| Frost heave is a common and typical frost damage phenomenon in frozen soil engineering,and it has become a recognized scientific and technical challenge.Excessive frost heave deformation will affect the normal service performance of engineering structures,and may even cause significant damage to the engineering infrastructure.The systematic mastery of the physical and mechanical properties of frozen soil,the clarification of the law of formation and growth of ice lens,and the development and improvement of the frost heave model system have important theoretical value and significance of disaster prevention and mitigation for deepening the research of frozen soil and preventing the engineering disasters of frozen soil.This study based on the background of frost heave problem of frozen soil engineering in cold regions,and combined the indoor tests,theoretical analyses and numerical simulations,the tensile strength and penetration characteristics of warm frozen silty clay in the Qinghai-Tibetan area,the weight order of the main controlling factors affecting frost heave,as well as frost heave control methods and mechanisms are systematically investigated.The main research results are as follows:(1)Based on the theory of elasticity,the stress distribution characteristics in the wall of thick-walled hollow cylinders are obtained,and the optimal specimen size of hydraulic fracture test and the expression of tensile strength calculation are given.By comparing the measured data with the traditional tensile strength test method of frozen soil,the feasibility and reliability of hydraulic fracture to measure the tensile strength of frozen soil are verified from both qualitative and quantitative perspectives.It is proposed that hydraulic fracture method can be an effective complementary method for the traditional tensile strength test method of frozen soil,and it is also suggested that necessary corrections need to be given to the measurement of tensile strength of warm frozen soil by using the indirect tensile method.(2)The mechanism of formation and growth of tensile strength of warm frozen soil is clarified,and it is recognized that the tensile strength of warm frozen soil is mainly due to the formation of ice and the increasing volume of ice in the frozen soil.The composition of the tensile strength of warm frozen soil is quantified,in which ice content(quantity)and ice strength(quality)contribute more than 90% to the tensile strength of warm frozen soil,and the prediction of the tensile strength of warm frozen soil can be predicted according to this result.Initial water content and temperature are the key factors that jointly limit the development and evolution of tensile strength in warm frozen soil.Where the initial water content mainly affects the tensile strength level of warm frozen soil,the temperature mainly controls the growth rate of strength.The comprehensive growth rate of tensile strength of warm frozen Qinghai-Tibetan silty clay is about 0.50 k Pa/(%·℃)under the conditions of the present test.(3)Fractal theory was used to provide an accurate mathematical description of the 3-D geometry of the seepage channel,and the seepage discharge equations for the non-standard pore channel were obtained according to Hagen-Poiseuille law,which leads to a model of the hydraulic conductivity of the saturated freezing soil under the fractal perspective.Considering the ice block effect of pore ice formation on water migration and the binding effect of clay particles on water,the equivalent hydraulic radius of the saturated freezing frozen soil seepage channel was corrected using unfrozen water saturation and adsorbed water coefficient.Meanwhile,using the mathematical conversion relationship between hydraulic radius and equivalent radius,the seepage discharge equation of saturated freezing soil was obtained,and the theoretical model of threefold fractal hydraulic conductivity of saturated freezing soil was finally obtained.(4)The hydraulic conductivity of saturated freezing soil is positively correlated with the fractal of pore distribution,and negatively correlated with the fractal of streamline tortuous.The hydraulic conductivity of frozen soil has a relatively complex mathematical analytical relationship with the fractal of pore distribution and fractal of streamline tortuous.The effect of temperature on hydraulic conductivity is essentially by affecting the pore structure and unfrozen water saturation.The pore microstructure parameters mainly control the upper limit value of the hydraulic conductivity,while unfrozen water saturation controls the change pattern of the hydraulic conductivity.It was found that the effect of adsorbed water coefficient on frozen soil hydraulic conductivity is equally significant,especially in the related model for calculating frozen soil hydraulic conductivity based on the soil freezing characteristic curve.Therefore,considering the effect of adsorbed water coefficient is crucial for accurate calculation of frozen soil hydraulic conductivity.In addition,using power functions to describe the mathematical relationship between frozen soil hydraulic conductivity and temperature has its range of applicability,it should be used with full theoretical model rather than simplified mathematical model when predicting hydraulic conductivity of low-temperature frozen soil.(5)Based on the optimized separated ice frost heave model,the various influence factors affecting the frost heave of Qinghai-Tibetan silty clay soils from the strongest to the weakest are obtained as hydraulic conductivity,cold end temperature,and overburden load,and the hydraulic conductivity is the most significant factor among the different frost heave influences,and it is found that the frost heave characteristics of the soils are determined by the multi-factors together.It is proposed that a comprehensive evaluation should be combined with the actual environment when classifying the soil as frost heave susceptibility,and it is considered that the evaluation is incomplete if only using the soil types as criteria.(6)Based on the indoor frost heave test of polyacrylamide modified silty clay,the frost heave mitigation mechanism was revealed,and it was found that the polyacrylamide and water can form a competitive adsorption on the surface of soil particles.Competitive adsorption is an important reason for the agglomeration of fine soil particles and the results of infrared spectroscopy showed that there is hydrogen bonding adsorption between soil particles and polyacrylamide,which confirms that the physical modification of silty clay by polyacrylamide is a physical modification. |
