Model Test Study on Group Under-Reamed Anchors Under Cyclic Loading

This study conducted laboratory model tests, integrated with Particle Image Velocimetry (PIV) technology, to investigate the evolution of the uplift bearing capacity of an under-reamed anchor group subjected to cyclic loading. The tests considered various working conditions, including different spacing ratios (S/D = 4, 5, 6, where S was the center-to-center spacing and D was the diameter of the under-reamed body), varying cyclic amplitude ratios (λ = 0.3, 0.5, 0.6, 0.7, 0.8) and different cycle times (M = 1, 5, 10, 30). PIV was utilized to observe the displacement field of the surrounding soil, revealing the group effect of the anchors and the variation in their uplift capacity under diverse cyclic amplitudes and cyclic times. The results indicated that the load–displacement curves could be delineated into three distinct stages: elastic, elastoplastic, and plastic. Notably, the group effect primarily initiated during the elastoplastic stage and developed significantly within the plastic stage. The cyclic amplitude ratio was identified as a key factor influencing the uplift capacity. Furthermore, compared to results from single pull-out tests, both the vertical displacement of the surrounding soil and the shear strength of the sidewall adjacent to the under-reamed body decreased following cyclic loading. Finally, the influence of the cyclic times depended on the occurrence of anchor failure; in the absence of failure, the anchor maintained satisfactory performance even after multiple cycles.