Experimental Study On Thaw-Induced Weakening Of Coarse-Grained Frozen Soil

The total area of the global permafrost region accounts for about 25%of the land area.The distribution area of frozen soil in China accounts for more than 70%of the land area,of which permafrost accounts for more than 20%,mainly distributed in the northern mountainous areas of Northeast China,the western mountains and the Qinghai-Tibet Plateau.Thaw-induced weakening is a common disease problem in the practice of engineering construction,operation and resource development in coarse-grained permafrost regions for many years.At present,the research on the thawing process and strength decay law of coarse-grained frozen soil is far from mature.In this thesis,the ultrasonic velocity and NMR signal intensity of coarse-grained frozen soil at different temperatures were measured based on ultrasonic method and NMR method,and the thawing law of coarse-grained frozen soil was explored.The significant temperature range of coarse-grained frozen soil under factors such as particle size and density was clarified.The strength decay law and thermal thawing deformation law of coarse-grained frozen soil were revealed by low temperature triaxial test.The research results lay a foundation for improving the law of heat-induced thaw settlement and strength change of coarse-grained frozen soil,and are of great significance for engineering construction and heat-thaw disaster prevention and control in cold regions of China.The main results are as follows:（1）The thawing process of coarse-grained frozen soil was measured based on ultrasonic method and NMR method.The acoustic wave velocity began to change at about-3°C,and the wave velocity decreased sharply in the range of-3°C～2°C,and became stable after the temperature exceeded 2°C.The unfrozen water content and T₂ distribution curve began to change from-5°C,and increased sharply in the range of-5°C～0°C,and tended to be stable after the temperature exceeded 0°C.（2）The temperature range measured by the ultrasonic method is about 2°C higher than that measured by the nuclear magnetic resonance method,the coarse-grained frozen soil measured by the ultrasonic method has a hysteresis phenomenon during the melting process;furthermore,it is indicated that the critical temperature range of coarse-grained frozen soil thawing is-3°C～2°C.A novel suggestion can be proposed that geothermal should be kept under-3℃to avoid thawing settlement and weakening of frozen soil.（3）The strength characteristics of coarse-grained frozen soil under different temperature,confining pressure and particle size were studied by constant temperature loading triaxial compression test.At the same temperature,the initial slope and peak strength of stress-strain curve of coarse-grained frozen soil increase with the confining pressure.Under the same confining pressure,the peak strength decreases with the increase of temperature,especially when the temperature rises from-3°C to-2°C,the decrease of stress-strain curve is the most significant compared with other heating gradients.Under different confining pressures,the stress-strain relationship of particle size less than 5mm changes from strain softening to strain hardening with the increase of temperature,and the frozen soil with particle size less than2mm shows strain softening characteristics.Under the same temperature and confining pressure conditions,the change trend of frozen soil with particle size less than 2mm is slightly higher than that of frozen soil with particle size less than 5mm.（4）The cohesion c and internal friction angleφof coarse-grained frozen soil with different negative temperatures and two particle sizes are obtained,and both decrease with the increase of temperature.The relationships between cohesion c and temperature,internal friction angleφand temperature were obtained by fitting.（5）The unloading test using triaxial device was conducted to test the axial deformation of coarse-grained frozen soil in the process of keeping the axial pressure constant and unloading the confining pressure.The axial deformation of frozen soil with two particle sizes is less than 0.6%between-3°C and-1°C.The unloading process at 0°C reveals a relatively large impact on the failure and deformation of the sample,and the deformation amount is about 1.1%.It is concluded that coarse-grained frozen soil will not produce large deformation and failure when unloading confining pressure under negative temperature.（6）The axial deformation of coarse-grained frozen soil increases under the effect of temperature rise,but the rangeability of the axial deformation decreases with the increase of confining pressure.The curves of axial strain with temperature rise process（-3°C～2°C）can be divided into two stages.The first stage is-3°C～0°C,and the deformation of coarse-grained frozen soil is small at this stage(0%≤₁)≤0.25%).The second stage is 0°C～2°C,and the axial deformation increases rapidly at this stage,and there is an obvious mutation point(0.25%≤₁)≤3.13%).There is an exponential function relationship between deformation and temperature of frozen soil with two kinds of grain sizes（R²>0.97）.