Accumulated deformation and microstructure of deep silty clay subjected to two freezing-thawing cycles under cyclic loading

The problem of the settlement and deformation of soft clay is becoming more and more prominent under the subway vibration loading. Due to the complexity of the construction environment in the soft soil areas, the artificial freezing method is often used to reinforce the soil, and sometimes it is also subjected to twice freezing. The Shanghai subway line crosses vertically and horizontally. The deepest excavation depth reaches 33.1 m, so the construction of the subway tunnel will develop to a deeper stratum. In this paper, the GDS dynamic triaxial tests were conducted to study the accumulated deformation of the gray silty clay of layer no.8 in Shanghai, 60 m in depth. Considering the influence of vibration time, frequencies, freezing temperatures, and freezing-thawing cycles, the axial strain of the deep silty clay before and after freezing-thawing was analyzed. The results show that the strain of the soil subjected to one freezing-thawing cycle and two freezing-thawing cycles increases by 39.6% and 72.8% comparing with the unfrozen soil, respectively. With the increase of vibration time, the rates of increasing of the axial strain gradually decrease. The higher the frequency and the lower the freezing temperature, the more deformation produces to the soil. The microstructures of the soil before and after freezing-thawing were obtained by mercury intrusion porosimetry (MIP) tests. There is a bottleneck with the pressure about 100 psi in the mercury intrusion process. Under the same cyclic loading, the lower the freezing temperatures and the more the freezing-thawing cycles, the more the mercury intrusion volume and the cumulative pore area. The most probable pore diameter of the unfrozen soil is about 524 nm smaller than that of the thawing soil, which is about 1050 nm. The pores 10-100 nm in diameter account for a large part. This research is of great guiding significance for the construction of deep subway tunnels.