Effects of ultra-freezing/thawing cycles on the mechanical properties and microstructure of Shanghai clay

High temperature superconducting cable uses liquid nitrogen cooling auxiliary system to cool the superconducting cable body to the superconducting critical temperature, ensures that the cable meets the designed current-carrying capacity, exploring the physical and mechanical properties of rock and soil under the freezing and thawing cycles of liquid nitrogen possesses an important practical significance and engineering practical value. In order to explore the changes in shear strength parameters and internal pore characteristics of Shanghai Clay under different freezing and thawing cycles of liquid nitrogen, indoor direct shear tests and mercury intrusion tests were carried out for Shanghai remolded clay that had undergone different freezing and thawing cycles. The results showed that the Shanghai Clay had undergone extremely low temperature after freezing and thawing, its cohesive force gradually decreased while the internal friction angle showed an increasing trend. the mercury intrusion tests results show that the freezing and thawing of liquid nitrogen makes the pore ratio of small pores and ultra-micropores of soil samples decreased, while micropores increased, the internal pore diameter shows a trend that concentrated in the category of micropores; more contact areas were created as a result of the small pore diameter and void ratio decreased, resulting in the increase of soil friction angle. The reduction of micropores and ultramicropores reduced the cementation between clay particles and the cohesive force generated by electrostatic attraction.