Cooling Effect of Crushed Rock-Based Embankment along the Chaidaer-Muli Railway

Hindawi Publishing Corporation Advances in Materials Science and Engineering Volume 2015, Article ID 182437, 8 pages http://dx.doi.org/10.1155/2015/182437 Research Article Cooling Effect of Crushed Rock-Based Embankment along the Chaidaer-Muli Railway Ji Chen,1 Yu Sheng,1 Yaling Chou,2 Lei Liu,3 and Bo Zhang4 1 State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Lanzhou 730000, China 2 College of Civil Engineering, Lanzhou University of Technology, Lanzhou 730050, China 3 Qinghai Research Institute of Transportation Science and Technology, Xining 810000, China 4 China Railway First Survey & Design Institute Group Co., Ltd., Xi’an 710043, China Correspondence should be addressed to Ji Chen; Received 1 May 2015; Revised 20 July 2015; Accepted 28 July 2015 Academic Editor: Bert Blocken Copyright © 2015 Ji Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper presents an experimental study of the cooling effect of crushed rock-based embankment on slope wetlands along the Chaidaer-Muli Railway. The result shows that only the embankment shady side can be effectively cooled down in a warm permafrost region and the crushed rock-based embankment can cool the entire embankment in a cold permafrost region. The crushed rockbased embankment cannot eliminate the problems from the south-north slope. Slope wetland can influence the temperature field of the crushed rock-based embankment. On the uphill side, it will lead to degradation in some cases and development of permafrost in other cases, which depends on the topsoil water content and ground surface runoff. On the downhill side, it always leads to the warming of permafrost. For crushed rock-based embankment constructed on slope wetlands, it is necessary to adopt other stronger measures to eliminate the sunny-shady slope problems. 1. Introduction The crushed rock-based embankment can provide active protection for permafrost and has been widely used in the construction of railways in permafrost regions. In the former Soviet Union, the crushed rock-based embankment was studied since the 1960s. Crushed rock was adopted as the roadbed filler to maintain the permafrost table along the Baikal-Amur Mainline and good results were achieved [1]. In the 1960s, 1970s, and 1980s, experimental studies on the crushed rock-based embankment were also carried out in Fenghuoshan Mountain, Rehui coal mine, and Heilongjiang Province in China [2–4]. Field data revealed that equivalent coefficient of heat conductivity of the crushed rock-based embankment in winter is approximately 5–10 times that in the summer [5, 6]. Mean annual ground temperature (MAGT) under the crushed rock-based embankment is much lower than that under the ordinary embankment [2]. After the year 2000, crushed rock-based embankments have been adopted in many highway and railway engineering. There are 130 km crushed rock-based embankments for Qinghai-Tibet Railway and 72 km for Gonghe-Yushu expressway [7, 8]. To study the heat-transfer mechanisms, engineering effects, and crushed rock-based embankment design parameters, Goering et al. [9–12] performed extensive studies on the crushed rock cooling mechanism and related technical problems based on a combination of model tests, numerical simulations, and field experiments. His study revealed that crushed rock-based embankment reduces the amount of heat absorbed by the underlying permafrost from the atmosphere in the summer and increases heat releases in winter. Lai et al. [13] established numerical models with the assumption that the air temperature would increase by 2.0∘ C over 50 years to analyze how the permafrost temperature will change under the traditional ballast embankment and the crushed rockbased embankment. The results showed that the crushed rock-based embankment can provide a good cooling effect to the permafrost. 2 Advances in Materials Science and Engineering Table 1: The geographic, geologic, and engineering characteristics of the monitoring section of crushed rock-based embankment. Geomorphic unit Northern slope of Datong Mountain North bank of Datong river North bank of Xuzhigequ river North bank of Duosuoqu river North bank of Duosuoqu river Milestone DK40+000 DK74+000 DK94+900 DK123+150 DK123+250 EH/m 6.0 8.0 4.5 4.5 4.5 PT/m 1.0 1.0 1.3 1.4 1.0 SG/∘ <5 <5 7 3 3 Strike/∘ 320 300 345 260 260 MAGT/∘ C −0.65 −0.55 −1.31 −1.18 −1.18 Ice features Ice-saturated Ice-rich Ice-saturated Ice-saturated Ice-saturated Notes: EH: embankment height, PT: permafrost table, SG: slope gradient, and MAGT: mean annual ground temperature. Although many simulation and experimental results indicate that the crushed rock-based embankment has played positive roles in cooling the underlying permafrost, some researchers have questioned its wide application. Through field investigations, Wu et al. [14] showed that there is better thermal stability when the temperature near the permafrost table under the crushed rock-based embankment decreases greatly in cold permafrost regions. But, in warm permafrost regions, thermal balance of permafrost will be disturbed, and it is disadvantageous to the stability of frozen soil embankment. Therefore, it is worth discussing the applicability of the crushed rock-based embankment under different permafrost conditions including the MAGT and ice content. About 53 km crushed rock-based embankments are used along the Chaidaer-Muli Railway (CMR) in both cold and warm permafrost regions. Moreover, the railway extends from southeast to northwest and mainly runs across slope wetlands. The embankment sections are asymmetric and the sunny-shady slope phenomenon cannot be neglected. Therefore, the engineering effect of the crushed rock-based embankment becomes more complex in the CMR. 2. Materials and Methods 2.1. Site Description. The CMR is the first local railway of Qinghai Province, China, located in the north of the QinghaiTibet Plateau and in the central part of the Qilian Mountains (Figure 1). It extends 142 km from the Reshui coal mine to the town of Muli. The elevations along the railway range from 3600 m to 4100 m. According to the meteorological data of the past several years, the minimum and maximum air temperatures are −40∘ C and 17∘ C, respectively. The annual precipitation is approximately 500 mm. The railway traverses slope wetland, where the freezing and thawing grass hummocks are fully developed and the vegetation fraction is generally more than 90% (Figure 2(c)). The permafrost is discontinuous and unstable with an MAGT range of approximately −1.5∘ C to 0∘ C. The ice content in most permafrost regions is generally relatively high. Previous studies found that cooling effect of crushed rock-based embankment may be influenced by the MAGT. (...truncated)