Freeze-thaw Response Mechanism Of Integral Channel Lining And Model Research Of Saline Soil Foundation

The saline land formed by irrigation in Ningxia is about 17 X 104 hm2,which is a seasonal permafrost area,and the concrete lining of the saline land base channel is highly susceptible to serious diseases such as frozen heave,salt swelling,and salt corrosion under the action of seasonal freeze-thaw cycles.The systematic study of the synergistic effect between saline soil foundation,channel lining structure,lining material,and ambient temperature is a key scientific problem for the study of freezing and life increase of irrigation channel lining project,which is centered on the problem of freezing and thawing damage and salt corrosion of saline soil-based channel lining in the yellow irrigation area.Therefore,this research focuses on the practical problem of channel lining frost damage prevention and control in the seasonal permafrost zone,and focuses on the experimental study of canal base saline soil improvement and theoretical study of channel lining synergy,combining mechanics,mathematics,and engineering physics background,improving the traditional frost swelling force calculation method and multi-field coupling theory and establishing more accurate frost swelling force model and multi-field coupling model,so as to provide the corresponding theory and technology for channel construction and frost damage management in seasonal permafrost zone irrigation areas.Theoretical and technical support is provided for channel construction and frost damage management in seasonal permafrost areas.Through indoor experimental research,theoretical analysis and numerical simulation research,and field test verification,the following research results were achieved:(1)Through the indoor triaxial test,compression test,dry shrinkage test,XRD and SEM characterization analysis,the variation law of shear strength index of improved saline soils is affected by four kinds of curing agents such as cement,fly ash,silica fume,and magnesium slag;the strength formation mechanism of improved saline soils was revealed.The mathematical model between the strength index of improved saline soil and the amount of curing agents was established,and the extreme difference P-value was<0.0001;the freeze-thaw strength decay model of improved saline soil with four kinds of curing agents was established,and the variance R2 was mostly above 0.98.(2)Using the self-designed indoor freeze-thaw test apparatus,the variation of freezing and swelling force,salt and freezing swelling amount with depth,compaction coefficient,and water content of saline soils and improved saline soils of five different densities and three different water contents were revealed by eight freeze-thaw cycles.Mathematical models were developed for the relationship between the maximum frost swelling force and the maximum frost swelling volume of saline and modified saline soils with the compaction coefficient and the number of freeze-thaw tests,and equations were fitted for the relationship between salt and depth of saline and improved saline soils.The freeze-thaw test found that the freezing and swelling forces of saline soils and modified saline soils fluctuated several times when the temperature was between-15℃ and 0℃ during a single freeze-thaw cycle,revealing the mechanism of freezing damage that the channel liner damage occurs mostly in spring during freeze-thaw.Based on the rule that the maximum frost swelling force,salt migration,and maximum frost swelling of saline soil and improved saline soil increase and then decrease with the increase of compaction coefficient,the recommended compaction coefficient area of channel saline soil foundation and improved saline soil foundation was determined.(3)On the basis of the existing mathematical models of frost swell force,the mathematical models of frost swell force of channel lining slab with different inclination angles and the mathematical model of frost swell force of beam arch with elastic foundation were improved;the method of deflection inversion of frost swell force was proposed and the related mathematical model was established.The analytical solutions of the frost swelling force calculated by these three models were obtained through model solving,which revealed the change law of frost swelling parameters and channel lining geometric parameters of integral channel lining on saline soil foundation and obtained the control factors of channel lining frost swelling damage.The errors of the liner frost swelling force calculated by the established deflection inversion frost swelling force model and the improved elastic foundation beam arch frost swelling force model are 7.2%and 14.8%,respectively,from the measured values.Based on the rule that the maximum control tensile stress of the channel lining is equal to 10%of the allowable tensile stress of the lining concrete,the optimal thickness of the channel lining can be determined by using the three models.(4)Based on the response mechanism of monolithic channel lining in saline soil areas suffering from both salt swelling and frost heave,the characterization unit body was selected for the study,and the controlling equations of water-thermal-force-salt(HTSM,hydraulic-Thermal-Salt-Mechanic)multi-field coupling were established by using the continuous medium matter theory,conservation of matter and conservation of energy.Compared with the traditional coupled model,the advanced features of the coupled model proposed in this study are:the effects of three factors on water migration and salt migration,such as solute potential,non-isothermal conduction hydraulic coefficient,and soil wetting heat,are analyzed comprehensively,and the control equations of water and salt fields are improved by introducing these three factors;the effects of sulfate solute crystallization,ice-water phase change,wetting heat and water convective heat on the temperature field are considered to improve the control equation of temperature.The effects of ice water phase change and volume expansion of sulfate crystallization in saline soils on stress-strain analyzed,and the stress-strain control equations for saline soils under freezethaw cycles are improved.(5)A comparative analysis of the measured values of temperature,moisture,and frost heaving force of saline soils and improved saline soils with the numerical simulation results of HTSM multi-field coupling was conducted through field freeze-thaw tests and numerical model simulations,showing that the errors between the measured and numerical simulation values were mostly about 10%,with the maximum error being 13.5%,which proved the effectiveness of the HTSM multi-field coupling control equations.The comparative analysis of the measured values of channel lining stress with the calculated and numerical simulated values of the frost swell force model showed that the errors of the measured values of channel lining frost swell stress for saline soils with 10%and 14%water content and improved saline soils with the calculated and numerical simulated values of the frost swell force mathematical model were mostly about 10%,with the maximum error being 10.2%,which verified the validity of the frost heaving force model and further verified the effectiveness of the HTSM coupled model.The validity of the coupled HTSM model was further verified.