Relationships of Plasticity and Grain Size of Lake Agassiz Sediments

In a study designed to relate geological and soil mechanics properties, the grain size distribution and the plasticity and natural water content were determined on 71 samples of Lake Agassiz clay from Grand Forks and Fargo, North Dakota. Using the hydrometer method, the sediments were found to range in φ mean diameter from 6.44 (11.3 µ) to 11.53 (0.34 µ), with an average value of 9.50 (1.35 µ). The standard deviation of the grain size distribution varies from 1.50 to 4.36 φ units and averages 2.43. Thus the sediments have a wide range of size and sorting that centers about normally sorted, medium-grained clay. The plasticity characteristics, determined by the usual soil mechanics procedures, were found to have a wide range of values, with an average liquid limit of 83.6, plastic limit of 25.2, and plasticity index of 58.4, all in percentage water content. The natural water content varies considerably from the average value of 51 per cent of dry weight. In general, then, the Lake Agassiz sediments are highly plastic and have relatively high natural water content. These grain size properties and plasticity and natural water-content properties are closely related. High statistical correlation coefficients (averaging 0.89) were obtained for distributions of liquid limit, plastic limit, plasticity index, and natural water content versus φ mean diameter; high values of all these parameters being associated with high values of φ mean diameter (i.e., with fine grain sizes). The φ standard deviation of the grain size distributions varies inversely with the φ mean diameter and can therefore be correlated equally well with the other properties. However, it is shown that the φ mean diameter is the more significant grain size parameter. For comparison with other sediments, background information on mineralogy and particle shape is presented, based on X-ray diffraction, electron diffraction, and differential thermal analyses and electron photomicrographs.