Ten years of ground temperature data(2003–2013) indicate that the long-term thermal regimes within embankments of the Qinghai-Tibet Railway(QTR) vary significantly with different embankment structures. Obvious asymmetries exist in the ground temperature fields within the traditional embankment(TE) and the crushed-rock basement embankment(CRBE). Measurements indicate that the TE and CRBE are not conducive to maintaining thermal stability. In contrast, the ground temperature fields of both the crushed-rock sloped embankment(CRSE) and the U-shaped crushed-rock embankment(UCRE) were symmetrical. However, the UCRE gave better thermal stability than the CRSE because slow warming of deep permafrost was observed under the CRSE. Therefore, the UCRE has the best long-term effect of decreasing ground temperature and improving the symmetry of the temperature field. More generally, it is concluded that construction using the cooling-roadbed principle meets the design requirements for long-term stability of the railway and for train transport speeds of 100 km h?1. However, temperature differences between the two shoulders, which exist in all embankments shoulders, may cause potential uneven settlement and might require maintenance.
NIU Fu unLIU MingHaoCHENG GuoDongLIN ZhanJuLUO JingYIN GuoAn
For the purpose of enhancing air convection and controlling solar radiation, a new crushed-rock slope embankment design combined with a sun-shade measure is proposed. A newly designed embankment was constructed in the Tuotuohe section of the Qinghai-Tibet Railway and a field-testing experiment was carried out to determine its convection and temperature characteristics. The results show that distinct air convection occurred in the crushed-rock layer of the new embankment, especially in cold seasons, which was enhanced when it flowed upwards along the slope. This preliminarily indicated that the new design of the embankment slope was good for reinforcing air convection in the crushed-rock layer. The frequent fluctuations of the convection speed and the environmental wind speed were in good agreement, suggesting that the convection in the crushed rock primarily came from the ambient wind. It was also preliminarily determined that the new embankment had a better cooling effect and sun-shade effect for decreasing the temperature of the embankment slope compared with a traditional crushed-rock slope embankment, and the mean temperature difference between them was up to 1.7 °C. The mean annual temperature at the bottom boundary of the crushed-rock layer was obviously lower than that at the top boundary, and heat flux calculation showed that the shallow soil beneath the embankment slope was weakly releasing heat, all of which indicated that the new embankment slope design was beneficial to the thermal stability of the embankment. This study is helpful in providing some references for improved engineering design and maintenance of roadbeds in permafrost regions.
It has been proven that crushed rock layers used in roadbed construction in permafrost regions have a cooling effect. The main reason is the existence of large porosity of the rock layers. However, due to the strong winds, cold and high radiation conditions on the Qinghai-Tibet Plateau(QTP), both wind-blown sand and/or weathered rock debris blockage might reduce the porosity of the rock layers, resulting in weakening the cooling effect of the crushed rock layer(CRL) in the crushed rock embankment(CRE) of the Qinghai-Tibet Railway(QTR) in the permafrost regions. Such a process might warm the underlying permafrost, and further lead to potential threat to the QTR's integrity and stability. The different porosities corresponding to the different equivalent rock diameters were measured in the laboratory using water saturation method, and an empirical exponential equation between porosity and equivalent rock diameter was proposed based on the measured experimental data and an important finding is observed in our and other experiments that the larger size crushed rock tends to lead to the larger porosity when arbitrarily packing. Numerical tests were carried out to study impacts of porosity on permafrost degradation and differential thaw depths between the sunny and shady shoulders. The results show that the decrease in porosity due to wind-blown sand or weathered rock debris clogging can worsen the permafrost degradation and lead to the asymmetric thermal regime. In the traditional embankment(without the CRL within it), the largest differential thaw depth can reach up to 3.1 m. The optimized porosity appears in a range from 34% to 42% corresponding to equivalent rock diameter from 10 to 20.5 cm. The CRE with the optimized porosities can make underlying permafrost stable and 0 ℃ isotherms symmetric in the coming 50 years, even under the condition that the climate warming can lead to permafrost degradation under the CRE and the traditional embankment. Some practical implications were proposed to benefit the future desi
