Ninth International Conference on Permafrost ... - IARC Research

Engineering Effect on the Thermal Status of Shallow Ground in Permafrost RegionsZhi WenState Key Laboratory of Frozen Soil Engineering, CAREERI, CAS, Lanzhou Gansu 730000Yu ShengState Key Laboratory of Frozen Soil Engineering, CAREERI, CAS, Lanzhou Gansu 730000Wei MaState Key Laboratory of Frozen Soil Engineering, CAREERI, CAS, Lanzhou Gansu 730000Qingbai WuState Key Laboratory of Frozen Soil Engineering, CAREERI, CAS, Lanzhou Gansu 730000Bo HuangQinghai Provincial Highway Research and Survey Institute, Xining Qinghai 810008, ChinaIntroductionThe construction of an embankment in permafrost regionsmay induce substantial disturbance on the heat and masstransfer balance between the ground surface and atmosphere,which results in more heat absorption in the embankment.The temperature of the permafrost underneath increaseseven if permafrost thaws, causing serious problems forembankment due to thaw settlement on the Qinghai-TibetanPlateau (Wen & Sheng 2003). The thermal status of theembankment affects directly the thermal status of permafrostunder the railway embankment, which determines theembankment stability in the permafrost regions, especiallyice-rich permafrost regions (Yu & Lai 2002).Owing to important signification of the thermal regime ofthe upper soil layer on permafrost stability, many researchershave studied the thermal regime of the natural ground onthe Qinghai-Tibetan Plateau (Xu & Ma 1984, Zhang &Zhu 1998). However, there is scarce research on heatbalance and annual heat incomes and expenses of railwayembankments. To know about the influence of engineeringactivities on the thermal regime of the natural ground,dynamic monitoring using dataloggers made in CompbellCompany for temperature and heat flux in the upper activelayer was carried out during August 2002, making it possibleto use quantificational method analyses of the influence ofengineering activities.General Situation of the Test SitesThe test site is situated between Kekexili and Fenghuoshanalong the Qinghai-Tibetan Railway on the Qinghai-TibetanPlateau. Dynamic monitor sections of railway embankmentand natural ground were set up on August 2002. Eachsection may measure heat flux and temperature of shallowground simultaneously. Instruments used in two sectionswere self-calibrating heat flux sensor, temperature sensor,and datalogger and the readings were taken once half hour.The temperature probe may measure the temperature at thedepth of 2.0 cm, 5.0 cm, 10.0 cm, 20.0 cm, and 50.0 cmfrom ground surface. The depth of heat flux placed was 20.0cm from ground surface. Taking the difference of heat fluxat two sides of the embankment into consideration, therewas one self-calibrating heat flux sensor at each side of twosections and the data of heat flux were the even value of twosensors.Variation Character of Soil TemperatureAfter railway embankment construction, many conditionswere changed compared to natural ground, such as surfaceconditions (vegetation, albedo, etc.), soil moisture content,and soil component, which affected radiation absorptionof surface and its transfer process downwards certainly. Asseen in Figure 1, the soil temperature under the embankmentsurface at a depth of 50.0 cm was 2°C higher than that of thenatural ground and had same phase location under similarclimatic conditions (embankment testing site was close tonatural ground site). Apparently, the phase and variationcurrent of soil temperature was similar between two sites,which was due to some same conditions for two sites, suchas climatic conditions, etc., that had nothing to do withengineering activities. However, the soil temperature at theembankment site was 2°C higher than that of the naturalground site if other conditions stayed the same, whichwas affected by engineering activities distinctly. Afterengineering construction, vegetation in the natural groundwas destroyed, and the soils type, soils component, and itsmoisture content were changed, which increased the radiationabsorption and decreased the evaporation water content. Ina word, a high temperature boundary was brought to bearon the surface, which augmented the heat exchange andaccelerated the permafrost degradation. Due to engineeringactivities, thawed core below embankment came into beingand thawing settlement began.Temperature/ ℃13119722-Aug 24-Aug 26-Aug 28-Aug 30-Aug 1-Sep 3-Sep 5-SepTime/dNatural groundRailway embankmentFigure 1. The contrastive temperature curves between the railwayfoundation and the natural ground at 50 cm depth.339