Particle Size Analysis Techniques

Particle size analysis is a fundamental test performed in soil mechanics and other fields to determine the proportion of different particle sizes present in a sample. This analysis helps classify soils and predict their engineering properties. There are various methods for particle size analysis, depending on the range of particle sizes being analyzed.

Hydrometer Method for Particle Size Analysis

The hydrometer method is a sedimentation technique used specifically for determining the particle size distribution of fine-grained soils, namely silt and clay. Coarser particles like sand and gravel are typically analyzed using sieving methods.

Here's how the hydrometer method works:

• A soil sample is dispersed in water, often with a dispersing agent to prevent flocculation.
• The suspension is placed in a sedimentation cylinder.
• As particles settle out of suspension according to Stokes' Law, the density (or specific gravity) of the suspension decreases.
• A hydrometer is used to measure the changing density of the suspension over time at different depths.
• Stokes' Law relates the terminal settling velocity of a spherical particle in a fluid to its diameter, the difference in densities between the particle and the fluid, and the fluid's viscosity. By measuring the density of the suspension at different times, we can determine the concentration of particles still in suspension and, using Stokes' Law, estimate the maximum size of particles remaining in suspension at that time and depth.
• This allows for the determination of the percentage of particles finer than certain sizes, thus building the particle size distribution curve for the fine fraction.

Other Options Explained

• Pycnometer: A pycnometer is primarily used for determining the specific gravity (density relative to water) of soil solids, not the particle size distribution.
• One point method: This term typically refers to a simplified procedure for determining the liquid limit of a soil (a measure of its consistency), such as the one-point liquid limit test based on Casagrande's method. It is not a method for particle size analysis.
• Sedimentometer: While also a sedimentation technique for particle size analysis, a sedimentometer (or sedimentation balance) directly measures the mass of particles settled over time onto a pan or balance. The hydrometer method is another type of sedimentation analysis, focusing on the suspension density rather than settled mass. The hydrometer is a common tool specifically associated with this type of particle size analysis in soil testing.

Based on the standard methods for soil testing, the hydrometer is a key instrument used for particle size analysis of fine soils (silt and clay).