Earthwork and Paving (C-12) · Lesson 2 of 6

Relative Compaction: Reading a Nuclear-Gauge Density Test

"Relative Compaction: Reading a Nuclear-Gauge Density Test" is lesson 2 of 6 in the Earthwork and Paving (C-12) Learning Hub. It opens with "The Lab Sets the 100 Percent" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. The Lab Sets the 100 Percent
  2. Know the Percentage the Spec Requires
  3. Gauges Read Wet - Compare on the Dry Basis the Spec Names
  4. Calculate Relative Compaction and Check Moisture
  5. What to Do With a Failing Test

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Convert wet density to dry density" through "Retest at a new location".

What you'll be able to do

  • Explain the main idea of "Relative Compaction: Reading a Nuclear-Gauge Density Test" in plain language you can use on the exam.
  • Apply this step with confidence: "Gauges Read Wet - Compare on the Dry Basis the Spec Names".
  • Apply this step with confidence: "Calculate Relative Compaction and Check Moisture".
  • Apply this step with confidence: "What to Do With a Failing Test".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Relative Compaction: Reading a Density TestLAB CURVE (ASTM D1557) AND FIELD TESTSDry density, pcf68101214110120130Moisture content, %90% = 116.1 pcfMDD 129.0 pcf at OMC 10.0%Test 1Test 2Field points sit below the lab curveDRY DENSITY, THEN PERCENTGauge reads WET density + moisture wDry = wet / (1 + w/100) RC = dry / 129.0TEST 1: 128.6 pcf wet, w = 11.0%128.6 / 1.110 = 115.9 pcf dry115.9 / 129.0 = 89.8%TEST 2: 132.0 pcf wet, w = 10.4%132.0 / 1.104 = 119.6 pcf dry119.6 / 129.0 = 92.7%CBC 1804.6: 90% of D1557 (fill 12 in or less)Caltrans aggregate base: 95% (26-1.03E)Test 1 = 89.8% is under 90% and is 1 point wet of optimumAerate toward 10.0%, add roller passes, retest at a new spotTest 2 = 92.7% is at least 90% - this lift passesSources: 2025 CBC 1804.6 (ASTM D1557 named there); 2025 Caltrans Std. Specs 19-5.03A, 26-1.03ECC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: The Lab Sets the 100 Percent

Before fill goes down, the soils lab runs a moisture-density test on the actual soil. The curve gives two numbers: Maximum Dry Density (MDD, in pounds per cubic foot) and Optimum Moisture Content (OMC, in percent). The 2025 California Building Code words it this way in section 1804.6: "maximum dry density at optimum moisture content determined in accordance with ASTM D1557." The MDD is defined as 100 percent. Every field test is then reported as a percentage of that number, called relative compaction (RC). The Caltrans Standard Specifications (section 6-2.01A) say relative compaction is tested under California Test 216 or 231.

Step 2: Know the Percentage the Spec Requires

The percentage comes from the project documents. For shallow foundations on compacted fill, 2025 CBC section 1804.6 says the fill must comply with an approved geotechnical report; the exception is fill 12 inches or less in depth, which needs an in-place dry density not less than 90 percent of the ASTM D1557 maximum dry density, verified by special inspection under section 1705.6. Caltrans earthwork is 95 percent relative compaction in the zones listed in section 19-5.03B (for example the top 0.5 foot below the grading plane between the outer edges of shoulders) and 90 percent in other embankment areas (section 19-5.03C). Each layer of aggregate base must reach at least 95 percent (section 26-1.03E). Always read the grading plan and soils report first - they set the number for your job.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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