| Traffic line projects in permafrost regions are currently facing the great challenge of permafrost foundation degradation.Compared with bridge and tunnel engineering,subgrade engineering is most closely related to permafrost foundation.The degradation of permafrost will directly lead to deterioration of subgrade bearing capacity and thaw settlement deformation.This thesis is supported by the National Natural Science Found of China,“Research on distributed refrigeration anti-thaw settlement mechanism of subgrade in permafrost regions based on cooling load timing characteristics”(42001059).Firstly,by introducing a new index of “cooling load” to characterize the hourly characteristics of the heat budget of frozen soil,a calculation model of ground cooling load is established.Secondly,a full-season refrigeration system for protecting permafrost was designed and assembled.According to the combination of laboratory test,field test and simulation calculation,the working performance of artificial refrigeration system under different refrigeration temperature and intermittent operation control mode was carried out.Based on the heat exchange characteristics between the column cold source and the foundation,the refrigeration mechanism of the refrigeration system under long time scale was explored.Finally,a whole process design and application scheme of artificial refrigeration anti-thaw settlement method for subgrade in permafrost area was proposed.The main research results obtained in this thesis include:(1)Based on the principle of compression refrigeration,a full-season refrigeration system that can meet the refrigeration requirements of subgrade engineering in permafrost regions is designed,and a distributed refrigeration scheme for subgrade in permafrost regions is developed.(2)Based on the Building Thermal Environment Design Simulation Toolkit(De ST),a calculation model of dynamic cooling load of foundation in permafrost region was established.Combined with the actual engineering case of Qinghai-Tibet Highway,the variation law of cooling load required to maintain the permafrost upper limit after subgrade filling is calculated,the sensitivity of cooling load and line direction was analyzed.The results showed that the maximum cooling load is 169W/meter,the average cooling load is 78 W/meter,the peak cooling load lags behind the peak atmospheric temperature by 12 days.(3)The compressor power selection in the artificial refrigeration system is guided by the calculation results of the ground cooling load,and the ground cooling indoor test of “column cooling source-foundation-environment” was carried out.The basic data of refrigeration temperature of artificial refrigeration system in open environment were obtained.When the refrigeration temperature was set to-5 °C,-15 °C and-25 °C,the average temperature of refrigeration tube wall was-6.44 °C,-14.33 °C and-23.78 °C.When the start-stop ratio was 1h:3h,1h:4h and 1h:5h,it was-8.29 °C,-7.73 °C and-6.07 °C.By comparing and analyzing the indexes of soil temperature,refrigeration influence radius and effective refrigeration coefficient of the model box,it is suggested that the reasonable refrigeration temperature of the frozen soil foundation refrigeration system is-25 °C under constant temperature operation,and the reasonable start-stop ratio is 1h:4h under intermittent operation.(4)Based on the construction project of Erzhi expressway in Qinghai,the field assembly and operation effect of the whole season artificial refrigeration system were studied.Based on COMSOL,a three-dimensional simulation model of subgrade hydrothermal coupling was established.Taking the design parameters and operation control parameters of the refrigeration system as variables,the long-term numerical calculation of different refrigeration mechanisms were carried out.The results showed that when the shoulder-vertical layout scheme was adopted and the cooling temperature is-15 °C,the time interval between the subgrade center and the slope toe on both sides to restore to the upper limit of natural frozen soil was 60 days,and the maximum difference value of the upper limit of frozen soil on the longitudinal section was 9.3 cm on average,which can effectively prevent the secondary diseases such as horizontal irregularity and longitudinal irregularity caused by differential thaw settlement on the transverse and longitudinal sections of subgrade. |
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