Summary of research on frost heave for subgrade in seasonal frozen ground

The building of railways on seasonally frozen ground is inevitable as China pursues economic development and the improvement of its citizens'living standards.However,railway construction in seasonally frozen soil areas is often faced with frost heave,leading to uneven subgrades which seriously threaten traffic safety.This article summarizes extant research results on frost heave mechanism,frost heave factors,and anti-frost measures of railway subgrades in seasonally frozen soil areas.

Different Responses of Vegetation to Frozen Ground Degradation in the Source Region of the Yellow River from 1980 to 2018

Frozen ground degradation under a warming climate profoundly influences the growth of alpine vegetation in the source region of the Qinghai-Tibet Plateau.This study investigated spatiotemporal variations in the frozen ground distribution,the active layer thickness(ALT)of permafrost(PF)soil and the soil freeze depth(SFD)in seasonally frozen soil from 1980 to 2018 using the temperature at the top of permafrost(TTOP)model and Stefan equation.We compared the effects of these variations on vegetation growth among different frozen ground types and vegetation types in the source region of the Yellow River(SRYR).The results showed that approximately half of the PF area(20.37%of the SRYR)was projected to degrade into seasonally frozen ground(SFG)during the past four decades;furthermore,the areal average ALT increased by 3.47 cm/yr,and the areal average SFD decreased by 0.93 cm/yr from 1980 to 2018.Accordingly,the growing season Normalized Difference Vegetation Index(NDVI)presented an increasing trend of 0.002/10 yr,and the increase rate and proportion of areas with NDVI increase were largest in the transition zone where PF degraded to SFG(the PF to SFG zone).A correlation analysis indicated that variations in ALT and SFD in the SRYR were significantly correlated with increases of NDVI in the growing season.However,a rapid decrease in SFD(<-1.4 cm/10 yr)could have reduced the soil moisture and,thus,decreased the NDVI.The NDVI for most vegetation types exhibited a significant positive correlation with ALT and a negative correlation with SFD.However,the steppe NDVI exhibited a significant negative correlation with the SFD in the PF to SFG zone but a positive correlation in the SFG zone,which was mainly limited by water condition because of different change rates of the SFD.

Frozen ground and periglacial processes relationship in temperate high mountains: a case study at Monte Perdido-Tucarroya area(The Pyrenees, Spain)

Seasonally frozen ground,mountain permafrost and cryogenic geomorphological processes are important components of the Pyrenean high mountains.This work presents the results of a study on the distribution of frozen ground in a marginal and paraglacial environment of temperate mountains.An inventory was made of landforms and indicators of frozen ground,and frozen ground was mapped accordingly.During 2014 and 2016 ground temperatures and thermal regimes were monitored,basal temperatures of snow-cover(BTS)were measured and a thermal map was drawn.Differential thermal behaviours were detected among different elevations and slope orientations.Periglacial processes are the most widespread,in which frost weathering and nivation,together with gelifluction and cryoturbation,are the most efficient processes;the latter two are generally linked to the presence of frozen ground.The fall in air and ground temperatures with altitude,slope orientations,and snowpack thickness and evolution determine ground thermal regimes.In the study area,three types of thermal regimes were established:climate-controlled,snowcover-controlled,and frozen ground-controlled.Seasonally frozen ground occurs across a broad range of elevation between 2650 and 3075 m asl,whereas possible permafrost only occurs above 2750 m asl.

Spatiotemporal variations of maximum seasonal freeze depth in 1950s–2007 over the Heihe River Basin, Northwest China

Investigation on spatiotemporal variations of maximum seasonal freeze depth （MSFD） over the Heihe River Basin is of great importance for systematic understanding of regional climate and environmental change, ecological-hydrological processes, water resources assessment, construction and resource development. Based on soil and air temperatures at the meteorological stations of the China Meteorological Administration （CMA） over the Heihe River Basin, MSFDs time series are structured into a composite time series over the 1960-2007 period. Evaluating the averaged MSFD time series for 1960 2007 reveals a statistically significant trend of 4.0 cm/decade or a net change of-19.2 cm for the 48-year period over the basin. The MSFD had significantly negative correlation with mean annual air temperature （MAAT）, winter air temperature, mean annual ground surface temperature （MAGST）, degree days of thawing for the air （DDTa） as well as for the surface （DDTs）, and degree days of freezing for the surface （DDFs）. While there was significantly positive correlation between DDF,. and MSFD time series, MSFD was deeper and changed greatly in the Heihe River source area. It was shallower in the east-central basin and gradually deepened in other sections of the basin. The MSFD distribution pattern in 2003-2005 is consistent with that of averaged degree days of freezing for air （DDFa） in 1960-2007. However, the maximum of MSFD may not be accurate, because there is no long term observation data in the deep seasonally frozen ground regions near the lower boundary of permafrost. With increasing elevation, averaged DDFa increased at a rate of 51.6 ℃-day/100m, therefore, the MSFG and the date reaching MSFG became deeper and later, respectively.

Water migration in subgrade soil under seasonal freeze-thaw cycles in an alpine meadow on the Qinghai-Tibet Plateau

Highway frost heave and thawing settlement caused by water migration towards the freezing front and ice lens development is widespread in the alpine meadow area of the southeast QinghaiTibet Plateau(QTP). A laboratory experiment on a highway reconstruction and expansion project in the QTP was carried out in this work to analyze the effects of fine particle content, initial water content, and the number of freeze-thaw cycles(FTCs) on frost depth, temperature gradient(Grad T), total water intake, and water intake flux. Based on the results of the laboratory experiment, a modified model of migration potential related to fine particle content, freeze-thaw history, and freezing time was established. The results show that, with the increase of fine particle content, the frost depth of soil decreases, the curve of total water intake over time is transformed from an Sshape to an arch, and the curve of water intake flux over time is transformed from a peak shape to descending shape. The variation trend of migration potential with freezing time and the freeze-thaw history is the same as that of water intake flux with freezing time and freeze-thaw history. The variation trend of soil intake flux can be used as a reference to determine the variation trend of soil migration potential. This study provides a reference for the design and construction of highway subgrade in the alpine meadow area of the QTP.

Responses of Climatic Change on the Lagging Time about Ground Temperature Reaching the Extremum at Shallow Layers Soil in Wuli

The air temperature of Wuli,which is located in seasonal frozen ground zone,is rising by 13 ℃ yearly.This paper discusses the days that each ground layers' temperature lags behind the surface temperature in reaching extremum.The results were shown:The time of each ground layers' lagging days was increasing;the lagging day in warm season was longer than that in cold season;the growth rate of lagging days in warm season was 0.5 d/y,while the growth rate of lagging days in cold season was 0.7 d/y.

Current state and past changes in frozen ground at the Third Pole:A research synthesis

The thermal state of frozen ground and its changes are important for understanding environmental change and supporting related applications to the Earth’s Third Pole,which is a hotspot area for science research.However,challenges remain in data and modelling,meaning that much information is unavailable,especially for the entire Third Pole region.Here,we provided basic statistical data regarding the current state of frozen ground and its changes over the 1960s–2010s across the entire Third Pole by integrating nearly all currently available ground observation data and high-quality spatial data using machine learning models and existing high-quality frozen ground data products.The results show that the current(2000–2018)areal extents of permafrost and seasonally frozen ground in the Third Pole are approximately 1.27×10^(6)km^(2)(1.15×10^(6)to 1.39×10^(6)km^(2))and 2.59×10^(6)km^(2),accounting for 28%and 58%,respectively.The areal extent of permafrost region is approximately 50%(23%–93%)larger than that of permafrost area(land underlain by permafrost),especially in some early maps.The corresponding regional average of the mean annual ground temperature is approximately−1.51℃(−1.75 to−1.27℃)in the permafrost area.The regional average of active layer thickness overlying the permafrost and the maximum frost depth for regions of seasonally frozen ground are 235 cm(233–237 cm)and 92 cm,respectively.From the 1960s to the 2010s,on average,permafrost in the Third Pole warmed at a rate of 0.17℃per decade,which was associated with increases in the maximum thaw depth at a rate of 4.42 cm per decade.The regional average of the maximum frost depth declined at a rate of 2.34 cm per decade over the same period.This synthesis highlights the differences between the two terms(permafrost region and permafrost area)and provides crucial information for frozen ground in the Third Pole with higher accuracy for the scientific community and the public.

季节性冻土区黏土冻结深度预测

合理确定冻结深度是季节性冻土区防冻设计的关键,为研究季节性冻土区黏土的冻结深度,统计了中国部分季节性冻土地区气温,获得了这些地区的气温特征,以此确定了室内试验的温度边界条件,开展了10%、20%、30%3种不同质量含水量的单向冻融循环试验。研究结果表明,10%、20%、30%3个试样的最大冻结深度分别为28、15、12 cm;鲁基扬诺夫公式预测值较Stephan公式、《建筑地基基础设计规范》公式、《冻土地区建筑地基基础设计规范》与《冻土工程地质勘察规范》公式的预测值更准确;利用未冻水体积含水率修正了《冻土地区建筑地基基础设计规范》的冻结深度计算值,其误差在6%之内,因而建议在季节性冻土区的黏土地区用该修正公式计算设计冻结深度。

Measurement of subsidence in the Yangbajing geothermal fields,Tibet,from TerraSAR-X InSAR time series analysis

Yangbajain contains the largest geothermal energy power station in China.Geothermal explorations in Yangbajain first started in 1976,and two plants were subsequently built in 1981 and 1986.A large amount of geothermal fluids have been extracted since then,leading to considerable surface subsidence around the geothermal fields.In this paper,InSAR time series analysis is applied to map the subsidence of the Yangbajain geothermal fields during the period from December 2011 to November 2012 using 16 senses of TerraSAR-X stripmap SAR images.In the case of the TerraSAR-X data,most orbital fringes were removed using precise orbits during the interferometric processing.However,residual orbital ramps remain in some interferograms due to the uncertainties in the TerraSAR-X orbits.To remove the residual orbital ramps,we estimated a best-fit‘twisted plane’for each epoch interferogram using quadratic polynomial models based on a network approach.This method removes most of the long-wavelength signals,including orbit ramps and atmospheric effects.The vertically stratified component(Topography Correlated Atmospheric Delay,TCAD)was also removed using a network approach.If the influence of seasonal frozen ground(SFG)is not taken into consideration,our results show that the subsidence rate around power plant I(the south plant)is approximately 20 mm/yr with a peak of 30 mm/yr.The subsidence rate around power plant II(the north plant)is approximately 10 mm/yr,when accounting for the influence of SFG on the power plant and its surrounding ground surface.Our results show that ground motion is caused by seasonal frozen ground and is strongly related to the temperature change.