This study investigates the strain development of saturated silty soil of Yellow River under varying initial consolidation inclination angles zeta by principle stress rotation tests. The results revealed that distinct patterns in axial, circumferential and torsional shear strains show the influence of zeta on the mechanical response of silty soil. Notably, the axial strain exhibits compressive behaviour at zeta=90 degrees during the first cycle, while the circumferential strain displays tensile behaviour. Anisotropy initiates at zeta=90 degrees and around 60 degrees for other zeta angles. Different values of zeta exhibit stabilization trends in strain fluctuations, with zeta=90 degrees and zeta=75 degrees showing intriguing similarities. The case of zeta=45 degrees stands out, with the highest fluctuation and strain amplitude. Torsional shear strain similarities are observed among most zeta angles except for zeta=90 degrees and zeta=60 degrees. Volumetric strain emphasizes the significant impact of consolidation angle inclination on anisotropic characteristics. With the increase of the initial solidification angle, the hysteresis curve shifts to the left, indicating cyclic creep characteristics, with negligible shear strain for the case of zeta=60 degrees. As the cycle period increases, the hysteresis loop contracts, indicating the continuous strengthening and eventual stabilization of shear stiffness. This comprehensive exploration provides valuable insights into the complex behaviour of saturated silty soil under rotational stress conditions, highlighting the role of initial consolidation inclination angles in shaping its mechanical response.

Undrained behavior of sandy soil with fines content is a challenge in geotechnical research. In this article, the effect of low clay content (plastic Kaolin) on the anisotropic behavior of sand is studied. In the technical literature, there are different data about the effect of fine particles (generally high percentage), but there are not enough studies on low fines content (especially plastic fines) and anisotropic conditions. For this purpose, 30 undrained tests are performed using a torsional shear hollow cylindrical apparatus (TSHCA) with constant (alpha o) and (b) values on Firoozkuh sand. The specimens had Kaolin contents of 0, 3, 5, 7 and 10%, and the inclination angle (alpha o) is varied from 15o to 60o. The specimens are prepared by dry deposition method and are consolidated under P'c= 100 and 200 kPa. The results of the experiments show that increasing the (alpha o) leads to more contractive behavior in sand. By adding clay particles to the host sand up to 3%, the peak strength of the specimen is increased (7% and 6% for alpha=15 degrees and 30 degrees, respectively), and then with the increase of clay content up to 10%, the strength of the specimen is decreased (33% and 22% for alpha=15 degrees and 30 degrees, respectively). But at alpha = 60o, with the addition of 5% clay, decrease in the peak strength is observed (about 15%) and with a further increase in the clay content, unlike the angles of 15o and 30o, increase in the peak strength of the specimen is observed, so that at 10% clay, the strength of the specimen is higher than the host sand (about 7%), which can be attributed to the cohesion nature of the clay particles. With the increase of clay content, anisotropy degree is decreased. In other words, with the increase of fines content, the anisotropic behavior is decreased.

This study uses a hollow cylindrical apparatus to explore the effects of principal stress rotation on saturated silty soil, focusing on the static characteristics affected by cycle counts, intermediate principal stress coefficient ( b ), and rotational angle (alpha). As the principal stress axis rotates, strain fluctuations decrease and stabilize, with consistent strain trends observed across various b values. Anisotropy appears around 60 degrees during the first cycle, significantly impacting radial strain while torsional shear strain remains less affected. Distinct hysteresis loops in shear stress-strain relationships reveal initial unclosed forms due to plastic strain accumulation, transitioning to closed loops with increased cycling, and showing noticeable variations in shear stiffness. As b values rise, stiffness degrades, influenced by both b values and a angles. Volumetric strain shows a linear increase for two cycles before decelerating, with b =1 demonstrating anisotropy at 60 degrees and other values at 90 degrees. Minimal contraction occurs for b =0 after the tenth cycle, while b =0.5 sees significant volume reduction. Higher b values also reduce non-coaxial behavior, linked to the initial principal stress orientation. These findings enhance the understanding of silty soil behavior under stress rotation, offering valuable insights for geotechnical engineering applications.