Thermal regime of conventional embankments along the Qinghai–Tibet Railway in permafrost regions
Highlights
► Thermal regime of conventional embankments along the railway has been investigated. ► Investigation includes shallow ground temperatures and underlying permafrost changes. ► Three detrimental thermal performance of conventional embankment were summarized. ► The reasonable height range of conventional embankment was validated by filed data.
Introduction
From an engineering viewpoint, the bearing strength of permafrost is a function of its temperature (Esch and Osterkamp, 1990, Lunardini, 1996). Therefore, in order to reduce the risk of structure failure in a permafrost environment, thermal stability of the ground has to be the main goal (Harris et al., 2009). As a major linearity engineering on the Qinghai–Tibet Plateau (QTP), the Qinghai–Tibet Railway (QTR) was a landmark project in permafrost regions (Zhang et al., 2008). The successful construction of the railway benefited from detailed geotechnical investigation, advanced design idea, high-quality construction process and precise monitoring system during all project phases (Ma et al., 2011). In unstable permafrost regions, the railway adopted the advanced design idea, “cooled roadbed”, to proactively lower underlying ground temperature in the contexts of construction activities and climate warming (Cheng, 2003, Cheng, 2005, Ma et al., 2002). A number of active cooling measures including crushed rocks, thermosyphons, ventilation ducts, awnings and dry bridges were applied to cool the permafrost beneath embankment, and all of them performed satisfactorily at present (Cheng et al., 2008, Feng et al., 2006, Ma et al., 2006, Ma et al., 2009, Niu et al., 2006, Wu et al., 2008, Wu et al., 2010, Zhang et al., 2005). But, except these embankments with cooling measures, there were still some sections of the embankment constructed with conventional technique, which only placed gravel pad directly upon the undisturbed ground surface without any cooling measures, called as conventional embankment (Wu et al., 2007). The thermal regime of conventional embankment (CE) determines to a large extent the embankment performance relative to settlement and stability, and therefore need detailed investigation.
In order to prevent decline of underlying permafrost table after construction phase and formation of talik during the operation phase, CE should be built within a reasonable height range (Cheng et al., 1983, Wu et al., 1988, Wu et al., 1998). In cold permafrost regions (mean ground temperature close to − 11 °C), a gravel embankment thickness of about 1.5 m is usually adequate to prevent thawing of the underlying permafrost (Berg and Quinn, 1977). But on the QTP, permafrost is characterized by high mean annual ground temperature (MAGT), ranging between 0 and − 4.0 °C, and consequently by a weak thermal stability (Wu et al., 2010a). So only in regions with MAGT or mean annual air temperature (MAAT) lower than certain values, the reasonable height range of CE does exist. According to field observations of freezing and thawing degree days and n factors along the Qinghai–Tibet Highway, Ding and He (2000) pointed out that the certain value of MAAT was − 3.8 °C for embankment constructed with fine-grained soil. Combining with observation of experimental embankment at Beiluhe Basion, Zhang et al., 2005, Zhang et al., 2006 conducted some numerical simulations and proposed that the certain values of MAGT and MAAT were − 0.6 °C and − 3.5 °C, respectively. However, up to now, all these values had not been validated by engineering practice. Additionally, in locations where it is impractical to build a thicker embankment, placement of insulation in thinner embankment provides an alternative. Insulation layer was initially installed in roadways and airfield runways in North American (Esch, 1973). At Beiluhe Basin, some experimental embankments with insulation layer were constructed, but the results showed it was not suitable for warm permafrost regions with high embankment surface temperature (Sheng et al., 2003, Wen et al., 2008).
Another problem of CE that researchers and engineers need to take into consideration is thermal effect related to embankment side-slopes (Cheng et al., 2003). After embankment construction, the two side-slopes with different aspects receive different amounts of solar radiation and thermal difference between each other is the result. This thermal difference could be considerable since the combination of low latitude, high elevation, thinner atmosphere and higher atmosphere transparency makes the QTP one of the areas with strongest solar radiation on the earth (Chen et al., 2006). Based on filed experiments at Beiluhe Basin, Hu et al. (2002) studied the relationship between solar radiation and temperatures of the two side-slopes with different embankment orientations and proposed a method to calculate both of them. Sheng et al. (2005) observed temperatures of two side-slopes of an experimental embankment at Beiluhe Basin for one year, and concluded that high temperature of south-facing slope in winter contributed to a large extent to the thermal difference between south- and north-facing slopes. Chou et al., 2008a, Chou et al., 2008b researched the thermal difference between the two side-slopes and their influence on underlying permafrost table distribution by numerical simulations. Wu et al. (2010b) examined the freezing and thawing periods of side-slopes, average temperatures of active layer and permafrost tables beneath crushed rock embankments and CEs with different orientations along the QTR.
However, researches above always focused on active layer thickness and underlying permafrost table distribution but seldom cared about deep permafrost temperature beneath embankment. Investigation of crushed rock embankments along the QTR founded that the deep permafrost had some warming trends even when permafrost tables rise obviously in some regions (Ma et al., 2008, Mu et al., 2010, Wu et al., 2006b). These warming trends may be more considerable beneath CEs and consequently undermine the embankment stability. Secondly, researches about CE above generally conducted at some specific regions, such as Beiluhe Basin. Thus results from these researches could not be easily applied to other regions along the QTR since the complicated environment conditions and permafrost distribution on the QTP. In this paper, the thermal regime of CEs along the QTR in 550 km of permafrost regions was comparatively investigated based on ground temperature data compiled from 13 monitoring sites. Shallow ground temperatures within embankment, permafrost table variations and dynamic changes of deep permafrost temperatures beneath embankment were investigated to evaluate the thermal performances of CEs and their influences on long-term embankment stability. The investigation is essential for precise diagnosis of embankment failures and adoption of appropriate remedial measures, and meanwhile provides high-quality in-situ data series for numerical simulation in future.
Section snippets
Monitoring system and sites description
An important prerequisite to guaranteeing the longevity of infrastructure in permafrost regions is a sufficiently detailed preliminary study, and the most essential step is to determine the permafrost presence and its ice content (Bommer et al., 2010). The preliminary geotechnical investigation of permafrost along the QTR initiated during the 1960s and lasted about 40 years (Wu et al., 2002). In light of these field investigations and experiences from previous engineering constructions, − 1 °C was
Shallow ground temperatures within embankment and natural ground
After embankment construction, topography, surface cover (vegetation, snow, or water), and thermal properties of the substrate, differed radically from conditions existing prior to the construction. These changes would disrupt the energy balance at ground surface and result in changes of shallow ground temperature (Smith, 1975). Here, two monitoring sites located in cold and warm permafrost regions, respectively, were chosen to comparatively analyze ground temperatures at 50 cm depth within
Discussions
Analyses of ground temperatures within and beneath embankment indicated that, at present, three main detrimental performances of CEs presented and may need relevant remedial measures in future. Firstly, due to thermal effect of side-slopes, shallow ground temperatures within right and left embankment shoulders differed obviously from each other, and as a consequence, an asymmetrical ground temperature filed developed beneath CE. This asymmetrical temperature filed will lead to development of
Conclusions
The following points summarized the investigation of thermal regime of CEs along the QTR in permafrost regions.
- (1)
Shallow ground temperatures with embankment shoulders and nature ground differed obviously from each other. Average of the differences between ground temperatures at 50 cm depth within right and left shoulders was less than 1 °C in warm seasons but greater than 2.7 °C in cold seasons.
- (2)
In regions with MAGT < − 0.6 to − 0.7 °C, CE did have a reasonable embankment range; permafrost tables under
Acknowledgment
We would like to express our gratitude to two anonymous reviewers for their constructive comments and suggestions. This work was supported by the Western Project Program of the Chinese Academy of Sciences (No. KZCX2-XB2-10), Program for Innovative Research Group of Natural Science Foundation of China (No. 40821001), National Natural Science Foundation of China (Nos. 40801024, 41001041, 41071048), and the Innovation Research Foundation of Chinese Academy of Sciences (No. KZCX2-YW-QN307).
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