| The Xining-Chengdu high-speed railway(XCHR)crosses the Zoige Wetland,located in the north-east edge of the Qinghai-Tibet Plateau and the upper reaches of the Yellow River.The cold climate and frost-heave sensitive subgrade soil are critical for roadbed construction,which causes the large deformation,threatening the train running safety.Although the frost heaving mechanisms and preventions of embankment for Harbin-Dalian high-speed railway and Lanzhou-Xinjiang high-speed railway in seasonally frozen region have been widely investigated,the frost heaving characteristics and engineering treatment for the humus clay in Zoige Wetland is still not been studied.To guarantee the stability of railway embankment and protect the wetland in the freezing seasons,it is necessary to investigate the mechanism of frost heave of roadbed soil and develop the corresponding anti-freezing technologies for high-speed railway(HSR)embankment in Zoige Wetland.Such research would have great significance for the freezing prevention design and freezing damage treatment of XCHR.This paper systematically investigate the frost heave characteristics of natural ground,the water migration behaviors and freezing prevention optimization of coarse-grained soil(CGS),the segregated frost heave mechanism of saturated freezing clay,and the anti-freezing effect of composite embankment by using field monitoring,indoor test and numerical analysis.Some main findings are as follows:(1)The frost heaving characteristics of natural flat ground and hillslope in Zoige Wetland were monitored.The ground temperature development,frost heave variation,soil water content and groundwater level of these two grounds were analyzed.The in-situ monitoring presented that the maximum frost heave amount and maximum frost heave ratios at flat ground with the natural water content of 38.1%and reached 25.64 mm and 6.10%,these at hillslope with the natural water content of 22.7%reaches 3.20 mm and 2.67%,respectively.The groundwater level of flat ground maintains at the depth of 1.5 m,while that of hillslope is deeper than the depth of 3.0 m.The difference of groundwater level is the main reason for the discrepancy of frost heave amount between flat ground and hillslope.(2)The indoor tests were conducted to analyze the frost heave characteristics of natural ground clay and the replaced coarse-grained soil(CGS).The further tests were conducted to evaluate the influences of fines content,water content and grain gradation on the frost susceptibility of CGS.Combined with the engineering conditions,the rational engineering treatments were proposed.Under the same condition,the frost heave ratio of natural clay is25.73%after the 96h of frost period,while that of CGS is 1.60%after the 144h of frost period.The different frost heave ratios were mainly caused by the diverse patterns of water migration between clay and CGS.The migrated water from the outside water supply into frozen soil would change into ice lens in the freezing clay,while that change in to the pore ices and ice layer under the upper cold pedestal in the CGS.It is suggested that the anti-frost methods of HSR need to be fully considered,including strict control of fines content and water content,priority of using poorly-graded filling.The replacing CGS filling layer should also incorporate with other measures of waterproof and drainage measures to disrupt local rainwater accumulation.In addition,the slope foot of embankment should use the frost-resistant berm.(3)A two-dimensional thermal-hydro-mechanical model is proposed to simulate the ice lens formation in saturated freezing soil.The model gave the equation of pore pressure.The main variables of model during the freezing process were analyzed in detail.The pore water pressure is largest in the outside water supply,then in the unfrozen zone,and smallest in the frozen zone.As a result,the water transfers from the outside to the frozen zone through crossing the unfrozen zone.Because the water transferred and finally changed into ice lens,the displacement gradient of unfrozen zone is least,then that of in-situ frozen zone,and that of segregated frozen zone is largest.The vertical strains of unfrozen zone,in-situ frozen zone and segregated frozen zone are about-0.006,0.12 and 0.6,respectively.(4)The above model was further developed to predict the influences of load,hydraulic conductivity and soil types on the distribution of ice lens.Both higher overburden load and smaller hydraulic conductivity bring less heave,fewer ice lenses.The difference of frost heave ratio between two distinct types of contacted soils is 1.45%at freezing 96 h when suffering the same cold condition,which can cause the uneven deformation of embankment in cold region.Even though the uneven heaving exists,the two distinct types of contacted soils still show partial similar characteristics of ice lens distribution.Their ice lenses are prone to connect under the effect of water transversally migrating across the contacted interface.(5)A finite element model was developed to study the anti-frost effect of composite embankment in seasonally frozen zone.The composite embankment with CGS and insulation layer could not only reduce the maximum frost depth and maximum frost heave amount of embankment,but also could reduce the heave amount difference between left should and right shoulder,compared with the common embankment with CGS.That is to say,the sunny-shady slopes effect of composite embankment in seasonally frozen zone could be partly mitigated.The heave amount of embankment decreased with the increase of thickness of insulation layer and increased with the increase of thermal conductivity of insulation layer.However,the heave amount of embankment would not significantly decrease with the further increase of thickness and decrease of thermal conductivity when the thermal resistance of insulation layer is greater than 1.5m~2·K/W.The effective thermal resistance of insulation layer is 1.5m~2·K/W. |
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