Porosity of crushed rock layer and its impact on thermal regime of Qinghai-Tibet Railway embankment

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 design, construction and maintenance of CRE in permafrost regions.

Variations in the northern permafrost boundary over the last four decades in the Xidatan region, Qinghai–Tibet Plateau

The distribution and variations of permafrost in the Xidatan region, the northern permafrost boundary of the Qinghai-Tibet Plateau, were examined and analyzed using ground penetrating radar(GPR), borehole drilling, and thermal monitoring data. Results from GPR profiles together with borehole verification indicate that the lowest elevation limit of permafrost occurrence is 4369 m above sea level in 2012. Compared to previous studies, the maximal rise of permafrost limit is 28 m from 1975 to 2012. The total area of permafrost in the study region has been decreased by 13.8%. One of the two previously existed permafrost islands has disappeared and second one has reduced by 76% in area during the past ~40 years. In addition, the ground temperature in the Xidatan region has increased from 2012 to 2016, with a mean warming rate of ~0.004℃ a^(-1) and ~0.003℃ a^(-1) at the depths of 6 and 15 m, respectively. The rising of permafrost limit in the Xidatan region is mainly due to globalwarming. However, some non-climatic factors such as hydrologic processes and anthropic disturbances have also induced permafrost degradation. If the air temperature continues to increase, the northern permafrost boundary in the Qinghai-Tibet Plateau may continue rising in the future.

Performance comparison of permafrost models in Wudaoliang Basin,Qinghai-Tibet Plateau,China

Knowledge of the spatial distribution of permafrost and the effects of climate on ground temperature are important for land use and infrastructure development on the Qinghai-Tibet Plateau(QTP). Different permafrost models have been developed to simulate the ground temperature and active layer thickness(ALT). In this study, Temperature at Top of Permafrost(TTOP) model, Kudryavtsev model and modified Stefan solution were evaluated against detailed field measurements at four distinct field sites in the Wudaoliang Basin to better understand the applicability of permafrost models. Field data from 2012 to 2014 showed that there were notable differences in observed ground temperatures and ALTs within and among the sites. The TTOP model is relatively simple, however, when driven by averaged input values, it produced more accurate permafrost surface temperature(Tps) than the Kudryavtsev model. The modified Stefan solution resulted in a satisfactory accuracy of 90%, which was better than the Kudryavtsev model for estimating ALTs. The modified Stefan solution had the potential of being applied to climate-change studies in the future. Furthermore, additional field investigations over longer periods focusing on hydrology, which has significant influence on permafrost thaw, are necessary. These efforts should employ advanced measurement techniques to obtain adequate and extensive local parameters that will help improve model accuracy.

Cryostructures and ground ice content in ice-rich permafrost area of the Qinghai-Tibet Plateau with Computed Tomography Scanning

Permafrost is an important part of the cryosphere,playing an integral role in the hydrologic cycle,ecology,and influencing human activity.Melting of ground ice can drastically change landscapes and associated thaw subsidence may induce instability of infrastructure.The terrain conditions on the Qinghai-Tibet Plateau are complex,and the spatial distribution of ground ice is highly variable,so knowledge of its abundance and variability is required for impact assessments relating to the degradation of permafrost.This study examined 55 permafrost samples from warm,ice-rich permafrost region in Beiluhe Basin,Qinghai-Tibet Plateau.The samples were examined using Computed Tomography scanning,and the ice content and cryostructure were determined.The results indicated that:1)variation in volumetric ice content was considerable(0%-70%),with a mean value of 17%;2)seven cryostructures were identified,including crustal,vein,lenticular,ataxitic,reticulate and layered cryostructure;3)volumetric ice content varied by cryostructure,with the highest associated with layered and ataxitic cryostructures.Volumetric ice contents were lowest for samples with pore and lenticular cryostructures.This work provides detailed ground ice content and will be helpful for assessing thaw subsidence and infrastructure stability on Qinghai-Tibet Plateau.

保温法结合弃渣换填防治季节冻土山区软基冻害的适用性

为更好地解决季节冻土山区突出的软基冻害难题,提出在弃渣碎石换填层上铺设保温板的“保温换填法”。基于工程实例,运用带冰水相变的考虑路基阴阳坡效应的有限元数值解法,模拟分析XPS保温板在不同的埋深、厚度,以及考虑冻融循环作用和地下水对XPS保温板导热系数不利影响(最不利工况)的条件下,路基不同部位最大季节冻深和路基阴、阳坡路肩横向热差异在30年内随时间的变化。结果表明:“保温换填法”在抬升路基不同部位的最大季节冻深和控制路基阴、阳坡路肩横向热差异方面都具有良好的效果,但具体效果与路堤高度有关,路堤高度越小效果越好;综合考虑经济性和长期保温效果,路面下0.8 m是XPS保温板合理的埋设深度,8 cm厚是合理的铺设厚度,按《公路路基设计手册》公式确定的6 cm厚在季节冻土山区路基中偏于不安全;同时,“保温换填法”也适用于季节冻土山区地下水位埋深浅的地段。

换填与降排水措施对寒区沟谷软弱路基冻结特征的影响

基于甘肃南部宕昌-迭部二级公路,选取了2个典型寒区沟谷软土路基试验段,监测了2个冻融期内路基温度、含水量、变形以及地下水位,分析了弃渣换填深度与降排水措施对路基冻结特征的影响。分析结果表明:在监测的2个冻结期内,换填深度为2.0m的试验段K18+180的冻结深度比换填深度为1.0m的试验段K18+330的冻结深度大0.12~0.16m,说明换填深度越大,冻结深度越大;K18+330段初始地下水位为3.4m,仅设置地表排水沟时,冻结期间地下水位稳定在3.4m左右,距冻结面的最小距离为1.7m,说明设置排水沟时地下水位在冻结期间基本没有变化;K18+180段初始地下水位是1.3m,在设置了渗沟降水措施后,冻结期间地下水位稳定在2.0m左右,距冻结面的最小距离为0.2m,地下水位降低了约0.7m,因此,渗沟降水可以降低地下水位,防止路基冻胀;K18+180段路基中心2个周期监测的最大冻胀分别为3.4、4.2mm,而K18+330段相应位置的最大冻胀分别为10.7、14.0mm,后者均是前者的3倍多,说明换填深度越大路基冻胀越小;《公路路基设计规范》(JTG D30—2015)规定的二级公路容许冻胀为50mm,软土路基容许工后沉降为500mm,K18+180、K18+330段路基的最大沉降分别为1.5、1.8mm,最大冻胀分别为4.2、14.0mm,远远小于规范值,表明试验段路基的稳定性良好,采用换填与降排水措施能有效控制路基冻胀。

Data-driven spatiotemporal projections of shallow permafrost based on CMIP6 across the Qinghai‒Tibet Plateau at 1 km^(2) scale

The degradation of near-surface permafrost under ongoing climate change on the Qinghai‒Tibet Plateau (QTP) is of growing concern due to its impacts on geomorphological and ecological processes, as well as human activities. There is an increased need for an in-depth understanding of the evolution of permafrost temperature (Ttop) and active-layer thickness (ALT) at a fine scale on the QTP under climate change. This study evaluated the permafrost thermal development over the QTP for the period 1980–2100 at a 1 km^(2) scale using a physically analytical model accounting for both climatic and local environmental factors based on multi-source data. The model results were validated against thermal borehole measurements and baseline maps. The modeled current (2001–2018) permafrost area (Ttop ≤ 0 ℃) covers 1.42 × 10^(6) km^(2) (ca. 56.1% of the QTP land area), 10.1% of which thawed over the historical period 1981–2000. To assess how the ground thermal regime could develop in the future, we utilized the multi-model ensemble mean of downscaled outputs from eight climate models under three Shared Socio-economic Pathways (i.e., SSP126, 245, and 585) in CMIP6 to force the permafrost model. Model results suggest that the current (2001–2018) permafrost extent is likely to dramatically contract in the future period (2021–2100), as indicated by consistent Ttop warming and ALT increasing due to climate changing. About 26.9%, 59.9%, 80.1% of the current permafrost is likely to disappear by the end of the 21st century under SSP126, SSP245, and SSP585 scenarios, respectively. The simulation results may further provide new opportunities to assess the future impacts of climate warming on environments and engineering development over the QTP.