Experimental Study On The Rheological Properties Of Dense Cohesive Sediments Under Shear Loadings

The cohesive sediments movement is extremely complex, which is influnced by many factors. Up to now, there are still many fundmental theoretical problems unrevealed. In order to reveal the mechanism of cohesive sediments‘ flow and deformation, more precise and reliable experiment data are desperately needed. The study on the instinct properties of cohesive sediments also requires multiscale and multi interdiscipline. Hence, this dissertation has the following main research contents based on the rheology methodology and theories of mechanics of sediment transport:Firstly, the rheological properties and response of dense cohesive sediments under steady shear loadings were investigated. The process of solid-liquid phase transition of dense cohesive sediments was revealed in detailed. The rheological response of dense cohesive sediments under low shear stress, low shear rate and strain was obtained, respectively. The influences of different types of shear loadings, shear time, water content and particle size on rheological properties were also respectively investigated.Secondly, the rheological response of natural dense cohesive sediments under oscillatory shear loading was investigated. Typical nonlinear viscoelastic response of dense cohesive sediments under large amplitude oscillatory shear was revealed. The effect of oscillatory frequency on viscoelastic characteristics of cohesive sediments was discussed. To further investigate and qualify the nonlinear viscoelastic response of natural cohesive sediments, the nonlinear stress waveforms were analyzed by using the fast Fourier transform method. The rheological model for cohesive sediments under oscillatory shear loading was provided. The wave-induced mud fluidization mechanism was revealed.Thirdly, the characteristics of stratification and rheological properties of nearbed cohesive sediments in Xihu Lake of Hangzhou, China were studied. The basic properties of the nearbed sediments and their rheological properties under steady shear loadings were measured. The characteristics of stratification of nearbed cohesive sediments were analyzed based on the measurements at five different locations in the lake. The wave conditions and wave-induced shear stress were estimated. The incipient bed shear stress was estimated based on empirical equation. It is found that the value of incipient bed shear stress was very close to that of the yield stress.Fourthly, this dissertation comparatively studied the rheological properties of natural and artificial dense sediments subjected to steady shear and mechanical vibration loadings. The microstructures change of the dense cohesive sediments was observed and compared before and after the mechanical vibrations by using environment scanning electron microscope. The effect of vibration frequency on apparent viscosity of dense sediments was analyzed. Based on the results, several potential applications of sediment fluidazition by mechanical vibrations were highlighted.Major contributions are summarized as:(1) The mechanism of solid-liquid transition of dense cohesive sediments in steady shear loadings were revealed, the rheological response of dense cohesive sediments to low shear loadings were observed, the parameters of Herschel-Bulkley model and Carreau model for describing rheological behavior of dense cohesive sediments were obtained.(2) The response of nonlinear stress wave to large strain amplitude oscillatory shear was obtained, the rheological model for dense cohesive sediments under oscillatory shear was proposed, the mechanism of wave-induced mud fluidazition was revealed from the point of sediment rheology.(3) The relationship between yield stress and incipient shear stress was further proofed, the importance of rheological properties applied into cohesive sediment movement was revealed.This dissertation provided some reliable rheological data of dense cohesive sediments and the rheological model under shear loadings. The academic achievements of this dissertation would have important significance for understanding the rheological behavior of dense cohesive sediments, and provide a theoretical support for engineering practice.