Study On Continuous Non-destructive Pore-ice Monitoring Methods And Local Frost Heave And The Segregation Mechanism Of Unsaturated Silty Clay

Unsaturated silty clay is a common frost susceptible soil.Its frost heave deformation easily induces engineering diseases such as subgrade deformation,culvert cracking,and pile foundation uplifting in cold regions,threatening the construction and operation of cold region projects in China.At present,there are two bottlenecks in the study of frost heave mechanism of unsaturated silty clay:(1)most of the current frost heave test and model studies are proposed for saturated soil.The frost heave mechanism of unsaturated soil is unknown,and there is a lack of theoretical models suitable for unsaturated soil;(2)the current frost heave test method is not suitable for microscopic measurement,so it is impossible to continuously monitor the process of water migration,segregation ice accumulation,and soil local deformation during frost heave.Therefore,the test data cannot meet the needs of testing and improving the theoretical model of frost heave.This study addresses the above bottlenecks,proposes a new continuous non-destructive observation method of pore ice content and local deformation of frost heave for the refined observation of frost heave hydrothermal coupling process,and reveals the frost heave mechanism of unsaturated silty clay based on experimental phenomena,the main findings and conclusions of the paper are as follows.(1)A non-destructive and continuous measurement technique of pore ice content based on transient heat source method is proposed to realize the measurement of pore ice content in the temperature range of-20～-0.5℃ for silty clay,sandy soil and standard sand,and the average error of pore ice content measurement of the new method is 2%.The perturbation of local temperature field of in-situ permafrost during the measurement is less than 0.3℃,and the performance of the new method has been significantly improved compared with that of the same type of sensors.(2)An image preprocessing method combining the histogram equalization method and the automatic threshold method are proposed to solve the grayscale mutation problem caused by ice and water phase change and frost cracking in the application of digital image correlation(DIC)observation technique for frost heave observation,and realize the non-contact continuous measurement of sub-millimeter frost heave deformation.The method is successfully applied to the observation of frost heave deformation in unsaturated chalky clay,and the measurement accuracy is better than 0.3mm.(3)The tests revealed two important characteristics of the segregation frost heave of unsaturated silty clay,namely: 1)the segregation ice grows in the form of ice columns,destroys the soil structure,and thus forms segregation cracks are not completely filled with ice;the degree of saturation at the segregation cracks in this test is about 0.6-0.8;2)the continuous segregation ice growth occurs when the soil temperature is between the freezing point and the SFCC inflection point.As long as this condition is met,the pore ice will continue to grow and thus form a crack;at the same time,when the soil meets this condition,the crack in the frozen area will also continue to grow.This study provides a refined observation method for the segregation and frost heave of frozen soil,and reveals the segregation frost heave mechanism of unsaturated silty clay.These results can provide detailed data support for the establishment and verification of frost heave models for unsaturated silty clay,and are of great significance to the improvement of frozen soil mechanics theory.