The integration of grain-size distribution and plasticity parameters for characterizing and classifying unconsolidated fine-grained sediments

Grain-size distribution (GSD) characteristics are often used to classify different types of sediment and to understand how variations in depositional processes affect mechanical behavior. However, the current grain-size-based classification systems have not been able to distinguish fine-grained sediments with varying silt and clay content. In addition, the current classification systems do not consider mineralogy in the classification scheme or the subsequent implication of mineralogy in the mechanical behavior of the sediment. This study proposes a new grain-size-based scheme to classify unconsolidated inorganic sediments that cover a wide range of natural depositional environments with a special emphasis on fine-grained deposits. The proposed classification assumes a linear relationship between percent fines and the silt fraction. This relation describes the behavior of the GSD curve, which analysis showed to be controlled by the ratio of the clay fraction to the silt fraction. The proposed scheme allows the clay mineral composition to be included, and allows the mechanical behavior to be inferred as well. The classification scheme shows that plasticity indices (i.e. liquid limit and plasticity index) are reliable proxies for clay mineralogy. In addition, the study uses the activity of clay as a quantifiable factor that discriminates between the major clay-mineral groups. By combining grain-size characteristics and plasticity, the proposed classification extends the utility of the scheme beyond simply classifying the sediment class, towards inferring the potential mechanical behavior of sediments having various GSD characteristics and mineralogy.