What is the soil compaction testing?
Soil compaction testing is a simple and inexpensive ground improvement technique that works on all types of soil. The natural ground condition of a site might not always possess the ideal strength required for supporting the intended buildings or infrastructure. One might encounter issues with one or multiple subsoil layers, which would require improvement. The weak subsoil layer can either be replaced with better-performing ones, or modified using certain ground improvement techniques. Either way, soil compaction must be done to enhance the quality.
In soil compaction, the soil is densified by applying external pressure using vibratory rollers. To further enhance the compaction, water is added as a lubricating agent between soil grains. Good geotechnical properties are achieved when the soil is compacted at or near its Optimum Moisture Content (OMC), whose value is determined by carrying out multiple soil compaction tests in the laboratory with varying energy levels either using the standard Proctor (ASTM D-698) or the modified Proctor (ASTM D-1557).
Key Considerations for Soil Compaction Testing
1. Proper Sample Collection
- Sample collection is essential for soil compaction testing.
- Samples should be taken from multiple locations within the project area.
- Previously compacted soil samples must not be used.
2. Particle Size Consideration
- Proctor tests are applicable for soils with 30% or less by mass of particles retained on the 3⁄4-inch (19.0 mm) sieve.
- Soil particle size must be considered for accurate test results.
3. Use of Fresh Samples
- Previously compacted soil samples should not be used.
- New samples must be collected and compacted for accurate testing results.
4. Layering During Compaction
- Each sample should be compacted in three layers, with each layer being of approximately equal thickness.
- The final layer should slightly extend into the collar but should not exceed 1⁄4-in. (6-mm) above the top of the mold.
- Each sample should be compacted in three layers, with each layer being of approximately equal thickness.
5. Correct Rammer Technique
- Ensure that the rammer reaches its full height during each blow.
- Listen for the proper sound of the rammer striking the top for confirmation.
- Move the rammer slightly along the circumference to improve compaction.
6. Careful Trimming After Compaction
- After removing the extended collar, carefully trim the excess layer.
- The soil beneath the trimmed layer should not crack or be disturbed.
7. Use of Soil Extractor
- A soil extractor can help remove the sample from the compaction mold more easily and completely.
- Use circular or square rings for conducting consolidation and direct shear tests.
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Soil Compaction Test Procedure
1. Record Preliminary Data
- Record the dimensions (diameter and height) of the compaction mold.
- Note the mass of the empty compaction mold and base plate.
- Choose the water content for the sample.
2. Apply Grease to the Mold
- Apply a thin layer of grease on the inner surface of the mold.
- Place the collar over the mold to prepare for compaction.
3. Layer Compaction Process
- Begin by placing the first layer of soil in its loose state.
- Compact with 25 blows (for Standard Proctor) or 52 blows (for Modified Proctor).
- After each layer, scrap the top surface before adding the next layer.
Also Read: What is Soil Remediation?
4. Trimming and Measuring
- After compacting the final layer, remove the extended collar and trim the soil to achieve a flat surface.
- Measure the weight of the compaction mold with the compacted sample.
Further Testing After Compaction
Extract the sample from top and bottom for further experiments like:
- Optimum Moisture Content
- Consolidation test
- Direct shear test
Plotting the Compaction Curve
- Plot moisture content against dry density.
- Use the compaction curve to determine maximum dry density and optimum moisture content.
Speeding Up the Process: Nuclear Densometer
For fast-paced construction projects, a nuclear densometer is used at the site to evaluate soil compaction.
- The nuclear densometer provides a faster method of checking compaction than traditional lab tests.
Ataur Rahman
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