Knowledge Resource Center for Ecological Environment in Arid Area
| 风沙及升温对青藏铁路块石路基服役性能的影响研究 | |
| 其他题名 | Long-term Service Performance of Block-stone Embankment of Qinghai-Tibet Railway under the Scenario |
| 陈琳 | |
| 出版年 | 2015 |
| 学位类型 | 硕士 |
| 导师 | 俞文兵 |
| 学位授予单位 | 中国科学院大学 |
| 中文摘要 | 随着人类活动，尤其是工程活动的加剧及气候变暖的影响，导致青藏高原线性工程两侧多年冻土退化、植被退化、沙漠化趋势增强。目前，铁路沙害呈现出迅速增长的趋势，青藏公路、铁路全线处于不同沙漠化区域中，受到不同程度的沙害。在气候变暖和风沙堆积填堵条件下，块石路基这一“主动冷却”路基核心技术的服役性问题亟待解决。本文在研究风积沙的热物理特性基础上，通过现场试验、室内模型试验和数值方法对风积沙条件下冻土地温变化、块石结构的降温性能及块石路基的长期服役性进行了评估和预测。为应对不利环境下块石路基结构降温性能下降的问题，提出了在块石路堤坡脚处斜插热管的补强措施，并通过数值模拟方法对其效果进行了初步验证。以期为多年冻土区沙害地段块石路基的病害治理以及拟建青藏高速公路风沙穿越区病害预防方案选择提供科学依据、为评价预测青藏高原沙化对冻土环境影响提供参考。得到的主要结论如下： (1) 风积沙颗粒相互支承，呈点与点的接触，颗粒间孔隙很大，磨圆度高，分选好，呈类球形。表面的撞击坑、擦痕使得颗粒的比表面积增大，连通性增强，孔隙率增加，保水性较差，渗透性较强；干燥状态的风积沙相互接触面积减小，空气填充孔隙，其换热的主要方式是固体颗粒的导热，导热系数较低；而湿润状态下，正温时孔隙由水填充，间接增大了风积沙的接触面积，导致导热系数增大；负温时孔隙内的水变成冰，从而导致导热系数进一步增大。 (2) 为了探究风积沙对多年冻土温度的影响，通过室内模型试验与数值模拟方法对风积沙等不同下垫面类型对多年冻土温度的影响过程及机理进行了研究。模型试验结果表明，草皮能够有效的削弱辐射灯发射的长波红外辐射，具有一定的隔热保温作用，地表草皮覆盖有利于多年冻土的赋存；地表沙化后，土体底部温度增高。沙层具有隔热保温作用，湿沙吸放热能的能力强于干沙；薄沙层覆盖条件下土体底部温度较厚沙层覆盖条件下低。数值模拟结果表明干、湿沙状态下，沙层覆盖后人为冻土上限均抬升，且随着覆盖厚度增加，抬升幅度逐渐增大；而干沙层覆盖后下部冻土温度升高，湿沙覆盖后下伏冻土温度降低。 (3) 块石结构冷却技术的长期性现场试验结果表明封闭块石层内部传热方式主要为自然对流，且自然对流仅发生在冷季，对流强度不仅与块石层顶底温差有关，而且受块石层有效对流高度的影响。随着填堵厚度的增加，块石层内有效对流高度逐渐减小，临界温差值逐渐增大，自然对流越不容易产生，块石的降温性能减弱，对块石层的服役性能极为不利。初始年平均气温为-3.20 ℃，气温升温率为0.026 ℃?a-1条件下，块石层内部瑞利数随运行时间的增长而逐渐减小，自然对流减弱，冻土上限降低；对于1.5 m块石层被风积沙填堵0.6 m后，能够抵御升温1.04 ℃的影响，若气温进一步升高，块石层降温效果减弱，多年冻土将处于退化状态。 (4) 在年平均气温为-4.5 ℃条件下，开放块石路基受外界风的作用，其内部流速较大，对流换热方式以强迫换热为主，路基下土体的降温效果明显，土体储存冷能较多，能够有效发挥冷却路基、降低多年冻土地温的作用，并且路基下冻土热状况稳定，能够抵御50年气温升高2.6 ℃的影响。开放块石路堤坡脚块石路层被风沙填堵覆盖后，降温效果显著下降，冻土上限下降，年平均地温升高，路基下冻土处于较高温度状态，对路基热稳定产生不利影响。 (5) 高温多年冻土区，风积沙填、堵覆盖对开放块石路基降温效果影响较大，路基中心下部冻土上限显著下降，年平均地温上升。低温冻土区，风积沙填、堵覆盖对块石路基下冻土上限影响较小，但年平均温度上升幅度较大。对于不同含冰量类型的冻土而言，随着含冰量的增加，冻土上限下降幅度越小，而在年平均气温相同的情况下，年平均地温随含冰量的变化幅度较小。在气候变暖和风沙作用下，少冰冻土对外界环境变化的响应较大，冻土上限明显下降。 (6) 对于封闭块石路基而言，薄沙层覆盖对路堤坡脚下部冻土上限影响较小，厚沙层覆盖后路堤坡脚下部冻土上限抬升明显。干沙层具有隔热保温作用，干沙层下存在着热积累，干沙层在抬高人为上限的同时，下伏冻土温度也普遍升高，这对在高温高含冰量区冻土路基的热稳定性极为不利；而湿沙层覆盖后路堤坡脚下部冻土上限也有较大抬升，且下伏冻土温度降低。 (7) 为应对不利环境下块石路基结构降温性能下降的问题，提出了在路堤坡脚处插入热管的补强措施。热管制冷效果显著，路基内部以及热管周围多年冻土的温度得到显著降低，人为冻土上限明显抬升，大幅提高了路基的热稳定性，可以缓解风积沙填堵覆盖造成的局部升温问题。在年平均增温率为0.052 ℃.a-1，年平均气温为-4.5 ℃条件下，通过对气候变暖和风沙背景下热管-块石复合路基未来温度场进行预测分析，发现热管-块石复合路基能够有效的降低路基下土体温度，能够抵御气候变暖和风沙层的影响，维持路基下多年冻土稳定，确保冻土路基的长期热稳定性。 |
| 英文摘要 | The combined influence of climatic warming and increasing anthropogenic activities, particularly engineering construction in permafrost regions, accelerate the degradation of permafrost and steppe vegetation, and expand areas of desertification. Recently, the aeolian sand damages have occurred at some sites along the Qinghai-Tibet railway and highway. Block-stone embankment as a mainstream technology has been extensively used to keep permafrost stable on the Tibet Plateau. Now a new problem has to be faced, i.e., aeolian sand fills the pores spaces of rock layer and what will this bring to effect variation of lowering permafrost thermal regime beneath block-stone embankment? On the basis of Qinghai-Tibet railway engineering, we conducted a series of programs on the effects of desertification on the ground temperature of permafrost and impacts of aeolian sand to cooling effects and the long-term thermal stability of block-stone embankment in permafrost regions by means of in-situ experiments, model tests and numerical simulation. Then we propose reinforcement measures of a two-phase closed thermosyphons stalled in the slope of embankment to maintain the thermal stability of block-stone embankments, and several simulated cases were conducted to predict reinforcing effect of thermosyphons. It is expected to provide key technical supports and a scientific basis for reinforcing measures of effect variation of lowering permafrost temperature beneath block-stone embankment after block-stone layer is filled by sand and the infrastructure design of Qinghai-Tibet high-grade highway. The main conclusions are as follows: (1) Thermal conductivity is the key parameter to predict the effects of sand sediments on the ground temperature of permafrost. The variation mechanism thermal conductivity of aeolian sand and scanning electron microscope / energy dispersive X-ray (SEM / EDX) laboratory tests were studied. The granulometric analysis of grain shows that the grain sizes are 75–500 μm, evenly graded poor, and it has few clay and gravel -free ingredients. The aeolian sand grows and forms a sand dune without any regular shape in natural state, and with large particle porosity, high grain roundness and good sorting. The grain surface has obvious impact craters, which noticeably increase the specific surface area and porosity. The mutual contact area of grain is also reduced, then the pores are filled by air, all of these lead to the decrease of the value of thermal conductivity. (2) Model tests and numerical simulations were conducted to investigate the impact of aeolian sand on the ground temperature in permafrost regions. Physical results revealed that the grass layer was found to play a key role in protection of the underlying permafrost, because it can effectively weaken the influence of radiation of infrared radiation lamp. After the ground surface was covered by aeolian sand layer, the range of annual ground temperature and heat exchange below the aeolian sand layer was all reduced greatly. Numerical results showed that artificial permafrost table uplift after the aeolian sand accumulation, the thicker the sand layer is, the lower artificial permafrost table is. In addition, when the water content of aeolian sand layer is zero, ground temperature in the deeper layer is gradually higher than that of natural surface. While the ground temperature is lower when the sand layer is damp. The ground temperature decrease slightly with the increase of the thickness of aeolian sand layer. (3) In order to investigate the thermal regime of permafrost beneath aeolian sand-filled rock layer, the cooling characteristics and cooling mechanism of aeolian sand-filled rock layer were studied in in-situ experiments and numerical simulations. Both field experiment and numerical simulation reveal that natural convection in rock layer only occurs in winter season. With the increasing thickness of sand in rock layer, the critical temperature difference between the no sand-filling part of increases, and the Ra number decreases and natural convection intensity weakens gradually. Under the climate warming rate of 0.052 ℃?a-1 scenario, the cooling effect of rock layer can counteract the negative effect of climate warming and rise up the permafrost table in about the former 20 years in the area where the mean annual air temperature is -3.20 ℃. But finally, the permafrost will degrade continuously under greater warming scenario. (4) Based on a coupled model of convective heat transfer and heat conduction considering phase change and outside wind sand. The results show that extensive convection can be produced in the block-stone layer of open block-stone embankment and the cooling effect and long-term thermal stability of an open block-stone embankment are better under the climate warming rate of 0.052 ℃?a-1 scenario. After the block-stone layer filled with aeolian sand, it is clear that the cooling characteristic of block-stone embankment weaken by analyzing velocity variation in the rock layer, then the permafrost table decrease gradually. (5) The numerical results show that, with the climate warming rate of 0.052℃?a-1 scenario in the area where the mean annual are temperature (MAAT) is -4.5 ℃, the maximum thaw depth and the mean annual ground temperature beneath the block-stone embankment increase obviously after block-stone layer is filled by aeolian sand. While the corresponding figures for the area where the MAAT is -5.5 ℃, 6.5 ℃ is smaller. For different types of ice content in frozen soil, with the decrease of ice content, the artificial permafrost table decrease faster, while the annual mean ground temperature changes insignificantly. (6) Cooling mechanisms of closed embankments significantly depend on natural convection. After the surface of closed rock layer buried by thick aeolian sand layer, the artificial permafrost table beneath the block-stone embankment rise significantly. While the influence of thin aeolian sand accumulation on artificial permafrost table is smaller. In addition, when the water content of sand layer is zero, ground temperature in the deeper layer beneath the block-stone embankment is gradually increasing. While the ground temperature decrease when the sand layer is damp. (7) Numerical simulations are conducted to predict reinforcing effect of thermosyphons. When the block-stone embankment is reinforced with thermosyphon, artificial permafrost table gradually rises and ground temperature beneath the embankment decrease distinctly. In addition, numerical results show that the cooling effect of embankment with combined block-stone layer and thermosyphon is better. The embankment can overcome the influence of the global warming and aeolian sand damages, and ensure the thermal stability of embankment. |
| 中文关键词 | 青藏铁路 ; 块石路基 ; 风积沙 ; 升温 ; 多年冻土 |
| 英文关键词 | Qinghai-Tibet railway Block-stone embankment Aeolian sand Climate warming Permafrost |
| 语种 | 中文 |
| 国家 | 中国 |
| 来源学科分类 | 岩土工程 |
| 来源机构 | 中国科学院西北生态环境资源研究院 |
| 资源类型 | 学位论文 |
| 条目标识符 | http://119.78.100.177/qdio/handle/2XILL650/287540 |
| 推荐引用方式 GB/T 7714 | 陈琳. 风沙及升温对青藏铁路块石路基服役性能的影响研究[D]. 中国科学院大学,2015. |
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