CHARACTERIZATION OF THE SOLID-FLUID TRANSITION OF FINE-GRAINED SEDIMENTS

Characterization of the strength of fine-grained sediments as they evolve from an intact seabed material to a remolded debris flow is essential to adequately model submarine landslides and their impact on pipelines and other seabed infrastructure. In the current literature, two distinct approaches for modelling this material behavior have been considered. In the soil mechanics approach, fine-grained soils are characterized by the undrained shear strength, s(u). The critical state framework proposes a relation between s(u) and the water content, or void ratio of the soil. In addition, rate effects and strain softening effects are described by multiplying a reference value of s(u) by a function of the shear strain rate or the accumulated shear strain respectively. In the fluid mechanics approach, slurries of fine-grained material are characterized by a yield strength and a viscosity parameter, which describes the change in shear stress with shear strain rate. Empirical relationships have been proposed, which relate the yield strength and the viscosity to the sediment concentration. This paper demonstrates that the two modelling approaches are essentially similar, with only some formal differences. It is proposed that the strength of fine-grained sediments can be modelled in a unified way over the solid and liquid ranges. To support this unified approach, an experimental campaign has been conducted to obtain strength measurements on various clays prepared at different water content. The testing program includes fall cone tests, vane shear tests, miniature penetrometers (T-bar and ball) and viscometer tests. Rate effects and remolding effects are investigated over a wide range of water contents spanning the domains of behavior that are usually defined separately as soil and fluid. The present paper focuses on analyzing the results of fall cone, vane shear and viscometer tests. Analysis of the results shows that the variation in shear strength over the solid and liquid ranges can be described by a unique function of water content - suitably normalized - for a given soil. Furthermore, the effect of strain rate and degree of remolding can be accounted for by multiplying the basic strength parameter by appropriate functions, which are independent of the current water content.