Mechanism And Control Standard Of Differential Frost Heaving Deformation Of Changji Expressway Widening Subgrade

The expansion of roads in the seasonal frozen area will change the temperature and humidity distribution of the original subgrade.Due to the difference in soil quality between the new and old subgrades,the subgrade will produce uneven differential frost heave under freezing,which will cause longitudinal cracking and freezing drum disease on the road surface,affecting road durability and driving safety.Domestic and foreign scholars have accumulated some research achievements on the numerical models of subgrade frost heaving,the prediction of frost heaving and the allowable frost heaving of roads.However,it has not been involved in the expansion of the differential frost heaving mechanism and control standards of the subgrade.Therefore,this paper relies on the expansion project of Changchun Longjia airport to Jilin section of Hunwu expressway.Based on the combination of in-situ monitoring,laboratory test,theoretical analysis and numerical simulation,the mechanism and control standards of differential frost heaving deformation of expressways in the seasonal frozen area were studied.The main research contents and the results obtained are as follows:In order to study the distribution law of water and temperature in widening subgrade under the action of freezing and thawing,based on the analysis results from in-situ monitoring data,the thermal-hydro coupling model for widening subgrade in the seasonal frozen area was established.The temperature,humidity and deformation monitoring were carried out for six consecutive months for two typical subgrade sections of Changji expressway.The analysis found that the temperature of the soil within the depth of three meters below the top surface of the subgrade is significantly affected by the change of outside temperature.In addition,the frost heaving deformation of the new and old subgrades reached a maximum at the end of February.The higher the moisture,the faster the freezing rate of the subgrade,and the greater the amount of frost heaving.According to the calculation results of the thermal-hydro coupling model,the first year after the subgrade is widened,the temperature and humidity fields will redistribute obviously and reach equilibrium in the fourth year.Aiming at the differential frost heaving deformation calculation of widening subgrade under freezing,based on the energy conservation law,mass conservation law,Fourier equation,Darcy equation and elasticity theory,the thermal-hydro-static coupling model of widening subgrade is established and validated.In the process of calculating in-situ water freezing into ice,it is necessary to consider the relationship between subgrade resilience modulus and negative temperature and unfrozen water content.According to the calculation results,the differential frost heaving deformation characteristics of the two typical subgrade sections have certain similarities.In addition,due to the redistribution of temperature and humidity fields,the frost heaving deformation of the top surface of the subgrade will decrease significantly in the second year after the subgrade is widened.In order to obtain a reasonable prediction method for differential frost heaving,through fitting and parameter analysis of subgrade frost heaving deformation,the differential frost heaving deformation prediction formula for the low-fill and shallowexcavation typical subgrade sections of Changji expressway is proposed.Furthermore,the model of the internal force response of the pavement based on the differential frost heave deformation is established.It has been found through calculation that the most prone to cracking of the road surface above widening subgrade often occurs on the old road and near the junction of new and old subgrade.By calculating the pavement stress of two typical subgrade sections,the frost heave of subgrade under the most easily crackable position of pavement is proposed as the control index.Then the control standards of differential frost heaving of widening subgrade for Changji expressway is determined.During the construction process,the frost heaving of subgrade in the most disadvantageous position of road surface can be reduced by laying geogrid,insulation board and adding compound improver in the filling of subgrade,improving the frost resistance of widening subgrade.