Investigation on the Bonding Behavior of the Strand–Grout Interface in Ground Anchors

Although the long-term behavior of ground anchors depends fundamentally on interfacial behavior, the independent effect of the strand–grout interface on load loss has not been comprehensively investigated. This study establishes a physical model testing method that isolates the strand–grout interface and systematically investigates both short-term and long-term load loss behavior. Pull-out tests and long-term monitoring tests were conducted using grout uniaxial compressive strength (qu = 18–30 MPa) and bond length (Lb = 900–1500 mm) as primary design variables. Long-term monitoring confirmed that prestress loss at the strand–grout interface is induced by the progressive pull-out displacement of the strand over time, following a logarithmic decay pattern. The load reduction coefficient n was significantly more sensitive to Lb than to qu; n increased sharply from 0.015 to 0.069 as Lb decreased. Anchors with insufficient bond length exhibited secondary load reduction behavior that disrupted the stable log-linear decay, posing significant risk to long-term performance. Based on RMSE analysis of the fitted logarithmic model, a minimum monitoring period of approximately 50 days is recommended for reliable long-term prediction when bond length is adequate. These findings identify qu and Lb as the governing parameters, providing a quantitative basis for optimizing prestress design and enhancing the long-term reliability of anchor systems.