With the rapid development of marine engineering, large−diameter steel pipe piles are increasingly used in infrastructure construction, such as bridges, docks, and offshore wind power projects. Therefore, studying the shear behavior of the sand–steel interface is of great importance. In this study, the traditional vane shear apparatus was improved by utilizing its torsional shear actuator, adding an overlying pressure fixing device, and applying lateral pressure through a compressive spring. The original cross plate was replaced with a cylindrical steel rod to simulate the shear behavior of the large−diameter pile–sand interface under different stress states. Experimental results show that this apparatus effectively solves the problem of soil loss due to the shear gap in both the ring shear and direct shear tests under smooth interface conditions. As the shear rate (2°/min, 4°/min, 6°/min) increased, the peak and residual shear stresses decreased, while the shear stress increased with vertical confinement pressure, accompanied by significant residual stress. As the relative density of sand increased from 27.4% to 72.2%, the shear behavior transitioned from contraction to dilation. Regarding surface roughness, the experiment identified a critical threshold: when roughness is below this threshold, it significantly affects the peak shear strength; when above this threshold, the effect is smaller, and failure shifts to the internal sand body. This study provides valuable insights into the mechanics of the sand–steel interface and contributes to optimizing the foundation design for marine infrastructure.

Shape memory alloy dampers (SMADs) are widely applied in structural vibration control due to their excellent superelastic properties. However, there has been no research on the random wind-induced vibration control of transmission tower-line (TTL) systems with added SMADs. To address this gap, this paper proposes an analytical framework for the wind-induced vibration control of TTL systems with SMADs under random wind loads. An analytical model for the coupled TTL system is developed. The constitutive relationship of the SMAD is derived using the statistical linearization method, and a vibration control approach for the TTL-coupled system with SMADs is proposed. The vibration response of the TTL–SMAD system under random wind loads is derived, and an extreme response analysis framework based on the first exceedance failure criterion is established. The results show that the optimal installation scheme for the SMAD achieves a vibration reduction of more than 30%. When the damper’s stiffness coefficient is approximately 1, the SMAD effectively controls the vibrations. Moreover, a service temperature of 0 °C is found to be the optimal control temperature for the SMAD. These findings provide important references for the application of SMADs in the vibration control of TTL systems.