Investigation on solid-liquid transition of soft mud under steady and oscillatory shear loads

An experimental study on the rheological properties of soft mud under steady and oscillatory shear was conducted. The phenomenon of solid-liquid transition is clarified by steady-state and dynamic tests. With an increase of shear load magnitude, the rheological property of soft mud could be classified into three stages: solid stage, solid-liquid transition stage, and liquid stage. This transition is found to be related to changes of the particle network structure during shear. Transitions between the three stages could be defined by two yield stresses stated as static yield stress and fluidic yield stress. The first yield stress corresponds to the transition from solid stage to solid-liquid transition stage, associated with breakup of the initial particle network structure. The second yield stress indicates the complete breakup of the aggregate. Beyond the fluidic yield stress the flow units becomes individual particles and the soft mud behaves like a viscous fluid. To establish a relation between the rheological behavior of mud and its measurable quantities (density and oscillatory frequency), samples of different density (1107–1546 kg/m³) were tested under various load combinations. Results indicate that both the static and fluidic yield stresses show exponential growth with density. For samples of medium density (1356–1168 kg/m³), there will be a sudden loss of strength associated with a decrease of stress at the beginning of the transition stage. Oscillatory frequency (0.05–2.05 Hz) has a neglectable influence at the solid stage, but plays an important role at the other two stages. Moreover, the nonlinear responses of soft mud under large amplitude oscillatory shear were also studied from the stress waveform and Lissajous pattern by the Fourier transformation method. Results show that nonlinearity dramatically increases at the transition stage.