Mechanism And Experimental Study On Pot-cover Effect Under Freezing-thawing Cycle

'Pot-cover Effect' refers to the phenomenon of moisture accumulation beneath the pavement under condensation or desublimation because the vapor transfer driven by the temperature gradient in the soil is blocked by the pavement.In the vast seasonal frozen soil area,Pot-cover Effect of soil leads to the frequent occurrence of engineering diseases of road,railway and airport runway.In order to prevent and control the engineering diseases of airport runway in the seasonally frozen area and improve the theory of vapor transfer related to the Pot-cover Effect,we conducted a field investigation on the Pot-cover Effect of the runway of Shenyang Taoxian International Airport.Based on a large number of laboratory tests and combined with theoretical research,we studied the water increasing law of subgrade soil.The main achievements are as follows:(1)A test platform of Pot-cover Effect was developed.The Pot-cover Effect Apparatus with Freezing-thawing Cycle(PEAFC)was developed.It is found by test that the water content measured by FDR(Frequency Domain Reflectometry)sensors will be affected by temperature drift and has a linear relationship with temperature.The water content calibration formula was established and the water content after calibration was reliable.The apparatus was improved to simulate the gravel-clay subgrade and collect the water generated by the Pot-cover Effect.(2)The influencing factors of Pot-cover Effect were studied systematically.A series of vapor transfer experiments of Pot-cover Effect under constant temperature gradient were carried out on the silty clay samples.The vapor transfer law of clay under different temperature gradient,initial water content,dry density,test time and water replenishing method were studied.The influencing factors on Pot-cover Effect was obtained.The test results show that the freezing action is the key to increase the water content under the soil cover greatly.(3)The water increase mechanism of Pot-cover Effect of subgrade soil is revealed,which is freeze-thaw cycle.First of all,the soil samples simulated clay subgrade are studiedby freeze-thaw cycle tests.The results show that the water content of the clay subgrade under the freeze-thaw cycle will increase with the increase of cycle times.The overall water content of the subgrade soil will increase greatly and obvious frost heave and thaw settlement phenomenon will occur.Secondly,the freeze-thaw cycle test is carried out on the soil samples simulated gravel-clay subgrade.The results show that under the freeze-thaw cycle,the vapor in each cycle will condense into ice in the gravel layer,and all the condensing ice will melt into water and flow out in the melting period.As a result,the water in the subgrade soil increases with the increase of the cycle times.The above test results reveal that the freeze-thaw cycle is the water increase mechanism of Pot-cover Effect of the subgrade soil.(4)The water increase mechanism of Pot-cover Effect of subgrade soil is revealed,which is vapor cycle phenomenon.First of all,the calculation formula of vapor transfer velocity is derived.The critical saturation state is proposed and defined as the starting condition of the vapor cycle of Pot-cover Effect.Secondly,condensation test on soil sample simulated gravel-clay subgrade was carried out.The test results show that the water will increase continuously if the the increased water can flow out in time.The theoretical research and test results reveal that the vapor cycle phenomenon is the water increase mechanism of Pot-cover Effect of the subgrade soil.