Experimental investigation on the train-induced subsidence prediction model of Beiluhe permafrost subgrade along the Qinghai-Tibet Railway in China

Based on dynamic triaxial test at low temperature of the frozen clay from the Beiluhe permafrost subgrade along the Qinghai-Tibet Railway (QTR), residual deformation laws and dynamic subsidence prediction model of permafrost subgrade under train traffic were studied. First, time history curves of residual strain of frozen soil are obtained and analyzed under different temperatures, frequency, confining pressure and moisture content. And conclusions can be drawn that the axial strain rate is greatly affected by the amplitude of dynamic stress, as well as increases with dynamic to static stress ratio and temperature rising, while it decreases with the raise of frequency and moisture content. Hereby, the power functions were adopted to fit the relationships of axial dynamic strain rate vs. stress ratio, temperature, frequency, and moisture content, respectively. Simultaneously, the permafrost dynamic subsidence mechanism was interpreted rationally and the variation of fitting parameters was discussed. Furthermore, the long-term traffic loading subsidence model was established through observing the axis residual strain time histories of frozen specimens under the longtime cyclic loading and adopting the amendment of vibratory number of times. The model can comprehensively consider the effects of stress state, temperature, moisture content, and confining pressure of frozen soil, as well as the vibratory frequency and vibration number of longtime cyclic loading. Consequently, the model can be applied to the train-induced subsidence forecast investigation of permafrost subgrade. The paper has an important significance for rational safety evaluation on long-term operation of permafrost regions railway such as QTR. Meanwhile, the investigation provides basic data for the further research on dynamic damage constitutive model of frozen soil under train traffic and the gradual improvement of railroading criterion in cold regions.     

Characteristics of roadbed settlement in embankment-bridge transition section along the Qinghai-Tibet Railway in permafrost regions

The Qinghai-Tibet Railway (QTR) project was finished on July 1, 2006, and has served for over 3 years. Judging from the present situation, the roadbed is stable and train speed in permafrost regions achieves 100 km/h as expected during the designing. However, as half part of the roadbed was constructed over the permafrost characterized by high ground temperature and high ice content, slight changes of the permafrost might lead to roadbed problems, of which the settlement in embankment-bridge transition section is an obvious and special one. Investigated results of 164 bridges and accounting to 656 positions from the Xidatan Basin to the Chiqu Valley along the QTR in 2009 showed that the settlement was influenced by factors including bridge orientation, embankment slope direction, embankment height, ground temperature, ground ice content of permafrost and local subgrade soil type. For the average value of the settlement, it was greater at the northern end of a bridge than that at the southern end, and was greater in sunny-slope than that in shady-slope. It was greater in high ice permafrost regions than that in low ice regions, and was greater in high-temperature permafrost regions than that in low-temperature regions. Additionally, it increased logarithmically with the height of the embankment. In regions where the subgrade soils were dominated by silt, silty clay or fine sand, the settlement amount was higher than that in bedrock regions. Correlation analysis results showed that there were good relationships between the settlement and the slope direction, embankment height, ground temperature and ice contents when some of the later items were quantified. The correlation coefficients were 0.234, 0.213, −0.21 and 0.151 respectively, when the factors were quantified.     

Evaluation on thermal stability of embankments with different strikes in permafrost regions

The former studies on embankment in permafrost regions indicate that the maximum thawed depth on the embankment central line is regarded as the evaluation criterion, through which the roadbed thermal stability is evaluated. In fact, thermal effect problems associated with slope orientations result in the maximum thawed depth position being deviated in the roadbed transverse direction rather than thawed on the embankment central line. So the thermal stability of the permafrost embankment will highly have been changed. In this paper, the thermal stability of permafrost embankment is modeled with the assumption that the air temperature will be warmed up by 0.022 °C/a when the running time is 20 a and 50 a, respectively. Different roadbed surfaces, different embankment heights and different roadbed strikes are all analyzed. Surface material includes gravelly and asphalt. Embankment height ranges from 0 m to 5.0 m with a step of 0.5 m. For the embankment strike, east to west (EW), southwest to northeast (SW–NE), south to north (SN) and symmetric route are taken into consideration. There are several significant findings summarized from the calculated results. Firstly, for the embankments with the shady–sunny slope that there are hardly linear relations between the changes of natural permafrost table and artificial permafrost table and the heights of embankment. Secondly, there are three results when the running time is 20 a: a) with the completely symmetric thermal boundary conditions and under both the gravelly and asphalt surfaces, there are almost linear relations between the changes of permafrost table and the embankment height; b) under the gravelly surface, for embankments with sunny–shady slopes, there is no linear relation between the changes of permafrost table and the embankment height; c) under the asphalt surface, for embankments with sunny–shady slopes, if the embankment is not high enough there is a linear relation between the changes of permafrost table and the embankment height, but the thermal stability cannot be reached. Thirdly, when the running time is 50 a, the thermal stability of all embankments cannot be reached other than the symmetric strike under the gravelly surface. Finally, when designing an embankment, besides considering the minimum embankment height and the maximum embankment height, the Yin-Yang Slope effect should be taken into account.     

In-situ test study on the cooling effect of two-phase closed thermosyphon in marshy permafrost regions along the Chaidaer-Muli Railway, Qinghai Province, China

More than half of the recently built 142-km long Chaidaer-Muli Railway (CMR) in northern Qinghai Province, China travels across warm (≥−1 °C), ice-rich permafrost in wetlands on the southern flank of the Qilian Mountains. In comparison with the Qinghai-Tibet Railway from Golmud to Lhasa, the CMR traverses mostly across wetlands underlain by more ice-rich permafrost. Warm and ice-rich permafrost is sensitive to human activities and environmental changes, which can result in changes in the active layer thickness and permafrost temperatures, inducing instability and failure of infrastructures in permafrost regions. Thermosyphons were adopted in a quarter of the whole CMR route. For studying the cooling effect of the thermosyphon technique, two monitoring sites with different mean annual ground temperatures were installed since 2007. According to analysis of the ground temperature monitoring results from 2007 to 2010, the thermosyphon technique cooled the underlying permafrost and raised the permafrost table. The CMR has been put in operation since February 2010. The deformation monitoring data from 2008 to 2010 showed that the maximum accumulated settlement was 0.08 m and the minimum was 0.01 m. The settlements mainly happened in the initial months after the embankment construction was finished. In-situ monitoring results indicate that the thermosyphon technique has effect on cooling down the underlying permafrost and keeping the thermal stability of embankment in the unstable, marshy and ice-rich cold regions.     

武广高速铁路无砟轨道路基动力响应试验研究

在武广高速铁路典型路基断面埋设测试元件,分别于“联调联试”阶段和运营2年后进行了动车组列车荷载作用下的路基动力试验,实测了路基动应力、振动加速度、振动速度等动力响应。分析了路基动力响应与列车速度的关系、动力响应沿路基深度变化规律和路基动力特性在运营前后的变化情况。结果表明：基床表层顶面轨下位置动应力响应比中线处大;动态响应在基床表层范围内最为强烈且衰减较快;列车荷载速度对动应力幅值影响较小,对振动加速度幅值影响较大。运营2年后与“联调联试”阶段相比,基床表层顶面动应力幅值、振动速度幅值相差不大,而振动加速度幅值在“联调联试”阶段较大;两次试验测得的z=2.7m时(基床底层底面)路基动应力幅值和振动加速度幅值的衰减率依次为73.40%~79.30%和79.28%~86.90%,z=4.2m时（路基本体内）两者衰减率依次为82.99%~89.06%和92.78%~96.31%;而振动速度幅值,z=1.8m时（基床底层内）衰减率范围为75.62%~80.80%。     

Estimates on thermal effects of the China-Russia crude oil pipeline in permafrost regions

Using the finite element method, thermal effects of the Chinese-Russian crude oil pipeline in permafrost regions were estimated under two construction modes (conventional burial and aboveground embankment), two thermal control techniques (bare and insulated pipe), and three climate conditions along the route (marked under the natural mean ground surface temperatures of − 0.5, − 1.0, − 1.5 °C, respectively). The results show that with the silt embankment on gravel berm and the insulated pipe, the thaw of permafrost under the pipeline could be prevented during its service life. However, thawing of the permafrost under the conventionally buried pipeline cannot be prevented regardless of climate conditions or thermal insulation around the pipe. So it is strongly recommended that over-excavation and/or thermosyphons be applied for the conventional burial construction mode, especially in the warm and ice-rich permafrost sections to mitigate frost heave and thaw settlement in the subsoil.     

Experimental investigation of the dynamic behavior of frozen clay from the Beiluhe subgrade along the QTR

The dynamic behavior of frozen clay obtained from the Beiluhe permafrost subgrade along the Qinghai-Tibet Railway (QTR) was investigated through cryo-dynamic triaxial experiments. The effects of several key factors, including temperature, moisture content, frequency and confining pressure on the dynamic behavior, were analyzed. It was found that a hyperbolic model could describe the dynamic behavior of the frozen clay well. The maximum dynamic shear modulus of the frozen soil decreases as the temperature rises and as the confining pressure decreases; the reference shear strain magnitude decreases as the temperature and confining pressure increase and as the moisture content decreases; the maximum damping ratio increases as the temperature, frequency and confining pressure increase and as the moisture content decreases. There exists a critical moisture content (around 18%) that maximizes the dynamic shear modulus; at the critical frequency (about 6 Hz), the dynamic shear modulus is at its maximum, and the reference shear strain magnitude and the damping ratio attenuation exponent are at their respective minima. Moreover, functions were proposed to quantitatively describe the relationship between frozen clay dynamic parameters and various influencing factors.     

Characteristics and mechanisms of embankment deformation along the Qinghai-Tibet Railway in permafrost regions

Based on field monitoring datasets, characteristics of embankment deformation were summarized along the Qinghai-Tibet Railway in four permafrost regions with different mean annual ground temperatures (MAGTs). Then, further analyses were carried out at some typical monitoring profiles to discuss mechanisms of these embankment deformations with consideration of detailed information of thermal and subsurface conditions. The results indicated that in regions with MAGT <− 1.5 °C, embankments only experienced seasonal frost heaves, and of which the magnitudes were not significant. So, the embankments in the regions performed satisfactorily. Whereas in regions with MAGT ≥− 1.5 °C, both traditional embankment and crushed rock embankment experienced settlements, but characteristics and mechanisms of the settlements were different for the two kinds of embankment. For crushed rock embankment, the magnitudes of settlement and differential settlement between right and left embankment shoulders were not significant and increased slowly. In respect that upwards movements of permafrost tables and better thermal stability of permafrost beneath embankment, mechanism of settlements on the embankment was inferred as creep of warm and ice-rich layer often present near permafrost table. While for traditional embankment, particularly in warm and ice-rich permafrost regions, the magnitudes of settlement and differential settlement between right and left embankment shoulders were significant and still increased quickly. Considering underneath permafrost table movements and permafrost temperature rises, mechanisms of settlements on the embankment included not only creep but also thawing consolidation of underlying permafrost. Therefore, some strengthened measures were needed to ensure long-term stability of these traditional embankments, and special attention should be paid on temperature, ice content and applied load within the layer immediately beneath permafrost table since warming and thawing of the layer could give rise to considerable settlement.     

蜂窝锥壳结构动力学特性试验研究

通过模态试验对蜂窝结构锥壳结构的纵向固有频率和阻尼比进行了识别,并与数值仿真结果进行了对比。通过对比发现:复合材料以及蜂窝结构的应用能够降低该结构的固有频率并增加阻尼比;通过振动台试验,对锥壳-质量系统的动力学响应进行了研究,试验结果表明该锥壳结构对于85Hz以上的振动能够有效隔离,从锥壳下端至上端振动幅值衰减大于40%,具有理想的隔振效能。     

Development of freezing-thawing processes of foundation soils surrounding the China-Russia Crude Oil Pipeline in the permafrost areas under a warming climate

The proposed China-Russia Crude Oil Pipeline (CRCOP) will be subjected to strong frost heave and thaw settlement of the surrounding soil as it traverses permafrost and seasonally frozen ground areas in Northeastern China. The freezing-thawing processes, the development of the maximum frozen cylinder in taliks and thawed cylinder in permafrost areas, and the variations in the maximum freezing depths under the pipeline in taliks and thawing depths in different permafrost regions near Mo'he station, the first pumping station in China, were studied in detail using numerical methods in this paper. The inlet oil temperature at Mo'he station was assumed to vary from 10 to − 6 °C in a sine wave form during the preliminary design phase. Research results showed that the freezing-thawing processes of soils surrounding the buried pipeline had distinct differences from those in the undisturbed ground profile in permafrost areas. In summer, there was downward thawing from the ground surface and upward and downward thawing from the pipeline's surface once the temperature of the oil rose above 0 °C. In winter, downward freezing began from the ground surface but upward and downward cooling of the cylinder around the pipeline didn't begin until the temperature of the oil dropped below 0 °C. However, in the undisturbed ground profile, downward thawing from the ground surface occurred in summer and downward freezing from the ground surface and upward freezing from the permafrost table occurred in winter. The maximum thawing depths and thawed cylinder around the pipeline in warm permafrost enlarged with elapsing time and decreasing water content of the soils. In taliks, the maximum freezing depths and frozen cylinder around the pipeline kept shrinking with elapsing time and increasing water content of the soils. The freezing-thawing processes and development of the thawed and frozen cylinders around the pipeline were muted by any insulation layer surrounding the pipeline. Insulation had better thermal moderating on the heat exchange between the pipeline and the surrounding soils during the early operating period. But its role slowly weakened after a long-term operating. Research results will provide the basis for assessment and forecasting of engineering geological conditions, analysis of mechanical stability of the pipeline, foundation design, and pipeline construction and maintenance.     

基于高速铁路路基冻胀的轮轨动力响应研究

季节性冻土区高速铁路无砟轨道路基冻胀,影响了列车运行的安全性、舒适性以及无砟轨道主体结构的服役性能。为研究路基冻胀和高速行车荷载组合效应下的轮轨动力响应,建立了车辆-轨道-路基冻胀耦合动力学模型,对路基不同冻胀幅值、冻胀位置和行车速度下CRTSⅠ型板式无砟轨道轮轨动力响应及轨道结构受力进行分析。结果表明:冻胀发生区段轮轨动力响应增大,列车以350 km/h运行时的安全性和舒适性满足冻胀管理标准要求,但轮轨力随冻胀幅值和速度的增加而增大;轨道板和底座板振动加剧,在计算冻胀波长和幅值范围内,离缝处轨道板振动加速度峰值超过动态验收标准要求,容易引起离缝处CA砂浆层及路基基床表层伤损破坏,且轨道板、底座板振动加速度随行车速度增加而增大;轨道结构动应力和列车荷载传递关系密切,路基冻胀状态下列车荷载引起轨道板和底座板处于交替和交变的拉压受力状态,需要在设计中提出控制裂纹的措施,行车速度对短波冻胀时轨道结构受力影响较小。     

秦沈客运专线路堑段基床结构动态测试分析

通过对我国第一条铁路客运专线———秦沈客运专线路堑段基床表层的动态测试,得到了厚度分别为40cm与60cm的基床表层在不同车速下的动应力与加速度值,分析了它们在基床表层横向与深度方向的分布规律及其与速度的关系。     

Numerical comparison of flexible pavement dynamic response under different axles

A three-dimensional finite element model was utilised to examine the flexible pavement dynamic response under single, tandem and tridem axles at different speeds. Using two different hot-mix asphalt (HMA) layer thicknesses, 15.2 and 25.4 cm, the dynamic effects of moving axles were investigated on critical responses. These responses include the tensile strain at the bottom of asphalt layer, compressive strain on the top of subgrade and tensile and compressive strain on the surface layer. In this study, the HMA layer and other layers were characterised as linear viscoelastic and elastic material, respectively. Since this research focuses specifically on the time and dynamic effects, considering the transient dynamic loading and inertia forces, implicit dynamic analysis was done. The important findings are as follows. (1) Strains induced by tridem axles could be greater than tandem axles or even equal at different speeds. (2) It cannot be stated that axles always induce greater critical response value to road systems at lower speed because at higher speed they can also induce greater critical response value in pavements than that at lower speed. (3) Changing trend and changing rate of strains with speed are strongly affected by pavement thickness. In general, the effects of different axle configurations are strongly affected by moving speed and surface layer thickness.     

强夯施工对塑性混凝土防渗墙的振动测试分析

提出强夯施工时塑性混凝土防渗墙的振动测试方案。试夯数据表明,加速度控制指标0.1g比速度控制指标5cm/s更加严格;给出5个不同能级对应第一、二遍点夯的安全保护范围。基于Hilbert-Huang Transform信号方法的时频分析可知,不同夯击能级引起塑性混凝土防渗墙墙体测点的振动主频均小于20Hz;强夯施工对埋深仅为5m的塑性混凝土防渗墙的振动影响,瑞利波占主导地位,表现为墙体上端率先振动,且振动强度最大;塑性混凝土防渗墙沿墙高的垂向应力分布比较均匀;瑞利波由于受地基分层的频散影响,当主频降低时,波速和波长增加,最大瞬时能量和主频对应能量小幅增加     

重载铁路钢轨隐伤病害处振动特性分析EI北大核心CSCD

钢轨隐伤病害在重载铁路中大量存在,病害发展后期会导致病害位置整个轨道-路基动态响应增大,严重影响轨道-路基正常服役状态。为研究重载铁路典型隐伤病害产生后对整个轨道-路基的时频域影响特征,开展了现场静态测试与病害原因分析,以及分别进行了病害断面和正常断面的动态行车测试对比试验,得出了病害断面的时频域及振动传递特征。结果表明:病害断面钢轨轨头存在局部低塌、剥离掉块及多裂纹病害,轨头表面会存在明显的裂纹扩展区,其呈Y型扩展并有贯通整个轨头的趋势。病害断面轨道-路基振动明显较正常断面大,其中钢轨、轨枕、道床及路基振动大小均值分别是正常断面的26倍、23倍、13倍及5倍;病害断面轨道-路基频谱峰值存在转向架和邻轴距为基频10 Hz左右的周期性调制频率;病害断面轨枕下道砟粉化严重导致振动能量在2000 Hz附近几乎无衰减,而正常断面在2000 Hz附近由上至下各层之间振动能量依次减小;车速增大对病害断面钢轨振动冲击影响最强烈,对其下部轨枕及路基影响在45~75 km/h内逐渐减弱。     

Cooling effects study on ventilated embankments under the influence of the temperature differences between the sunny slopes and the shady slopes

Based on theories of heat and mass transfer, three-dimensional theoretical and numerical models were presented to analyze the temperature characteristics of embankments in permafrost regions. Conditions of air convective within crushed rock layer and the background of the climate warming in the future were considered. In addition, temperature differences between the sunny and shady slopes of embankments, as well as the uneven distribution of air temperatures inside the ventilated duct were also considered in the models. Cooling effects and temperature field characteristics of three kinds of embankments were analyzed and compared. Results of traditional embankment indicated that there were significant differences in permafrost tables between the sunny and shady slopes, and the underlying permafrost was in serious degradation. A large asymmetric 0 °C melting bulb was formed in and beneath the embankment over winter time. The analysis indicated that the duct-ventilated embankment could lower the temperature of underlying permafrost, and elevate the permafrost table under the embankment. The duct-ventilated embankment could also adjust the differences of permafrost tables, and improve the asymmetry of the temperature fields between the sunny and shady slopes. Due to climate warming, however, a 0 °C melting bulb still developed at the sunny slope foot of the duct-ventilated embankment. To prevent the 0 °C melting bulb from occurring, a closed crushed rock revetment was applied on the sunny slope, and the results indicated that the cooling effect of duct-ventilated embankment with the closed crushed rock revetment was better than that without the closed crushed rock revetment. Soils in and beneath the duct-ventilated embankment with closed crushed rock revetment could refreeze completely over winter time and this kind of embankment could also effectively adjust the differences of permafrost tables between the sunny and shady slopes.     

高速定转子均质机转子叶片振动模态分析

采用对接加载模态综合分析法,将定转子均质机关键元件转子系统结构简化为叶-盘系统,对单个转子叶片进行有限单元划分,分析得出了转子转速为4 500 R／MIN时振动频率响应分布趋势;前10阶扩展模态分布具有一定的规律性,在初始4阶模态状态时频率波动小,且与系统固有频率近似;从第5阶模态以后分布波动较大, 且波动幅度很接近,约为20HZ。分析结果有利于定转子均质机关键元件转子系统结构的优化设计。     

Effect of frequency on fatigue crack growth behavior of magnesium alloy AZ61 under immersed 3.5 mass% NaCl environment

Fatigue crack growth test of AZ61 magnesium alloy was carried out under immersed NaCl environment at frequencies of 15, 5 and 0.5 Hz under a stress ratio of 0.1. In order to investigate the effect of frequency on fatigue crack growth behavior in detail, additional tests at frequencies ranged from 15 to 0.01 Hz were conducted under a constant ΔK of 3.25 MPa m^1/2. Effect of frequency was clearly observed in low ΔK region, where fatigue crack growth rate decreased with decreasing frequency. Crack closure would be a dominant factor for the frequency effect observed under immersed NaCl environment at frequencies ranged from 15 to 0.5 Hz. However, fatigue crack growth rates at frequencies lower than 0.05 Hz were higher than those at frequencies higher than 0.5 Hz. The accelerated fatigue crack growth rates at frequencies lower than 0.05 Hz would be attributed to the corrosion attack at the crack tip.     

Hydrothermal processes of Alpine Tundra Lakes, Beiluhe Basin, Qinghai-Tibet Plateau

Most of the alpine tundra lakes, of average size 8,500 m², are widely spread in the Beiluhe Basin on the Qing-Tibet Plateau, where ice-rich permafrost exists. Approximately 70% of the lakes are elliptical in shape and 15% are elongated. About 80% of the lakes are unfrozen to the bottom throughout the year while a larger portion of it, about 60%, may be underlain by taliks that penetrate permafrost. The BLH-A Lake, a representative lake with 2-m deep water in the region, has been observed for about four years (from 2006 to 2009). Ice starts to cover on the lake-surface after mid-October, and its thickness increases to 60 to 70 cm by the end of cold season. The ice cover then starts to melt in later April and melts completely around mid-May. The lake-surface temperatures change sinusoidally with the air temperatures, but lagging about half a month. The water warms with the increase of the water depth, and the maximum annual temperature appears at depth of 1.5 m with a value of 14.3 °C on July 30, 2007. The lake-bottom temperatures are not isothermal at different depths for most time of a year. It may be related to the variable climate, little snow, and intensive solar radiation. The mean annual lake-bottom temperatures are about 5.5 °C in the deep pool with 2 m deep water and 4.3 °C in the shallow nearshore zone with 1 m deep water. The warm lake-bottom causes considerable disturbance to the permafrost. Surveyed data show that there is no permafrost under the lake when the mean annual lake-bottom temperature is over 5 °C.     

Dynamic properties of pultruded natural fibre reinforced composites using Split Hopkinson Pressure Bar technique

The paper presents results on dynamic mechanical properties of jute, and kenaf fibre reinforced composites at various strain rates using compression Split Hopkinson Pressure Bar technique. The stress–strain curves for both pultruded natural fibre reinforced composites at strain rates of nearly 1400 s⁻¹ are illustrated and then compared with statically determines stress–strain curve (1.0 × 10⁻³ s⁻¹). Results show that the strain rate does affect the value of dynamic compressive properties of both pultruded natural fibre composites. Higher dynamic compression modulus and 2.5% flow stress were recorded for higher strain rates as compared to lower strain rate over the range of strain rates investigated. Under dynamic loading, jute fibre reinforced composites recorded the highest value of dynamic response in terms of compression modulus, 2.5% flow stress and compressive strength than that of kenaf fibre reinforced composites. In addition, kenaf fibre reinforced composites is more severely damaged as compared to jute fibre reinforced composites for all tested strain rate.