Cyclic Behavior of Tianjin Soft Clay under Intermittent Combined-Frequency Cyclic Loading

Abstract

Subgrade soft clays are subjected to intermittent changeable-frequency cyclic loading and exhibit complex dynamic reactions compared with those under single-frequency cyclic loading. This research proposed a novel testing scheme to take the influence of combined-frequency subway train speed and intermittency into account. The testing procedure consisted of five loading periods in which occurred undrained combined (low–high–low) frequency cyclic loading and drained static loading, and post-cyclic loading consolidation settlement. A series of triaxial tests on Tianjin soft clay were conducted under intermittent combined-frequency cyclic loading conditions. The pore pressure, deformation, shear modulus and damping ratio, and the post-cyclic-loading consolidation settlement were investigated. The pore pressure and deformation increment decreased greatly when N (the number of cycles in each loading period) = 1,000 (200 × 5), but smoothly when N = 500 (100 × 5) from the first to fifth loading period. The intermittency made the soft clays more resistant to degradation of shear modulus and the influence of subway train speed on the shear modulus and damping ratio should be considered in dynamic design. The post-intermittent combined-frequency cyclic-loading consolidation settlement presented a stable increasing pattern when the cyclic stress ration (CSR) = 0.10. The subway train speed had an evident influence on the post-intermittent combined-frequency cyclic-loading consolidation settlement of soft clays.