Sieve Analysis & Hydrometer Protocols

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CivMasterPro
Highway QA/QC Mastery Series

Soil Mechanics Handbook
Chapter 1.3: Sieve Analysis & Sedimentation Protocols

๐Ÿ“‹ Main Directory ๐Ÿ“ Current: Module 04 Next Lesson: Mod 05 ➡️

1. Mechanical Particle Size Analysis (IS 2720 Part 4)

Grain size analysis is the systematic process of separating a soil sample into fluid or mechanical size blocks to map its overall particle distribution. Under **IS 2720 Part 4**, the testing spectrum is broken into two primary execution brackets:

  • Sieve Analysis: Utilized for coarse-grained particles larger than $75\,\mu\text{m}$ (microns). This process uses nested wire-cloth test screens sorted hierarchically by aperture size.
  • Sedimentation Analysis: Reserved for fine-grained particles smaller than $75\,\mu\text{m}$ (silts and clays) that cannot be mechanically sorted using dry physical meshes.

2. Dry Sieving vs. Wet Sieve Wash Protocols

A frequent field auditing error is performing dry sieving on fine-grained borrow soils that contain cohesive silt or clay binders. When clay aggregates dry, they bond to larger sand fractions, producing artificially coarse gradation data.

๐Ÿšฟ Cohesive Wash Mandate (Wet Sieving)

If a soil sample contains more than 5% fine material passing a $75\,\mu\text{m}$ mesh, a wet sieve analysis is mandatory. The sample must be slaked in water with a deflocculating chemical agent (such as Sodium Hexametaphosphate at 2 g/L) for a minimum of 1 hour to break down cohesive skin structures. The slurry is then washed through the $75\,\mu\text{m}$ sieve until the wash water runs perfectly clear before drying and performing final mechanical shaking.

3. Sedimentation Mechanics & Stokes' Law

Fine particles smaller than $75\,\mu\text{m}$ are analyzed using hydrometer sedimentation testing. This process relies on **Stokes' Law**, which states that terminal settling velocity ($v$) depends directly on particle diameter ($D$) when spherical elements drop through a uniform fluid medium:

$$v = \frac{\rho_s - \rho_w}{18\eta} g D^2$$

Where $\rho_s$ is particle mass density, $\rho_w$ is water fluid density, $\eta$ is dynamic water viscosity, and $g$ is acceleration due to gravity. By recording hydrometer density changes over log-scaled settling durations, lab technicians can compute the exact distribution of sub-75 micron silts and clays.

4. Gradation Curves & Uniformity Metrics ($C_u, C_c$)

Plotting the cumulative percentage passing versus grain size on a semi-logarithmic scale yields a grain-size distribution curve. To qualify a granular borrow soil for base or subgrade layers, QA/QC criteria require calculating the Coefficient of Uniformity ($C_u$) and the Coefficient of Curvature ($C_c$):

$$C_u = \frac{D_{60}}{D_{10}}$$
$$C_c = \frac{(D_{30})^2}{D_{60} \times D_{10}}$$

Where $D_{60}$, $D_{30}$, and $D_{10}$ represent the particle diameters corresponding to 60%, 30%, and 10% finer by weight on the distribution curve.

  • Well-Graded Gravel (GW): Requires $C_u > 4$ and $1 \le C_c \le 3$.
  • Well-Graded Sand (SW): Requires $C_u > 6$ and $1 \le C_c \le 3$.
  • Failure to meet these structural brackets means the material is classified as poorly graded/uniform soil, which is prone to shifting under heavy traffic loads.

๐Ÿ”ฌ Interactive Sieve Slope & Gradation Evaluator

Input your curve sizing values ($D_{60}, D_{30}, D_{10}$ in mm) below to compute uniformity characteristics and verify structural grading compliance.

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