations of the frozen soils with different freezing temperatures comply with the hyperbolic model, in which model parameters a and b decrease with increasing freezing temperature. The dynamic shear modulus of the frozen soils decreases with increasing dynamic shear strains initially, followed by a relatively smooth attenuation tendency, whereas increases with decreasing freezing temperatures. The damping ratios decrease with decreasing freezing temperatures. Two linear functions are defined to express the linear relationships between dynamic shear modulus (damping ratio) and freezing temperature, respectively, in which corresponding linear coefficients are obtained through multiple regression analysis of test data.     

暴雨对土壤组成和性质的影响——以河南省泌阳暴雨中心区为例

本文以河南省泌阳暴雨中心区为例 ,在野外土壤调查和室内理化分析的基础上 ,分析讨论了暴雨对土壤组成和性质的影响。结果表明 ,暴雨中心区土壤因长期的暴雨冲刷 ,土层浅薄 ,发生层数目少 ,土体构型简单 ;砾石含量高 ,物理性粘粒含量低 ,质地类型多属多砾质砂土 ;土体中不出现肉眼可辨的铁锰结核 ;土壤淋溶作用强烈 ,呈微酸性反应 ;土壤有机质和速效氮含量较高 ,而速效钾含量偏低 ,有机质表聚化现象不明显 ;盐基淋溶率小于 1 0 0 ,土体SiO2 /R2 O3小于 7 0 0 ,脱硅和富铁铝化作用明显 ,土体风化程度较高。对暴雨中心区附近的土壤来说 ,上述土壤组成和性质均出现明显的差异。     

Probabilistic modeling of soil moisture dynamics in a revegetated desert area

Soil moisture is the key link between land hydrological and ecological processes which plays an important role in the terrestrial water cycle. As extreme weather events have increased in recent years, the stochastic simulation of soil moisture has gradually become the focus of ecohydrology research. Based on continuous monitoring of soil moisture data from 2008 to 2011, and historical precipitation data from 1991 to 2011, combined with the Rodriguez-Iturbe soil moisture dynamic stochastic model, soil moisture dynamics and its probability density function in a revegetated desert area was simulated. Results show that annual soil moisture dynamic changes of the revegetated desert area during the growing season complied with rainfall distribution; soil moisture probability presents a single-peak distribution in the plant rhizosphere layer (0–60 cm). The peak width in the 20 cm topsoil was wider than in other soils, and the distribution presented the strong fluctuations and multiple aggregates. The peak widths of 40 cm and 60 cm soil moisture probability distribution were small, which are in accordance with simulated results of the Rodriguez- Iturbe model. This confirms that the Rodriguez-Iturbe model has good applicability and can well simulate the statistical characteristics of soil moisture in an arid revegetated desert area.     

柴达木盆地大柴旦盐湖区盐生植物根-土复合体力学强度试验研究

本文以柴达木盆地大柴旦盐湖区作为试验区,分别筛选出5种优势盐生植物,即海韭菜(Triglochin maritimum Linn.)、芦苇(Phragmites australis Trin.)、赖草(Leymus secalinus Tzvel.)、毛穗赖草(Leymus paboanus Claus.)、无脉苔草(Carex enervis C. A. Mey.)作为试验供试种,分别对5种盐生植物根-土复合体试样和不含根系素土试样进行了室内直接剪切试验和三轴压缩试验,对比了不含根系素土试样与根-土复合体试样的抗剪强度指标;在此基础上,评价了5种盐生植物根系增强土体抗剪强度作用。试验结果表明：(1)当含水量和密度值分别为29.01%和1.65g/cm3时,5种盐生植物根-土复合体试样的抗剪强度指标粘聚力c值为10.38kPa-19.19kPa,且显著大于素土试样的粘聚力c值5.95kPa,内摩擦角 值未表现出变化规律;(2)相对于不含根系的素土试样,由直剪试验和三轴试验得到的5种盐生植物根-土复合体试样的粘聚力增长幅度分别为16.8%~108.7%、19.2%~113.6%;(3)由直剪试验与三轴试验结果表明,区内5种盐生植物根-土复合体试样抗剪强度大小变化顺序表现出一致的规律性,即由大至小依次为：芦苇、海韭菜、赖草、毛穗赖草、无脉苔草、素土。该项研究结果为区内进一步开展盐生植物增强土体抗剪强度机理等方面的研究具有理论研究价值和实际指导意义。     

四川紫色土地区鹤鸣观小流域分布式侵蚀产沙模型

从四川省南部县鹤鸣观小流域Ⅱ号支沟为研究区,构建了适合紫色土地区小流域分布式侵蚀产沙模型。该模型以20m×20m栅格为空间步长,以10min为时间步长,定量分析鹤鸣观小流域Ⅱ号支沟水土流失程度,模拟了各时段每个栅格次降雨侵蚀产沙过程,计算了每个栅格次降雨径流量、侵蚀量与沉积量,并且运用递归算法计算出整个流域次降雨侵蚀产沙量,模型能够评价流域下垫面各因子空间分布不均匀性和人类活动的影响。在鹤鸣观小流域Ⅱ号支沟进行了模型的检验,模拟过程与实测结果符合较好。