Study on the Non-Coaxiality of Fiber-Reinforced Lightweight Soil Induced by Principal Stress Rotation under Traffic Loading

Abstract To investigate how fiber-reinforced lightweight soil (FLS) responds dynamically to stresses resembling those from vehicular loads, hollow cylinder torsional shear tests were conducted to simulate such conditions. The study also focused on the non-coaxial deformation characteristics of FLS under principal stress axis rotation. Key factors such as stress path, cyclic stress ratio (CSR), and torsional shear ratio ( η ) were analyzed, and predictive models were established to describe the directional evolution of principal strain increments, principal stress orientation, and non-coaxial response, characterized by the non-coaxial angle ( β ). The main findings are as follows: (1) The heart-shaped loading path significantly affects the non-coaxial response of FLS. As the principal stress direction angle increases, β exhibits a fluctuating downward trend, which differs distinctly from the pattern typically observed under conventional cyclic triaxial loading. (2) By examining the evolution of stress and strain increment directions within one loading cycle for different CSR values, it was found that the principal stress direction angle decreases approximately linearly, whereas the principal strain increment direction angle shows a nonlinear and fluctuating decline. (3) Larger η values intensify the variation of β and reduce its minimum value. (4) A fitting model was formulated to characterize the development of principal strain increment direction, principal stress direction, and β , and its predictive accuracy was verified against experimental data. This study lays the groundwork for analyzing non-coaxial responses under complex stress states and provides a useful reference for the design of roadbeds and the assessment of soil stability under traffic-induced loading.