Frost Heaving Process And Simulation Of Concrete Lined Canals In Seasonally Frozen Soil Regions

In the seasonally frozen regions of northern China,concrete lining of field canals is prone to be damaged under the action of the base soil frost heave,which affects the normal operation of the canals.How to select a suitable frost heave theory and model to simulate canal base soil is still to be studied.Boundary conditions like,solar radiation and heat conductivity boundary,changing groundwater level,and freezing constraint relationship between base soil and the lining,are also insufficient to study the lining damage process of field canals.Therefore,quantitatively analyzing the lining damage process is difficult.Exploring the frost heaving mechanism of base soil and selecting appropriate theories using for constructing the frost heave model is helpful for improving the model’s accuracy.Constructing a suitable heat-waterdisplacement boundary condition can help quantify the impact of different factors on the lining damage,further to provide a reference for efficient measures to avoiding this.This study analyzed the applicability of different frost heaving theories in modeling the frost heaving mechanism of canal base soil.The error sources of present frost heave models were analyzed and improved to satisfy the needs of frost heave model for simulating canal base soil.Basing on the solar radiation and thermal convection boundary calculation theory used in the crop model,the authors built solar radiation and heat conductivity boundary of the field canals.Changing groundwater level was modeled by adding a source term to the water transfer equation.Basing on the stress analysis and direct shear experiments,freezing constraint model between base soil and the lining was built.Combining the boundary condition model and the optimized frost heave model,a model for analyzing the lining damage of field canals was established,which was furtherly verified by outdoor experiments.Factors to influence the lining damage were analyzed with the model,which provides efficient measures for avoiding it.The main results are as follows:(1)The applicability of different theories for soil freezing,water migration,and frost heave displacement to describe the frost heave mechanism of canal base soil has been explored.The freezing of canal base soil is non-thermal equilibrium.Using the thermal equilibrium hypothesis results in a large deviation when computing the relationship between soil temperature and water pressure.Hydrodynamic theory ignores the changes of soil structure during the freezing process,which leads to a large simulated water content in the freezing front.Thus,it is not suitable for accurately simulating the base soil frost heave.The frozen fringe theory can only be applied to the one-dimensional problem of soil frost heave,also cannot be used to study the base soil frost heave of filed canals.Frost heave displacement of canal base soil is mainly caused by water migration and freezing.The proportion of in-situ frost heave is very small.A quantitative relationship is exist between frost heave displacement,external water supply,and the thickness of ice lenses.(2)The error characteristics of different methods for modeling temperature,water,and displacement fields of the frost heave model were quantified and the error sources were analyzed.The calculation method of thermal conductivity and thermal capacity is the main source of temperature field error.The thermal conductivity calculated by the geometric average method will result an overvalued temperature at the initial stage of freezing.The hydrodynamic model does not consider the impact of ice segregation on soil structure and water migration,which results in high uncertainty for simulating under different temperature and overburden pressure conditions for the same soil type.The most used two frost heave quantitative methods produce error exceeding 20%because inaccurate relationship between pore strain and frost heave displacement.This error can be easily reduced to 1%by the formula modified in this paper.The simulation error of temperature field is relatively small and stable,basically within±0.25 ℃;The simulation error of water field is obvious and most unstable.The maximum error of water filed at freezing front can reach over 20%.Due to the relatively accurate temperature simulation,the simulation error of freezing depth is basically within 10%.The error of the frost heave model is easily masked by inaccurate water migration simulation results and inaccurate frost heave quantification methods.(3)The improvement haves done on the mechanism of the resistance of segregation ice on water flow in the hydrodynamic theory,and the hydrodynamic model.Based on the experimental evidence of hydraulic conductivity of frozen soil,segregation theory and Gibbs Thompson effect,the resistance of segregation ice on water flow is analyzed.The reason for the overvalued of water migration in freezing front using hydrodynamic models under open saturated freezing system is explained,which shows an impedance factor is necessary when simulating water migration of freezing systems with ice segregation.The physical formula of an impedance factor is derived by using the initial porosity and the accumulative curve of particle size grading.The segregation impedance factor can model the accelerated decrease of hydraulic conductivity caused by pore expansion,which is more accurate compared with the previous empirical impedance factor.The segregation impedance factor combines the mechanism of segregation ice in the frozen fringe theory with the hydrodynamic model.Combining the hydraulic conductivity of the VG model,the segregation impedance factor can well simulate the water migration under different temperature and overburden pressure freezing conditions.This method combines the advantage of the application of hydrodynamic models in two-dimensional space and the advantage of more accurate frozen fringe theory to simulate ice segregation,which improves the accuracy and application ability of the frost heave model for canal base soil.(4)The improvement has been done on the modeling methods of solar radiation,convection,varying groundwater level,and freezing constraint model between base soil and the lining.A model for simulating the concrete lining damage of field canals has been constructed and verified through outdoor experiments.The solar radiation and thermal convection calculation theory in crop models have a good performance to simulate the temperature boundary conditions of the field canal.The water simulation result of frost heave model using a varying groundwater level condition has a good agree with the monitoring result.The freezing constraint model between base soil and the lining can accurately simulate the time when the lining stress release,thereby calculating the maximum stress state generated by the lining during the base soil frost heaving process.The simulated location of the maximum lining stress is close to the crack location observed in the field.When the temperature reaches the lowest at the initial stage of freezing,the frost heave force on the monitoring lining reaches the maximum value.When the maximum frost heave displacement occurs,the frost heave force on the monitoring lining has decreased to a lower level.During the process of frost heaving of the base soil,freezing constraint between base soil and the lining can be broken.The frost heave displacement of the base soil is a necessary and insufficient condition for the lining damage of filed canals.(5)The sensitivity of the lining damage model to factors of meteorological,groundwater level,canal direction,and lining structure is analyzed,which provides measures for avoiding the lining damage.The most sensitive factor for concreate lining damage in field canals is the minimum temperature at the initial stage of freezing.The lower the temperature,the greater the freezing force constraint.A small frost heave of the canal base soil(2mm)can lead to the deformation of the lining reaching the limit stress.Even in field canals where the groundwater level is around 3m,the frost damage to the concrete lining cannot be avoided.The large frost heave caused by high groundwater levels has far exceeded the need for lining damage,but if the freezing constraint is small,the lining stress will be released at the initial stage of freezing to avoid damage.The difference in solar radiation caused by the shady and sunny slopes of canals has a significant impact on the maximum stress on the lining.The maximum stress on the shady slope of east-west trending canals can reach twice the maximum stress on the slope surface of north-south trending canals.The simulation shows that the maximum stress of the new full curve lining structure(parabolic and catenary type)is reduced by about 20%compared to the widely used U-shaped lining structure.However,the monitoring results show that this effect is not obvious,and it is not enough to avoid frost heave damage of concrete lining simply by the new curved lining structure.Reducing the freezing constraint force between the base soil and the concrete lining can more effectively reduce the maximum stress of the concrete lining than reducing the frost heave displacement of base soil.