Abstract:  A series of triaxial tests was carried out under coupled cyclic-seepage loads for Tianjin coastal soft clay, and the variation of permeability in different loading stages was analyzed systematically. Combined with SEM and MIP, the evolution mechanism of permeability was elaborated. The results showed that under the coupling cyclic-seepage loads, the permeability of soft clay present three variation stages. With seepage loads increasing, the permeability increased initially and then decreased, but just reverse for increase of critical dynamic stress ratio. The evolution of permeability was caused by the adjustment of micro-features of pore shape, size, and distribution: for initial vibration, large compression of super large pores (D>2.5 μm) between particles occurred and pore shape varied little, leading to the linear reduction of permeability; at the medium loading stage, the large particles were broken, and the newly formed small particles were densely filled between the large particles, inducing a large number of “compact and zigzag” small pores (0.05 μm <D <0.1 μm), which led to the slow decrease of permeability; at later loading stage, the structure was compact and stable, and permeability was basically unchanged. For the unstable deformation, the “strips” like macropores were re-formed between the particle clusters (aggregates), and the permeability increased. The results can provide a theoretical basis for the determination of the permeability in the fluid-solid coupling analysis.

Key words: soft clay, cyclic-seepage coupling, permeability, scanning electron microscope (SEM), mercury intrusion porosimetry (MIP), evolution mechanism