Microscopic damage identification and macroscopic mechanical response of loess in seasonal frozen areas

Considering the influences of freeze-thaw cycles(FTCs)on the structural and mechanical properties of the natural loess, the intact loess samples with different moisture contents are prepared.The proton nuclear magnetic resonance(¹H NMR)and the scanning electron microscope(SEM)as well as the triaxial shear tests under FTCs are carried out, and the microstructual damage law and the macroscopic mechanical response mechanism of loess in seasonal frozen regions are studied.The test results indicate that the evolution of the soil microstructure is affected by FTCs significantly.Both the pore volume and the particle shape coefficient go through a fluctuation-growth period and an equilibrium-stable period successively.The pore volume increases markedly during the first six FTCs, and then slowes down after ten FTCs.The sample with high water content owns higher particle shape coefficient before and after FTCs.Compared with FTCs, the shape of stress-strain curve is governed by the moisture content more obviously.The degradation effect of FTCs on the failure strength of specimens is prominent, and is mainly embodied in the first ten cycles.The cohesion decreases exponentially with the increase of FTCs, while the overall attenuation amplitude decreases with the increase of water content, and the internal friction angle increases slightly.For the samples with high moisture content, fewer FTCs are required for the failure strength and cohesion tending to equilibrium.The frost-heaving stress and the moisture migration potential caused by phase change between water and ice loading-unloading repeatedly on the soil skeleton, induce the irreversible damage of microstructure, which is the potential mechanism of degradation of macroscopic mechanical properties of loess.