Experimental Investigation of the Deformation Characteristics of Natural Loess under the Stress Paths in Shield Tunnel Excavation

Natural loess with large voids and weakly bonded structures is widespread in the arid areas of the world, particularly northwestern China. Recent experimental studies have shown that the mechanical behavior of natural loess is stress path dependent. In practice, soils influenced by the construction of earthen structures may undergo various complex stress paths that are very different from the conventional stress paths commonly considered in geolaboratory experiments. Because of the limitations of current technologies, real stress paths are difficult to obtain from field tests or physical modeling. This study focused on the deformation behavior of natural loess from Jingyang, China, under the stress paths around a shield tunnel. First, the stress paths around a shield tunnel were determined from the numerical data obtained at different positions in a distinct-element simulation of shield tunnel excavation in sand (for simplicity) and by using a novel method referred to as the equivalent stress ratio method. Second, a set of undrained triaxial tests were conducted using the conventional and complex stress paths. The experimental results demonstrate that the deformation characteristics of the loess are different at different positions around the shield tunnel, and the largest deformation appears in the lateral zone (0°). This indicates that the lateral zone is the key zone to be controlled during shield tunneling. In addition, the relationship between the stress increments and the strain increments varies with the stress path. In a complete unloading path, the behavior of natural loess is largely elastic and linear. On the contrary, in a complete loading path or semiloading path, the behavior is largely inelastic and nonlinear, and this behavior is associated with the stress state and recent stress history. These results are valuable in establishing the constitutive relationships for natural loess under complex stress paths and may be useful for the construction of shield tunnels in loess areas.