Engineering construction in loess areas often requires improvement of loess. Basalt fiber-reinforced soil due to complexity of material composition often exhibits poorly predictable mechanical behavior. In this study, digital image technology-based unconsolidated-undrained (UU) triaxial shear tests were carried out on loess samples at three fiber lengths (L) and four fiber contents (η). Results prove the improvement of the shear strength of loess by basalt fiber inclusion, which varies in inverted u-shaped pattern with fiber length or fiber content, with the maximum at η = 0.6%, L = 12 mm. Digital Image Technology was employed to analyze the damage characteristics and strain field of the surface of the sample at different loading time. The volumetric strain of the reinforced sample decreases at higher fiber content or fiber length, from shear contraction to dilatancy. The unreinforced sample exhibits a typical brittle failure mode with visible shear band, while plastic failure for reinforced samples with an overall bulging failure mode. A statistical damage constitutive model of fiber-reinforced loess was established with limited parameters calibrated. The rationality of the model was verified by comparisons of measured and calculated stress-strain data.

The importance of the hydraulic properties of unsaturated soil for geotechnical designs has been well understood. Many studies on the soil water retention curve (SWRC) and the relative coefficient of permeability under drying and wetting cycles have been performed. However, most of the previous investigations of hydraulic properties are limited to a single drying and wetting cycle or are based on multiple drying–wetting cycles with limited soil experiments. This might be because the measurement of both the SWRC and the coefficient of permeability of unsaturated soil is complex and time consuming. In this study, the hydraulic properties of silicon micropowder (SMP) and Guangxi Guiping clay (GGC) were studied for the same soil specimen under five cycles of drying and wetting in multistep outflow experiments. Significant hysteresis of SWRCs was observed under the first drying and wetting cycle for both soils. However, the hysteresis effect decreased as the number of drying and wetting cycles increased. Furthermore, the coefficient of permeability was estimated using the van Genuchten–Mualem (VGM) model. Results also show that hysteresis plays an important role in the coefficient of permeability for both soils subjected to multiple drying and wetting cycles.