多年冻土区铁路路基热状况对工程扰动及气候变化的响应

基于青藏铁路沿线长期地温监测资料,对天然场地及铁路路基下部的浅层地温、多年冻土上限及下伏冻土地温动态变化过程进行对比分析,研究多年冻土区铁路路基热状况对于工程扰动及气候变化的响应过程.监测结果表明,路基修筑后边坡热效应显著,由此导致路基下部多年冻土热状况的不对称分布,必须引起足够的重视.块石路基修筑后,下部多年冻土上限抬升显著,其中阴坡路肩下抬升幅度普遍较阳坡路肩下显著.普通路基修筑后,在年平均地温低于?0.6～?0.7℃的地区下部多年冻土上限有不同程度的抬升,而在年平均地温高于?0.6℃的地区下部冻土上限则出现了一定程度的下降,其中阳坡路肩下降幅显著.受块石层冷却降温作用,低温冻土区块石路基下部浅层冻土地温有明显降温过程,而在高温冻土区这一降温趋势只存在于阴坡路肩下.对于普通路基,多年冻土上限抬升后,浅层冻土地温存在一定的升温过程.对于气候变暖,低温冻土区多年冻土的响应主要集中体现在冻土升温上,而高温冻土区多年冻土的响应则主要表现为冻土上限下降,冻土厚度减小.基于上述监测结果,可将目前青藏铁路路基热状况分为稳定型(低温冻土区块石路基)、亚稳定型(低温冻土区普通路基及高温冻土区块石路基)和不稳定型(高温冻土区普通路基).     

气候变暖下青藏高原冻土路基地温场演化规律研究

冻土物理力学特性与温度密切相关,气候变暖背景下冻土路基地温场的分布和演化规律不仅会影响到路基的静力稳定性,还会影响到其在地震、车辆等动力荷载作用下的响应特征与稳定性。为此,基于现场实测路基坡面温度,系统开展气候变暖背景下青藏高原典型(东西、南北、45°)走向条件下冻土路基地温场分布及演化规律的模拟研究。结果表明,阴阳坡侧浅层土体冻结指数差异较融化指数差异更为显著,东西走向下阴坡冻结指数约为阳坡的2倍,而融化指数约为阳坡的0.83。阴阳坡侧路基本体及活动层季节冻融过程存在明显不同步,东西走向条件下阴坡冻结期(融化期)可较阳坡侧长(短)约1个月。路基修筑后,阴坡一侧路基下部人为上限均有一定的抬升,而阳坡仅南北走向有抬升。此后,在气候变暖及沥青路面吸热效应下,路基人为上限不断下降,最大速率可达20cm/a,且逐步出现融化夹层,其中阳坡融化夹层厚度普遍大于阴坡,差值最大可达2.5m。路基本体季节冻融过程的不同步、人为上限埋深及冻土地温分布的不对称性应在未来青藏高原冻土路基静力、动力稳定性设计和研究中予以考虑。     

多年冻土区青藏铁路路基的长期热状况

基于青藏铁路楚玛尔河试验段10年(2003~2013)的地温监测资料,对青藏铁路4种典型路基结构的长期热状况进行了对比分析.结果表明,不同路基结构的长期热状况表现出较大的差异.普通路基与块石基底路基地温场存在明显的不对称分布,表明以上两种路基结构不利于路基的长期热稳定.但块石护坡路基与U型块石路基的地温场分布则表现出了较好的对称性.尽管块石护坡路基下浅层冻土地温存在一定的降温过程,但深层多年冻土却呈现出缓慢升温趋势,显示U型块石路基的热稳定性要优于块石护坡路基.被监测的4种路基结构中,U型块石路基在降低多年冻土温度与提高路基地温场对称性方面表现出了最佳的长期效应.基于青藏铁路10年的监测结果,充分肯定了主动冷却路基设计思路在保护冻土路基长期热稳定性方面的有效性,同时采用冷却路基技术的青藏铁路也达到了时速100 km h?1的设计要求.尽管如此,由于坡向效应所导致的路基左右路肩下的热差异存在于所有监测的路基结构中,但不同结构的热差异幅度不同,并将可能导致路基发生潜在的非均匀性沉降变形,因此需要在后续的维护工程中进行调整.     

开放边界条件下青藏铁路抛石路基的降温效果分析

铁路道碴铺层和抛石层可以看作多孔介质, 其内部流体的对流换热为多孔介质的传热传质问题. 目前抛石路基作为一种特殊的新型路基结构已在青藏铁路得到广泛应用, 但其抛石层多为上下封闭、左右开放, 内部空气与外界连通, 对流与传热模式变的更为复杂. 因此, 基于多孔介质中流体热对流的连续性方程, 非达西流动量方程及能量方程, 针对青藏铁路的气温条件和地质条件, 对开放边界、不同路堤高度(抛石层厚度为1.5 m)的抛石路基内速度场及其在未来50年的温度场特征进行了研究与分析. 结果表明: 在外界风的作用下, 这种开放路基道碴和抛石层内的对流换热方式以强迫对流为主; 并且在年平均气温为-4.0℃, 考虑未来50年气温上升2.6℃的情况下, 其降温效果较好, 路堤下有低温冻土核生成; 并且较高路基降温效果好于较低路基, 这主要是由于较高路堤底部较宽, 对下部冻土产生的影响范围较大; 但由于主导风向的影响, 较高路基的下部温度场出现左右不对称现象, 这很可能会造成路基整体的横向不均匀变形, 而较低路基无此现象发生, 这应引起设计与施工部门的足够重视.     

青藏公路沿线冻土的地温特征及退化方式

青藏高原多年冻土(以下简称冻土)具有地域分布广、厚度薄及稳定性差等特征. 过去几十年的气候变暖背景下, 冻土广泛退化, 地温升高, 夏季最大融化深度加深, 冬季冻结深度减小. 冻土已经产生下引式、上引式和侧引式退化. 冻土层厚度减薄, 或者在某些地区彻底消失. 冻土退化模式研究在冻土学、寒区工程和寒区环境管理方面具有重要意义. 由南至北穿越560 km冻土区的青藏公路沿线(简称青藏线)冻土在青藏高原腹地具有很好的代表性. 在水平方向上, 冻土退化在多年冻土下界附近的零星冻土分布区、融区边缘和岛状冻土区表现得更为明显. 当最大季节融化深度超过最大季节冻结深度时, 冻土开始下引式退化; 通常形成融化夹层, 造成多年冻土和季节冻结层不衔接. 当多年冻土层中地温梯度减小到小于下伏或周边融土层时, 则产生上引式或侧引式退化. 下引式退化进程可分为4个阶段: (1) 初始退化阶段, (2) 加速退化阶段, (3) 融化夹层阶段, (4) 最终多年冻土彻底融化为季节冻土阶段. 当多年冻土中地温梯度降至下伏融土层地温梯度以下时, 则产生上引式退化. 3种类型冻土温度曲线(稳定型、退化型和相变过渡型)展现了这些退化模式. 虽然存在不同地段和类型的地温特征, 三种退化模式的各种组合最终将使多年冻土消融, 转变成季节冻土. 过去25年来, 青藏线冻土年平均下引式退化速率变化在6~25 cm, 年平均上引式退化速率在12~30 cm, 零星多年冻土区年平均侧引式退化速率为62~94 cm. 这些观测结果超过所报道的过去20年来阿拉斯加亚北极不连续冻土区4 cm的年平均退化速率, 蒙古国不连续冻土区的4~7 cm的年平均退化速率, 以及雅库悌共和国亚北极和阿拉斯加北极稳定性冻土区退化速率.     

多年冻土区道路工程病害类型及特征研究

多年冻土及多年冻土区恶劣的环境给工程构筑物的建设及维护带来了极大的挑战。以青藏工程走廊内的青藏公路、青藏铁路两大道路工程为研究对象,系统论述了其路基工程、桥梁工程、涵洞工程的主要病害类型及分布特征。现场调查显示,青藏公路80%的路基病害由多年冻土融沉所引起,主要表现为严重的不均匀沉降变形和纵向裂缝,且主要发生在高填方路基上。这两种病害与多年冻土地温及含冰量密切相关,地温越高,含冰量越大,病害越为严重。青藏公路桥梁工程的病害主要集中在附属工程及上部结构上,而涵洞工程病害则主要由冻胀、融沉作用、泥石流淤积、冰塞以及施工原因导致。青藏铁路沿线现场监测及调查结果表明,目前铁路路基病害主要为高温冻土区普通路基的（不均匀）沉降变形、纵向裂缝、路桥过渡段沉降变形、风沙灾害及块碎石材料风化引起的冷却路基结构失效等。此外,青藏工程走廊内广泛分布着包括冻胀丘、冰椎、冰幔、热融湖塘等不良冻土地质现象,当上述不良地质现象与工程接近时,会对道路工程的稳定性造成威胁,严重时可导致一些工程病害的发生。     

探地雷达在多年冻土工程地质勘察中的应用效果研究

由于融土和冻土之间存在明显的电性差异,使探地雷达成为研究多年冻土的有效手段之一.本文结合工程实例,对探地雷达在青藏高原多年冻土工程地质勘察中的应用效果进行了现场试验研究.分析了探地雷达探测多年冻土的物理前提条件,总结了多年冻土主要地质要素的雷达图像特征和探地雷达在多年冻土工程地质勘察中的实际应用效果.研究表明融土与冻土的雷达图像特征存在明显差异:融土雷达反射波为低频强宽振幅的稀疏波,波形较杂乱;冻土反射波为高频低振幅细密波,波形较为规则;探地雷达可较为准确地划分地层、识别多年冻土上限、确定多年冻土分布范围,但尚不能有效确定多年冻土的含冰量;另外,探地雷达对细颗粒土的探测效果要明显好于粗颗粒土.文章还指出了现场探测和图像解译分析中需要注意的事项和下一步需要研究解决的问题,建议在开展现场探测工作之前先在有钻探资料或天然地质剖面处进行对比试验,搞清测区内主要地层的物性参数,掌握有效波和干扰波的分布规律,从而提高雷达探测结果的准确性和可靠性.     

青藏高原多年冻土退化过程及方式

气候变暖势必引起多年冻土的退化,基于数值模拟结果,将多年冻土退化过程按地温的深度剖面曲线形态划分为初始阶段、升温阶段、0梯度阶段、不衔接阶段和消失阶段．青藏高原多年冻土多是晚更新世残留,而全新世期间总体上是一个退化过程．根据青藏高原几个典型地区多年冻土深孔测温数据,判断目前高原多年冻土在其退化历史中所处的地位：高山地区处于升温阶段;中低山地区处于升温阶段末期;高平原和河谷盆地的多年冻土处于0梯度阶段;连续多年冻土下界附近及岛状冻土地区,正处于由0梯度向不衔接阶段过渡,多年冻结层边缘在萎缩,处于消失阶段．多年冻结层消融（消失）存在自下而上和自上而下两种方向．在升温阶段,多年冻土层中的热通量小于来自下伏地层中的地热通量时,部分地热流用于多年冻土底板相变耗热,发生自下而上的消融,随着多年冻土层中的地温梯度减小,用于底板消融的热量增加,直到地温曲线完全达到0梯度时,所有的地热流都用于多年冻土层融化潜热消耗,但其上部同时存在“热补偿”和“季节补偿”作用可以延缓多年冻土的消失;对于低温厚层多年冻土,当地面温度升高至可以抵消热补偿效应时,活动层中出现热积累,厚度增加,直至出现不衔接现象,同时存在“季节反补偿”作用,加剧了这一过程．     

土工格栅在复合地基中对桩与土的作用机制数值分析研究

土工格栅加筋垫层复合地基近年来在处理软土路基中得到了广泛应用,但土工格栅在复合地基中对桩与土的作用机制,尚无系统研究。基于弹塑性摩尔库仑模型,采用PLAXIS有限元软件,详细分析了无土工格栅和有土工格栅在不同抗拉强度下对路基沉降、侧向位移,桩的轴力和剪力以及基础的应力扩散等影响,得到土工格栅能有效减少路基沉降及侧向位移等结论,对于路基工程中土工格栅强度的选取及其它应用具有一定的指导作用。     

地球物理测井在多年冻土厚度和地下冰调查中的应用

基于青藏高原多年冻土区三个钻孔的地球物理测井数据和钻孔编录资料,我们对多年冻土厚度和多年冻土层内地下冰与地球物理测井数据之间的关系进行了相关的分析研究.研究表明,当地层为土壤类型时,可以使用井径和侧向测井曲线来判断多年冻土层厚度;而当地层为致密的基岩时,不能使用上述两种测井曲线来判断多年冻土层厚度.此外,还可以使用长源距伽马-伽马曲线和侧向测井曲线来识别多年冻土层内部分地下冰层的位置,其前提条件是地下冰层具有一定的厚度,或即使厚度较薄,但连续出现.这一研究结果对于利用地球物理测井曲线来调查多年冻土情况具有一定的应用价值.     

The application of auto-temperature-controlled ventilation embankment in Qinghai-Tibet Railway

Auto-temperature-controlled ventilation embankment is an effective engineering measure for "cooling roadbed". Practice proves that this new method can sufficiently make use of natural cold energy. It has the advantages of higher efficiency, better cooling effect and feasibility in engineering practice, and wider application in various environment, etc. And also, it is comparatively cheap in project cost. Through practice in the field for half a year, the testing results show that, with the application of auto-temperature-controlled system, the artificial permafrost table has been raised by 65 cm. The artificial permafrost table was basically at the embankment bottom, and the action of freeze-thaw circle on engineering stability was effectively avoided. In the month with highest ground temperature, in the scope with 1-4 m in depth, including the majority of the embankment and the upper part in the original seasonal layer, the ground temperature decreased by 0.7℃. Through thermal flux calculation in the original seasonal layer, in the month with the maximum thermal flux coming into permafrost, it is found that the thermal flux reduces nearly by half. Coming into the cooling period for nearly a month, the ground temperature in entire auto-temperature-controlled embankment is close to zero, and the foundation is at negative temperature. But in a large region in the embankment and foundation the ground temperature was over 0℃ and varied from 0℃ to 0.39℃ in ordinary ventilation embankment.     

The application of auto-temperature-controlled ventilation embankment in Qinghai-Tibet Railway

Auto-temperature-controlled ventilation embankment is an effective engineering measure for “cooling roadbed”. Practice proves that this new method can sufficiently make use of natural cold energy. It has the advantages of higher efficiency, better cooling effect and feasibility in engineering practice, and wider application in various environment, etc. And also, it is comparatively cheap in project cost. Through practice in the field for half a year, the testing results show that, with the application of auto-temperature-controlled system, the artificial permafrost table has been raised by 65 cm. The artificial permafrost table was basically at the embankment bottom, and the action of freeze-thaw circle on engineering stability was effectively avoided. In the month with highest ground temperature, in the scope with 1–4 m in depth, including the majority of the embankment and the upper part in the original seasonal layer, the ground temperature decreased by 0.7°C. Through thermal flux calculation in the original seasonal layer, in the month with the maximum thermal flux coming into permafrost, it is found that the thermal flux reduces nearly by half. Coming into the cooling period for nearly a month, the ground temperature in entire auto-temperature-controlled embankment is close to zero, and the foundation is at negative temperature. But in a large region in the embankment and foundation the ground temperature was over 0°C and varied from 0°C to0.39°C in ordinary ventilation embankment.     

Impact of heat advection on the thermal regime of roads built on permafrost

In northern regions, transportation infrastructure can experience severe structural damages due to permafrost degradation. Water infiltration and subsurface water flow under an embankment affect the energy balance of roadways and underlying permafrost. However, the quantification of the processes controlling these changes and a detailed investigation of their thermal impacts remain largely unknown due to a lack of available long-term embankment temperature data in permafrost regions. Here, we report observations of heat advection linked to surface water infiltration and subsurface flow based on a 9-year (from 2009 to 2017) thermal monitoring at an experimental road test site built on ice-rich permafrost conditions in southwestern Yukon, Canada. Our results show that snowmelt water infiltration in the spring rapidly increases temperature in the upper portion of the embankment. The earlier disappearance of snow deposited at the embankment slope increases the thawing period and the temperature gradient in the embankment compared with the natural ground. Infiltrated summer rainfall water lowered the near-surface temperatures and subsequently warmed embankment fill materials down to 3.6-m depth. Heat advection caused by the flow of subsurface water produced warming rates at depth in the embankment subgrade up to two orders of magnitude faster than by atmospheric warming (heat conduction). Subsurface water flow promoted permafrost thawing under the road embankment and led to an increase in active layer thickness. We conclude that the thermal stability of roadways along the Alaska Highway corridor is not maintainable in situations where water is flowing under the infrastructure unless mitigation techniques are used. Severe structural damages to the highway embankment are expected to occur in the next decade.     

多年冻土区青藏铁路列车荷载作用下路基振动响应研究

针对多年冻土区青藏铁路列车荷载振动作用下的动稳定性,通过对北麓河和二道沟三个典型铁路路基横断面振动响应的二分量加速度观测,对比分析客运列车和货运列车引起的路基振动特性和衰减规律,研究不同防护形式路基的列车振动响应。结果表明,路基上的振动作用主要集中在40～80Hz频率范围内;防护形式对路基的列车动力响应有明显影响,热棒加碎石路基动力响应最小,其次为碎石防护路基,未采取任何防护的路基铁轨上的动力响应最大,建议对未采取防护的路基进行防护。分析结论为青藏铁路列车作用下的路基动稳定性评估提供实测依据,对多年冻土区的路基稳定性研究提供参考。     

Seasonal differences in seismic responses of embankment on a sloping ground in permafrost regions

Because of its direct influence on the amount of unfrozen water and on the strength of intergranular ice in a frozen soil, temperature has a significant effect on all aspects of the mechanical behavior of the active layer in which temperature fluctuates above and below 0 °C. Hence seismic responses of engineering structures such as embankment on a sloping ground in permafrost regions exhibit obvious differences with seasonal alternation. To explore the distinctive seismic characteristics of a railway embankment on the sloping ground in permafrost regions, a coupled water-heat-dynamics model is built based on theories of heat transfer, soil moisture dynamics, frozen soil mechanics, soil dynamics, and so on. A well-monitored railway embankment on a sloping ground in Qinghai–Tibet Plateau is taken as an example to simulate seismic responses in four typical seasons in the 25th service year. The numerical results show that seismic acceleration, velocity and displacement responses are significantly different in four typical seasons, and the responses on October 15 are much higher among the four seasons. When the earthquake is over, there are still permanent differential deformations in the embankment and even severe damages on the left slope on October 15. Therefore, this position should be monitored closely and repaired timely to ensure safe operation. In addition, the numerical model and results may be a reference for maintenance, design and study on other embankments in permafrost regions.     

Numerical analysis for cooling effect of open boundary ripped-rock embankment on Qinghai-Tibetan railway

At present, the Qinghai-Tibetan railway is being built, and it will pass across more than 550-km perma-frost regions. Therefore, the key to the stability of therailway embankment lies in solving the permafrost problem. Because global warming and existence of railway tend to degrade the permafrost in these re-gions[1], more difficulties and problems are induced in the construction and maintenance of railway. In the area where the mean annual air temperature is higher than a certain value, the …     

Liquefaction-induced deformation of earthen embankments on non-homogeneous soil deposits under sequential ground motions

Damage of embankments during earthquakes is widely attributed to the liquefaction of foundation soil. Previous studies have investigated the dynamic response of embankments by mainly considering uniform sand foundation and a single earthquake event. However, the foundation of an embankment consists of many sublayers of soil from liquefiable sand to relatively impermeable layer, and during earthquakes a mainshock may trigger numerous aftershocks within a short time which may have the potential to cause additional damage to soil structures. Accordingly, the investigation of liquefaction-induced deformation of earthen embankments on various liquefiable foundation conditions under mainshock–aftershock sequential ground motions is carried out by a series of dynamic centrifuge tests in this study. The liquefiable foundation includes uniform sand profile, continuous layered soil profile, and non-homogeneous soil profiles. Effects of various foundation conditions on embankment deformations are compared and analyzed. From the test results, it is found that the embankment resting on non-homogeneous soil deposits suffer more damage compared to the uniform sand foundation of same relative density. The test results also suggest that the sequential ground motions have a significant effect on the accumulated deformation of embankment.     

Ground-temperature controlling effects of duct-ventilated railway embankment in permafrost regions

Based on observed data from field-testing embankment of the Qinghai-Tibet Railway, ground-temperature controlling effect of duct-ventilated embankment is studied in this paper.The results show that ventilation ducts can effectively cool the soils surrounding the ducts of the embankment, and the heat budget of the ambient soils in a year shows as heat release. Temperature status of the permafrost below the embankment with ducts buried in the relatively high position is similar to that of the common embankment. The permafrost processes warming all along in the two freezing-thawing cycles when the embankment was constructed. However, the temperature of the frozen soils below the embankment, in which the ducts buried in the relatively low position, rises a little in the initial stage. After that, it cools down gradually. At the same time,ventilation ducts can effectively reduce the thermal disturbance caused by the filled soils. The frozen soils below the common embankment and that with high-posited ducts absorb heat all along in the initial two cycles. While the soils below the embankment with low-posited ducts begin to release heat in the second cycle. This phenomenon proves that the ventilation embankment with low-posited ducts shows efficient temperature-controlling effect. Such embankment can actively cool the subgrade soils and therefore keeps the roadbed thermally stable.     

Ground-temperature controlling effects of ductventilated railway embankment in permafrost regions

Based on observed data from field-testing embankment of the Qinghai-Tibet Railway, ground-temperature controlling effect of duct-ventilated embankment is studied in this paper. The results show that ventilation ducts can effectively cool the soils surrounding the ducts of the embankment, and the heat budget of the ambient soils in a year shows as heat release. Temperature status of the permafrost below the embankment with ducts buried in the relatively high position is similar to that of the common embankment. The permafrost processes warming all along in the two freezing-thawing cycles when the embankment was constructed. However, the temperature of the frozen soils below the embankment, in which the ducts buried in the relatively low position, rises a little in the initial stage. After that, it cools down gradually. At the same time, ventilation ducts can effectively reduce the thermal disturbance caused by the filled soils. The frozen soils below the common embankment and that with high-posited ducts absorb heat all along in the initial two cycles. While the soils below the embankment with low-posited ducts begin to release heat in the second cycle. This phenomenon proves that the ventilation embankment with low-posited ducts shows efficient temperature-controlling effect. Such embankment can actively cool the subgrade soils and therefore keeps the roadbed thermally stable.     

机车动荷载作用下多年冻土区铁路路基动力响应的试验研究

青藏铁路穿越多年冻土区因地温和含冰量的不同而采取了不同的路基结构形式,以减小或避免气温和工程扰动对其下部多年冻土的影响。为了把握多年列车振动荷载作用下多年冻土区不同结构路基的动力响应特征,对青藏铁路北麓河段典型结构路基进行了实时强震动测试,得到了多年冻土区铁路路基的振动加速度衰减规律,对于研究机车动荷载对青藏铁路多年冻土区路基变形的定量影响规律,保障青藏铁路安全运营具有重要的科学意义。