Initiation of erosion of cohesionless and cohesive soil mixtures: Effects of density and fine content on the erosional behavior of soil

The erosional behavior of seabed and bluff materials depends on the characteristics of the materials. These materials are rarely composed of only granular materials, instead they include various amounts of fine grain sediments, and have different densities (i.e., strength), among other characteristics. Due to the diversity in the properties of fine grain sediments (silt and clay) such as their inter-particle cohesion, electrochemical properties, and mechanical characteristics, especially when they are mixed with cohesionless sediments, the mechanism of erosion of soils becomes complicated. As a result, the traditional Sheilds diagram developed for granular materials cannot be used for determining the initiation of erosion of sand mixed with fine grain materials. The purpose of the present research is to modify the Shield diagram for predominantly sandy soils of various geotechnical properties and to develop a relationship between the amount of fine grain materials as well the soil density and soil's erodibility. For this purpose, we identified a bluff site in Montauk, Long Island in New York, from where soil samples were collected. The soil samples were analyzed for their mechanical properties, reconstituted with various fine contents and relative densities and tested in a hydraulic flume to characterize the soils' initiation of erosion. The initiation of erosion test was conducted in an open top recirculating flume of 5.0 m long, 0.076 m wide, and 0.25 m deep, at the Hydraulic Laboratory at Stony Brook University. The soil samples were composed of five different fine contents, 0%, 5%, 10%, 15%, and 20 %, and two densities, 1776 (kg/m³), and 1927(kg/m³) corresponding to relative densities of 39% and 68%, respectively. The experimental results complemented the Shields diagram. Furthermore, empirical formulas were developed for the critical shear stress as a function of fine content and relative density of the sand and fine grain material mixtures.