Effect of freeze-thaw cycles on soil engineering properties of reservoir bank slopes at the northern foot of Tianshan Mountain

The instability of soil bank slopes induced by freeze-thaw cycles at the northern foot of Tianshan Mountain is very common. The failure not only caused a large amount of soil erosion, but also led to serious reservoir sedimentation and water quality degradation, which exerted a lot of adverse effects on agricultural production in the local irrigation areas. Based on field investigations on dozens of irrigation reservoirs there, laboratory tests were carried out to quantitatively analyze the freeze-thaw effect on the soil engineering characteristics to reveal the facilitation on the bank slope instability. The results show that the softening characteristics of the stressstrain curves gradually weaken, the effective cohesions decline exponentially, the seepage coefficients enlarge, and the thermal conductivities decrease after 7 freeze-thaw cycles. The freeze-thaw effect on the specimens with low confining pressures, low dry densities and high water contents is more significant. The water migration and the phase transition between water and ice result in the variations of the soil internal microstructures, which is the main factor affecting the soil engineering characteristics. Sufficient water supply and the alternation of positive and negative temperatures at the reservoir bank slopes in cold regions make the water migration and phase transition in the soil very intensely. It is easy to form a large number of pores and micro cracks in the soil freezing and thawing areas. The volume changes of the soil and the water migration are difficult to reach a dynamic balance in the open system. Long-term freeze-thaw cycles will bring out the fragmentation of the soil particles, resulting in that the micro cracks on the soil surfaces are developing continuously. The soil of the bank slopes will fall or collapse when these cracks penetrate, which often happens in winter there.