Hydrometer Analysis is a specialized laboratory test used to determine the grain size distribution of fine soils, specifically those passing the 75-micron sieve. The analysis is based on Stokes' Law, which correlates the settling velocity of spherical particles in a liquid to their diameter.
Test Objectives
- Determine accurate percentages of silt and clay fractions
- Support comprehensive soil classification (USCS/AASHTO)
- Assess fundamental permeability and compressibility indices
Project Application
- Evaluates settlement potential for foundations on fine soils
- Informs the design of earth dams, cores, and embankments
- Predicts drainage behavior and frost susceptibility
Engineering Value
- Prevents structural failure from overestimating bearing capacity
- Identifies reactive or sensitive silt/clay layers early
- Guides specific ground improvement and stabilization strategies
Stokes' Law Principles
- $v = \frac{D^2 (\rho_s - \rho_f) g}{18 \mu}$
- Settling velocity ($v$) depends on particle diameter ($D$)
- Corrected for fluid temperature and meniscus effects
While sieve analysis is effective for coarse grains, fine silts and clays are too small to be mechanically separated. In this test, soil is mixed with a deflocculating agent and water to create a uniform suspension.
As particles settle out of suspension, the density of the liquid changes. By taking hydrometer readings at specific time intervals, we calculate the percentage of particles remaining in suspension. This allows our laboratory to extend the grain size distribution curve down to the micron level, providing a detailed breakdown of the colloidal and clay fractions that dictate the cohesive behavior of your site's soil.
Our Methodology
- 1 Preparation of soil suspension with a dispersive agent
- 2 Systematic recording of hydrometer density readings
- 3 Temperature-corrected analysis of particle settling rates
- 4 Integration with sieve analysis for complete gradation reports