Undrained three-dimensional behaviour of soft clay under horizontal one-way and vertical two-way cyclic loading

The undrained cyclic behaviour of clay under three-dimensional (3D) stress states is vital in marine geotechnical applications, especially concerning deformation and stability issues related to pile foundations under oceanic environmental loadings. However, the 3D cyclic behaviour of clay is rarely undertaken in the laboratory. This paper presents an experimental study on the undrained 3D responses of clay performed with a true triaxial apparatus, where horizontal one-way and vertical two-way cyclic stresses are applied to the specimen simultaneously. Emphasis is put on the effects of different combinations of horizontal and vertical stresses on the behaviour of strain, pore water pressure, mobilised friction, and modulus. Test results indicated that cyclic behaviour strongly depended on the 3D stress conditions or stress paths. By introducing a new cyclic stress ratio (CSRA), it was demonstrated that the cyclic shakedown behaviours of clay could be made independent of stress paths. An allowable CSRA, acting as a cyclic stress boundary ratio, was established at 0·08. Interestingly, an increase in the modulus of clay was observed during cyclic loading. This phenomenon contradicted the typically observed modulus degradation of clay under undrained cyclic loadings, adding a new perspective to our understanding of the material's behaviour.