We propose a test procedure to quantify the response of dry sand to cyclic compressional loading under constrained conditions. The test procedure is designed to represent the response of sand layers to upward propagating P-waves during an earthquake event. Such P-waves are prominent within the vertical component of earthquake ground-motions, which is often ignored or simplified in common practice of seismic hazard analysis, despite its potential damaging effects. In the proposed method, the lateral deformation is restrained within a triaxial device, through variations of the cell pressure, thus maintaining pure compression while allowing moderate to large axial strains. Both dry and saturated samples are tested, and the compressive stiffness is computed from the full stress-strain loops. We show that as long as drained conditions are maintained and volume changes are allowed - the response of a saturated sample to slow cyclic loading is representative of the response of dry sand to seismic loading, despite the differences in saturation and in strain rates. Finally, we compare the proposed method to cyclic loading within a rigid cell and discuss the differences and limitations that the new proposed method overcomes.

Although the root can enhance the soil's strength, vegetation cover landslide still occurs frequently under the rainfall. To elucidate the mechanism underlying the degradation of the shear strength of root-soil composites under the influence of moisture, we investigated trees from hilly slopes in southeastern China. The tensile mechanical properties of roots were tested under varying moisture conditions.The results of previous work on the friction characteristics of the root-soil interface under different soil water content were also considered. Furthermore, large-scale direct shear tests were performed to assess the strength characteristics of root-soil composites under different root cross-sectional area ratios (RAR) and moisture contents. Based on the widely used Wu model, and incorporating the failure modes of roots in root-soil composites and the mechanism of root-soil interface friction, a root-soil composite strength degradation model was established considering the effects of moisture. Moisture significantly affected the tensile strength of fine tree roots, with the tensile strength of fine roots being lower in the saturated state than in the fresh state. In contrast, coarse roots were almost unaffected by moisture. As the moisture content increased, the additional strength provided by the roots decreased, and the root efficiency (REp) decreased significantly. The model was validated against experimental data, and the calculated results were accurate. In root-soil composites, as moisture infiltrates, the tensile strength of the roots, soil shear strength, and root-soil interface shear strength decrease to different degrees. This results in reduced resistance to deformation in the root-soil composites, leading to a decrease in its strength.

The anisotropy of shear resistance depending on friction direction can be selectively utilized in geotechnical structures. For instance, deep foundations and soil nailing, which are subject to axial loads, benefit from increased load transfer due to greater shear resistance. In contrast, minimal shear resistance is desirable in applications such as pile driving and soil sampling. Previous studies explored the shear resistance by interface between soil and surface asperities of a plate inspired by the geometry of snake scales. In this study, the interface friction anisotropy based on the load direction of cones with surface asperities is evaluated. First, a laboratory model chamber and a small-scale cone system are developed to quantitatively assess shear resistance under two load directions (penetration -> pull-out). A preliminary test is conducted to analyze the boundary effects for the size of the model chamber and the distance between cones by confirming similar penetration resistance values at four cone penetration points. The interface shear behavior between the cone surface and the surrounding sand is quantitatively analyzed using cones with various asperity geometries under constant vertical stress. The results show that penetration resistance and pull-out resistance are increased with a higher height, shorter length of asperity and shearing direction with a decreasing height of surface asperity.

Understanding direction-dependent friction anisotropy is necessary to optimize interface shear resistance across soil-structure. Previous studies estimated interface frictional anisotropy quantitatively using contractive sands. However, no studies have explored how sand with a high dilative tendency around the structural surface affects the interface shear response. In this study, a series of interface direct shear tests are conducted with selected French standard sand and snakeskin-inspired surfaces under three vertical stresses (50, 100, and 200 kPa) and two shearing directions (cranial -> caudal or caudal -> cranial). First, the sand-sand test observes a higher dilative response, and a significant difference between the peak and residual friction angles (phi peak - phi res = 8 degrees) is obtained at even a lower initial relative density Dr = 40%. In addition, the interface test results show that (1) shearing against the scales (cranial shearing) mobilizes a larger shear resistance and produces a dilative response than shearing along the scales (caudal shearing), (2) a higher scale height or shorter scale length exhibits a higher dilative tendency and produces a higher interface friction angle, and (3) the interface anisotropy response is more pronounced during cranial shearing in all cases. Further analysis reveals that the interface friction angle and dilation angle are decreased with the scale geometry ratio (L/H). For L/H values between 16.67 and 60, the interface dilation angle varies between 9 degrees and 4 degrees for cranial first shearing and 3.9 degrees-2.6 degrees for caudal first shearing. However, the difference in dilation angle within the same shearing direction is less than 1 degrees.