An in-situ testing system with spherical-joint force transmission and fractional-order PID control for reproducing principal stress axis rotation in subgrade - Tiancai Cao; Xinzhuang Cui; Zhenhao Bao; Yefeng Du; Wei Lv; Qing Jin; Pengcheng Jia; Lei Wang; Xiaoning Zhang (Transportation Geotechnics, 2026)