Hydromechanical Analysis of Collapse Settlement of Loess during Field Immersion Tests: Field Investigations and Numerical Modeling

Loess is the most well-known collapsible soil. The field immersion test is one of the effective means to assess the in situ self-weight collapsibility of soil. However, the reason why huge differences appeared in collapse settlements in different regions remains unclear, and which factors largely determine the amount of collapse deformation of loess during field immersion tests are worthy of being discussed. In this study, a numerical model for hydromechanical (HM) behaviors of loess was developed, in which the effects of the porosity on the water retention behavior and intrinsic permeability were incorporated. A field immersion test associated with the monitored collapse settlement, water retention curve measurements, and the variation of self-weight collapsibility with depth were used to testify to the effectiveness of the numerical model. The model was then adopted to clarify the effects of compression index, initial suction, and yield stress on the coupled HM or collapse settlement behavior of loess in field immersion tests. The aforementioned three parameters are relatively easy to be determined from various sites and have obvious differences. Then, a total of 126 numerical cases were conducted by the HM model, with the range of the collapse settlement from 0.334 m to 1.136, which covers the most range of the collapse settlement of the field immersion test systematically summarized by previous researchers. The result shows that yield stress and compression index have a significant influence on collapse settlement. The initial suction has a slight influence on collapse settlement, which mainly affects the velocity of water transport. The model herein can be used in the assessment of the in situ collapsibility of loess, and when the basic physical properties of a site are determined, the numerical simulation database of this study can preliminarily determine the site's collapsibility grade and amount of the collapsibility.