It is particularly important to study the strength of unsaturated loess, and its accurate determination is crucial to the stability analysis of soil slope and foundation and calculation of earth pressure. In order to reveal the control mechanism of physical parameters on the shear strength of unsaturated loess, the intact and remolded loess were used as the research object; sandy silt, quartz flour, and quartz sand were used as contrast; the systematic direct shear tests of unsaturated loess, sandy silt, and quartz sand under different conditions of water content, dry density, and clay content were carried out. The results show that the cohesion, internal friction angle, and shear strength of unsaturated loess piecewise functionally decrease with the increase of water content, its shear strength increases linearly with the increase of dry density, and its internal friction angle shows an upward quadratic function relation with the increase of clay content. The law results of comparing sandy silt, quartz flour, and quartz sand with loess considering water content and dry density are the same; therefore, the equation of shear strength of unsaturated loess is proposed for practical engineering reference, and by the first derivative analysis of the equation, it is feasible to determine the control proportion of the three parameters on the shear strength of unsaturated loess. A stage-like difference between the three control proportions is observed, depending on the combination variations of water content and clay content.

In order to study the effects of dry density and initial water content on the permeability characteristics of loess, the saturated permeability tests of undisturbed loess and remolded loess were carried out in Lanzhou New Area. The anisotropy of undisturbed loess under two factors is analyzed and compared, and the permeability difference between undisturbed loess and remolded loess is also analyzed. The results show that the structure of loess has an important impact on the permeability characteristics, and the dry density is an important structural parameter. The natural structure of undisturbed loess has obvious anisotropy. Under different pore conditions and water contents, it will show the anisotropic behavior of permeability. The influence of water content on permeability is mainly reflected in the influence on the properties of cement between soil particle skeletons. Under different dry density conditions, the influence of water content on permeability is different. When the pore structure is relatively compact, the influence of water content is more obvious. In practical engineering, these two factors need to be comprehensively considered in order to accurately describe the permeability distribution of loess. This study can provide some theoretical guidance for the construction of relevant loess projects.

What the role of each phase medium plays and how their interactions do work should be essential problems to understand dynamic behaviours of soils. In order to disclose interactions between solid, water, and air phases of soils, we applied loess samples to analyse controlling effects of residual deformation on pore pressure based on three kinds of laboratory tests. We obtained the similarity and difference of mechanical behaviors of soil samples under different water contents and loading. Both process and cause of pore air/water pressures are independent of initial stress conditions or loadings. However, absolute values of pore water pressure depend on the confining pressure, whereas the pore air pressure is contrary. The uniformity of responding process and cause of pore pressure depend upon the interaction mechanism between solid particles and air/water media, but the different absolute values depend upon the permeability and compressibility of air/water.