Marginal Compaction Test on a Fill Pad
A builder has placed select granular fill in 8-inch lifts beneath a proposed slab-on-grade. The geotechnical technician reports a nuclear gauge reading of 96 percent of maximum dry density at 1.5 percent above optimum moisture, based on a standard Proctor. You observe that the lift was placed the previous afternoon, that the surface is dry and dusty, and that the gauge was taken at a single location near the center of the pad. The specified compaction is 95 percent under the standard Proctor. The builder wants to pour concrete tomorrow.
1. Is the single passing test sufficient to accept the pad? Identify the weaknesses in the testing and what you would require.
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Not sufficient on its own. Weaknesses:
- Single location. One gauge reading cannot represent an 8-inch lift across the pad; density varies with placement and moisture. Require a sampling pattern with a minimum number of tests per area/lift per the specification.
- Surface condition. A dry, dusty surface suggests moisture loss at the surface; the gauge's moisture reading near the surface may not reflect the lift as placed. Verify moisture at test depth.
- Depth of influence. Confirm the gauge is measuring the full lift and not a shallow zone; compare to the lift thickness and gauge depth setting.
- Proctor reference. Verify the standard Proctor curve is for the actual fill material, not a generic curve.
Require additional tests across the pad, at depth, with moisture verified, before acceptance.
2. The additional tests come back at 93-94 percent in two locations. What are your options and what governs your recommendation?
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The results are below the 95 percent specification in two spots. Options:
- Reject and rework - scarify, adjust moisture toward optimum, and recompact the deficient areas, then retest.
- Evaluate engineering significance - if the geotechnical engineer confirms the deficient areas are outside critical zones and the design tolerates it, a documented variance may be possible; this is the engineer's call, not the inspector's.
- Hold the pour until the deficiency is resolved or formally accepted in writing.
Governing principle: the inspector verifies against the approved plans and specification and does not waive them unilaterally. The recommendation should be to rework and retest the two locations, or obtain a written engineer's acceptance with supporting analysis, before concrete is placed. Document the failing results, the locations, and the disposition.