| China’s land area is 10.45 million km~2,of which 75%is frozen soil,and it is widely distributed in the arid areas of northern China.In the context of global warming,the alternation of cold and warm,temperature fluctuation and the increasing extreme weather events are the most important characteristics of Earth’s climate change.The frequency and intensity of extreme weather events such as cold wave increase,which makes the thermodynamic response of frozen soil significantly different.The concrete lining structure of the channel is thin and brittle,easy to be damaged by the uneven frost heave deformation of the foundation soil,and is a typical engineering structure in cold area with poor frost heave resistance.Most of the canals in the major irrigation areas in China’s drought and cold areas are damaged by frost heave,which seriously affects the normal operation of canal system engineering and the full play of engineering benefits.Therefore,clear understanding of the frost heave mechanism of foundation soil in cold regions and accurate prediction of the uneven frost heave deformation of lining channels under the background of current climate change can not only provide technical support and theoretical basis for the anti-frost heave design of channels,but also minimize the loss and waste of water resources and improve the engineering efficiency and management level.It has important social and economic benefits and practical significance for drought and cold water supply security and economic development.Therefore,in view of the fact that the calculation method that does not consider the change of freezing process and the frequency of freezing index fails to reflect the characteristics of climate change scientifically,and the thermodynamic response of canal foundation soil under climate change is unclear,this paper firstly theoretically improves the frost heaving model of frozen soil to improve the prediction accuracy of deformation forces of frozen soil channels.Then,the regional frequency calculation method of freezing index is studied.Finally,the thermodynamic responses of different cooling processes to frozen soil during the year are studied.The main research contents and achievements are as follows.(1)Based on the transverse-isotropic characteristics of the mesoscopic fabric of frozen soil with ice lenses,an equivalent mechanical model of the interaction between ice lenses and soil is established,which can satisfy the static equilibrium and deformation coordination between frozen soil and ice sheet,and the constitutive model of transverse-isotropic frozen soil and the analytical expressions of five elastic constants are derived.It makes up for the shortcomings of the frozen soil constitutive structure that rarely considers the cold structure of ice lens.(2)According to the heat conduction equation,soil water motion equation and generalized Hooke’s law of hydrodynamic frost heave model,an assigned weightξis introduced to describe the volume frost heave difference between the parallel and the perpendicular to the direction of temperature gradient,respectively,then a transverse-isotropic frost heave model based on heat-moisture-deformation coupled is derived.A numerical simulation platform of transversely-isotropic frozen soil was developed using the COMSOL multiphysics coupling software.The model was verified by the results of one-dimensional standard frost heave test and prototype canal frost heave test.The results of comparative analysis show that the proposed model is more accurate than the traditional isotropic permafrost frost heave model in predicting deformation and interfacial frost heave forces,and can better reflect the frost heave failure characteristics of frozen soil canals.(3)Aiming at the difficulty of the prediction of the frequency of freezing index for the area lacking of meteorological data,a regional freezing index method considering the deterministic influence of altitude and latitude was proposed.Using deviation coefficient C_vand skewness coefficient C_s as clustering factors,combined with geographical characteristics and climate zoning,15 freezing index categories in China’s cold region were given by fast clustering method,and the rationality test of regional consistency and uniformity is passed.Based on the frequency calculation method of the regional freezing index,the probability distribution of each region of the freezing index is obtained.Through goodness of fit evaluation,the optimal distribution of 72.54%stations is generalized extreme value(GEV)distribution.Based on the daily mean temperature data of 10 major stations in Ningxia region from 1960 to 2020,the calculation steps of the scale freezing index and its frequency distribution were given.The results show that the regional freezing index method can consider not only the deterministic factors such as altitude and latitude,but also the randomness factors with inter-annual variation,which is more scientific,reasonable and accurate.The error of the proposed method is mostly less than 10%,and its RSS and RMSE are smaller than the inverse distance weight interpolation method.(4)The thermal response of frozen soil to different kinds of temperature change was systematically studied by using the frost heave model proposed in this paper.According to the characteristics of air temperature change under the background of climate change,the spatio-temporal evolution of frozen soil under the combination of three annual temperature variation modes(traditional trigonometric function mode of continuous annual temperature change,stepwise abrupt change mode with no distinct seasons,and broken line abrupt change mode with extreme high and low temperature of annual temperature)and daily temperature variation amplitude of 0～10°C was studied.The results show that under the same freezing index condition,the daily temperature amplitude has the least influence on the trigonometric model and the most influence on the broken line abrupt change mode.The influence of daily temperature amplitude on the maximum freezing depth and the maximum frost heave decreases with the increase of freezing index.When the freezing index is small(that is,the amplitude of annual temperature is small),the frost depth and frost heave calculated by the trigonometric model are the largest,which is the most unfavorable temperature change process among the three models.On the contrary,when the freezing index is large,the broken line abrupt change mode is the most unfavorable temperature change process.(5)Taking the West Trunk canal of Ningxia as an example,the frost heave failure of the canal under the design values of one in 100 years,one in 50 years,one in 20 years and one in10 years were studied by using the frost heave model and the regional freezing index method proposed in this paper.The relationship between freezing index and annual temperature change mode is clarified,and the most unfavorable annual temperature change modes under different design standards were given,which overcomes the subjective arbitrariness of temperature boundary selection and provides theoretical basis for the later reconstruction of the project. |
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