Abstract: In order to explore the influence of freeze-thaw cycles on the mechanical properties and shear band evolution mechanism of copper tail fine sand in Tibet,triaxial shear tests under different freeze-thaw cycles and confining pressures were carried out,and digital image measurement technology was used to measure the strain of the whole surface.The study reveals the variation law of stress-strain characteristics,strength characteristics,elastic modulus and shear strength index of saturated fine tailings.The decay law of cohesion and internal friction angle is described by Logistic model.The influence of freeze-thaw action on the initiation time and initiation strain of shear band is quantitatively characterized,and its evolution path is ana lyzed.The results show that the confining pressure and freeze-thaw cycles have a significant antagonistic effect on the mechani cal behavior of tailings fine sand.The confining pressure increases the failure strength,promotes the plastic deformation and in hibits the attenuation of elastic modulus by enhancing the lateral constraint.The freeze-thaw cycle leads to the decrease of strength and elastic modulus by repeated frost heave and thaw settlement,which aggravates the tendency of brittle failure.With the increase of freeze-thaw cycles,the cohesion and internal friction angle are significantly attenuated due to the destruction of cementation structure and the grinding of particle edges and corners,which provides a prediction tool for the anti-freeze-thaw design of tailings pond engineering.The micro-mechanism analysis shows that the micro-cracks and weak surfaces induced by freeze-thaw action will develop preferentially,resulting in the early initiation time of the shear band and the decrease of the crit ical strain.The morphology also gradually develops from the initial concentrated distribution to the dispersed multi-band sys tem,showing the cumulative evolution path of the internal damage of the fine sand during the freeze-thaw process.

Key words: copper tailings sand,freeze-thaw cycles,shear strength,shear band,stress-strain curve