ASTM D4221
Full Name
ASTM D4221 – Standard Test Method for Dispersive Characteristics of Clay Soil by Double Hydrometer
Scope
ASTM D4221 is a test method that helps determine the dispersive characteristics of clay soils. Dispersive clays are prone to erosion and internal erosion (piping) when exposed to water. They are problematic in construction because they lose strength and stability when wet. This can lead to failures in earthen dams, embankments, and foundations. Understanding a soil’s dispersiveness is crucial for designing appropriate erosion control measures and ensuring long-term stability of geotechnical structures.
ASTM D4221 involves a double hydrometer test. Essentially, two identical soil samples are analyzed. One sample undergoes the standard hydrometer test (ASTM D7928/D422) which includes mechanical agitation and a dispersing agent to separate all the fine particles. The second sample is tested without mechanical agitation or a dispersing agent, allowing only the naturally dispersive particles to separate. By comparing the percentage of particles smaller than 2 μm in both tests, the degree of dispersion is determined. A higher percentage of fine particles in the second test (no agitation/dispersant) indicates a more dispersive soil.
Specifically, the test measures the percentage of particles finer than 2µm. This particle size is significant because clay particles, which primarily contribute to dispersive behavior, are generally smaller than 2µm. The comparison between the two hydrometer tests provides a direct measure of the soil’s inherent tendency to disperse in water without external forces.
This test method is most suitable for soils that meet specific criteria:
- The soil’s plasticity index should be greater than 4. Plasticity index is a measure of a soil’s plasticity, or its ability to deform without cracking. A higher plasticity index generally suggests a higher clay content.
- The soil should contain more than 12% of particles finer than 2 μm. This ensures that there is a sufficient amount of clay-sized particles for the test to be meaningful.
- The soil should plot on or above the “A” line on the plasticity chart (as per ASTM D2487). This chart helps classify fine-grained soils based on their plasticity characteristics. Soils plotting above the “”A”” line are typically clays.
It’s important to note that ASTM D4221 may not identify all dispersive clay soils. Therefore, it’s often used in conjunction with other tests like the crumb test (ASTM D6572), the pinhole test (ASTM D4647), or pore water extraction analysis (ASTM D4542) for a more comprehensive assessment.
Dispersive clays are characterized by a high concentration of sodium ions on the clay particle surfaces. These ions cause the clay particles to repel each other in the presence of water, leading to the breakdown of the soil structure. This dispersion makes the soil highly susceptible to erosion and piping, especially in hydraulic structures.
Dispersive clays often exhibit the following characteristics:
- High shrink-swell potential: they expand significantly when wet and shrink when dry, causing cracking and instability.
- Low resistance to erosion: they are easily washed away by water flow.
- Low permeability in an intact state: while seemingly impermeable when dry, they become highly permeable once dispersed, allowing water to flow through easily.
ASTM D4221 Accredited Laboratory
Applied Testing & Geosciences, LLC is accredited for performing tests using the ASTM D4221 standard.
Contact us today to discuss your testing needs and ensure the success of your project.
Resources
Keywords
dispersive clay; hydrometer analysis; particle size; sedimentation analysis; dispersion; erosion; piping; clays; gradation; deflocculation; jugging; plasticity index; soil fraction