Particle-Size Analysis Test
Concept
The Particle-Size Analysis Test, also known as a sieve analysis or grain size distribution test, is a fundamental geotechnical procedure used to determine the distribution of particle sizes in a soil sample. The goal of the test is to classify soil based on the size of its particles, which influences its behavior and suitability for various engineering applications. This test helps in identifying whether a soil is predominantly gravel, sand, silt, or clay. Standards for conducting particle-size analysis include ASTM D422 and ISO 17892-4.
Descriptions and Procedure
Apparatus Required
- Set of standard sieves with various opening sizes
- Mechanical sieve shaker
- Balance with an accuracy of 0.01 g
- Mortar and pestle or mechanical grinder (if necessary)
- Drying oven
- Brushes and spatulas
- Sample splitter (optional)
- Graduated cylinder and dispersing agent (for hydrometer analysis)
Procedure
Sieve Analysis (for Coarse-Grained Soils)
1. Sample Preparation
- Collect a representative soil sample. The sample size depends on the largest particle present but typically ranges from 500 grams to several kilograms.
- Dry the sample in an oven at 110 ± 5°C to a constant mass. Record the mass of the dry sample.
2. Sieving Process
- Weigh the dried soil sample.
- If the soil contains clumps, gently break them using a mortar and pestle or a mechanical grinder without crushing individual particles.
- Place the soil sample in the top sieve of a stack arranged in descending order of sieve size.
- Cover the sieve stack with a lid and secure it on the mechanical sieve shaker.
- Shake the stack for a specified period, usually around 10 to 15 minutes, to ensure adequate separation of particles.
3. Weighing Sieve Fractions
- After shaking, carefully remove each sieve from the stack.
- Weigh the soil retained on each sieve and record the masses.
4. Calculations
- Calculate the mass of soil retained on each sieve.
- Determine the cumulative mass retained and the cumulative percent retained on each sieve.
- Calculate the percent passing each sieve by subtracting the cumulative percent retained from 100%.
Hydrometer Analysis (for Fine-Grained Soils)
1. Sample Preparation
- Take a portion of the soil sample passing the No. 200 sieve (0.075 mm).
- Disperse the sample using a dispersing agent (e.g., sodium hexametaphosphate) mixed in distilled water.
2. Hydrometer Test
- Transfer the soil suspension into a graduated cylinder and fill it to the 1000 ml mark with distilled water.
- Stir the suspension thoroughly to ensure even distribution of particles.
- Insert the hydrometer into the cylinder and record the initial reading at specific time intervals (e.g., 0.5 min, 1 min, 2 min, etc.) up to 24 hours.
3. Calculations
- Use the hydrometer readings and the calibration chart to determine the particle sizes corresponding to the recorded times.
- Calculate the percent finer than each particle size using the sedimentation theory (Stokes' Law).
Data Interpretation
Sieve Analysis
Grain Size Distribution Curve: Plot the percent passing each sieve size on a semi-logarithmic graph with sieve size on the x-axis (log scale) and percent passing on the y-axis (linear scale). This curve helps classify the soil and understand its gradation.
Uniformity Coefficient (Cu) and Coefficient of Gradation (Cc): Calculate these coefficients to assess soil gradation:
- 𝐶𝑢=𝐷60/𝐷10
- 𝐶𝑐=(𝐷30)2/𝐷10⋅𝐷60 Where 𝐷10,𝐷30 and 𝐷60 are particle diameters corresponding to 10%, 30%, and 60% passing, respectively.
Hydrometer Analysis
- Particle Size Distribution: Extend the grain size distribution curve to include fine-grained fractions from the hydrometer analysis.
- Soil Classification: Combine results from sieve and hydrometer analyses to classify the soil according to the Unified Soil Classification System (USCS) or other relevant classification systems.
Summary Table
By plotting the percent passing each sieve size on a semi-logarithmic graph, we can visualize the soil's particle size distribution and classify it accordingly.
The Particle-Size Analysis Test is essential for understanding soil gradation, which influences its compaction, strength, and permeability properties. This information is crucial for the design and construction of foundations, earthworks, and other geotechnical structures.