Soil mineral particle size analysis: Validation of Stokes' Law
Particle size analysis (PSA) is one of the most important and fundamental parameters in the soil science as texture affects the hydrological, chemical, physical, and biological properties of soils. The accuracy of PSA measurements has direct effects on almost all inherent soil related properties, such as water holding capacity, and cation exchange capacity, as well as engineering properties such as Atterberg limits and shear strength. The current sedimentation-based measurement methods, including the hydrometer (HM), pipette (PM), and Integral suspension pressure methods (ISPM), are based on the application of Stokes’ law, however, application of Stokes’ law to PSA is limited by its assumptions. The overall goal of this research was to further understand some of the relationships between particles in suspension and particle-particle interactions when conducting PSA measurements. The interactions between particles were evaluated with various particle size fractions, using a Pario meter, which records the suspension pressure at 10 seconds intervals within a 24-hour time frame. These measurements show that the interactions between particles increased with increasing mass concentration in suspension and had significant effects on the measurements of PSA, especially when the soil samples contain large quantities of non-expandable clay and sand-sized particles. Removal of the sand fraction (>10 g/L) prior to conducting any sedimentation-based measurements is recommended to obtain a more accurate measurement of PSA. The effect caused by the presence of clay fractions is highly dependent on the type and concentration in suspension. Moreover, the accuracy of the ISPM and HM was evaluated by comparing measurement results with soil mixtures with known particle size composition. The results show that both methods overestimate the clay content and underestimate the silt content, but HM gives more accurate results compared to ISPM. The effect of density of the suspension solution was tested in experiments using clay- and silt-sized particles and recommended that they be specified in Stokes’ equation, especially for predicting the settling velocity of clay-rich soil samples.