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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 standard Proctor (ASTM D-698) and modified Proctor (ASTM D-1557).

Key Consideration for Soil Compaction Testing.

  • Proper sample collection is essential for soil compaction testing. Considering the fact that soil type and condition might vary, samples should be collected from multiple locations within the defined project area. The sample should be made free of any debris or organic matter. Extra care should be taken to ensure that samples or soil structure are not altered while collecting.
  • Particle size is another important precursor to the Proctor test to achieve accurate results. The proctor tests are applicable for soils having 30% or less by mass of particles retained on the 3⁄4-inch (19.0 mm) sieve.
  • Previously compacted soil samples must not be used.
  • Each sample should be compacted in three layers which after compaction should be distributed approximately equal in thickness. After compaction of the final layer (third layer), the sample should slightly extend into the upper collar, but must not exceed 1⁄4-in. (6-mm) above the top of the mold (i.e., the base collar). In case, the layer extends above the top of the mold by more than 1⁄4-in. (6-mm), the sample shall be discarded and the experiment should be performed again.
  • While blowing the sample, one must ensure that the rammer has attained its full height. The best way to check this is by listening to the sound of a rammer gently striking the top while pulling. 
  • Keep moving the rammer slightly along the circumference after each blow for better soil compaction.
  • After removing the extended collar, the extra layer should be carefully trimmed such that the layer beneath it does not crack or is disturbed, and a flat even surface is achieved.

Also Read: A Definitive Guide on Geotechnical Reports and Soil Testing Expenses

  • Preferably, a soil extractor should be used to get the sample out of soil compaction mold more easily and completely.
  • Due considerations must be taken while extracting a sample from the compacted mould using circular or square ring for conducting consolidation and direct shear test. For both these tests, it’s always better to remove the 1-inch of soil before extracting the sample. Oiling/ greasing the inner surface of the ring should help.

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What is the Soil Compaction Test Procedure?

  • Before starting the experiment, record the following data: 
    1. Dimensions (i.e., diameter and height) of the compaction mould, 
    2. Mass of empty compaction mold with base plate (i.e., without the extended collar), 
    3. Water content chosen for the sample.
  • Apply a thin layer of grease on the inner side of the mould and place the collar over the mould.
  • Begin the experiment by placing the first layer of the sample in its loose state and compacting with a rammer with 25 blows (for Standard Proctor), or 52 blows (for Modified Proctor).
  • Before placing the second layer, scrap the top surface of the already-compacted layer. Similarly, compact the third layer.
  • After compacting the final layer, remove the extended collar, and trim the soil with a straight edge without disturbing the layers below.
  • Measure the weight of the compaction mould (with base plate) with a compacted sample.

Also Read: What is Soil Remediation?

  • Extract the sample from top and bottom for further experiment, such as: 
    1. Optimum Moisture Content, 
    2. Consolidation, or 
    3. Direct shear test.
  • Plot moisture content against dry density, and use the compaction curve to determine maximum dry density and optimum moisture content.
  • Repeat the procedure with varying water content until a decrease in mass of soil is observed for at least two successive readings.
  • For fast paced construction projects, a nuclear densometer is used at site to evaluate the compaction of soil. It is a faster method of checking field compaction.

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Ataur Rahman

President at prycoglobal
Ataur Rahman is the president at Pryco Global Inc (PRYCO). He has more than a decade of experience in the leadership role, majorly into Oil & Energy and Heavy Civil & Commercial Construction Projects. He has turned around a couple of loss-making businesses unit into profit centers through effective Cost Management, Process Improvement, Business Strategy Realignment, Reorganizing Organizational Structure.
Ataur Rahman