Residual soil widely distributed in Fujian region has the characteristics of strong structure and easy softening in contact with water, which limits the possibility of its beneficial utilization. This study investigates the impact of humid and hot environment on the strength characteristics of residual soil, and how changes in soil microstructure are correlated with strength attenuation. Residual soil with particle size distribution from gravel to clay was subjected to repeated hygroscopic cycle tests. Subsequently, unsaturated triaxial consolidation drainage shear (CD) and nuclear magnetic resonance (NMR) tests were carried out on the samples undergoing 0-7 hygroscopic cycles, and the damage mechanism of the soil was analyzed from macroscopic to microscopic scales. Results showed that the soil shear characteristics were influenced by the number of hygroscopic cycles and had a correlation with stress level (confining pressure and target suction), the greater the cumulative irreversible deformation and the more pronounced shear dilation characteristics of the soil had after more hygroscopic cycles and higher stress levels. The shear strength index of unsaturated soil after repeated hygroscopic paths presented a decreasing trend, but the attenuation of internal friction angle and suction friction angle was limited, and the average values were 21.3 degrees and 14.7 degrees, respectively. The T 2 spectral distribution curve of soil was a trimodal pattern, and the content of small holes consistently decreasing as the cycling process progressed, while the percentage of macropores increased significantly. In view of the continuous dissolution of soluble minerals and cementing materials and the repeated release of suction in the soil, the internal particles of the soil were gradually loosened. Accompanied by the continuous expansion and penetration of intergranular pores, connecting cracks were ultimately formed. The above fatigue damage to the soil pore structure led to the attenuation of its macro-mechanical properties. Throughout the test, the saturated shear strength of the soil continued to decrease due to the interaggregate connection was always broken, while the destruction of the intergranular connection in the aggregate was relatively slow, and the internal friction angle in the soil implied a slow decrease and even stabilized at a later stage. The research results could provide a useful reference for a deeper understanding of the environmental damage effects on the soil macroscopic mechanical properties.

Air-fall pyroclastic soil deposits usually display a loose fabric composed of alternating layers of ashes and pumices. Such deposits, when lying on steep slopes, represent a major geohazard due to the occurrence of landslides. This is the case of the carbonate massifs in Campania (southern Italy), a wide landslide-prone area of approximately 400 km2 covered with pyroclastic soils. In such cohesionless deposits, the additional shear strength provided by soil suction in unsaturated conditions is important for ensuring slope stability and can be jeopardized by soil wetting during rainwater infiltration. This paper provides a comprehensive view of the hydraulic and shear strength characteristics of different layers of pyroclastic deposits at different sites in Campania, revealing a broad view of their similarities and differences. To that end, some datasets from previous studies and novel data are gathered, linking the index properties, the hydraulic behavior of the soils and the contribution of suction to the shear strength of the studied materials. Two types of ashes at different positions within the stratigraphic sequence are identified: ashes interbedded between pumice layers, where landslide failure surfaces usually occur, and altered ashes in contact with the bedrock, which affects water leakage from the overlying soil profile. The former show quite uniform characteristics, and this allowed testing some predictive models for the assessment of the unsaturated shear strength of pyroclastic ashes in the absence of direct measurements. In contrast, the latter may exhibit significantly different behaviors, with great variability in hydraulic and mechanical properties.