Sampling Frequency Shortfall on a Base Course Lot
A base course lot of 1,800 m² is being placed in a single lift. The specification requires one field density test per 400 m² or fraction thereof, plus one gradation and one Proctor verification per 1,000 m² or fraction thereof. The technician has run four density tests (one at each corner area) and one gradation. No Proctor verification was run for this lot; the last Proctor on the source was 14 months ago. One density test reads 96.5% and the other three read 99–101%. The contractor wants to close the lot. The engineer asks whether the lot can be accepted and what testing is still owed.
1. Compute the required number of tests for this lot and identify the shortfall.
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Density: 1,800 m² / 400 m² = 4.5, so 5 tests are required (fraction thereof rounds up). The technician ran 4, so one density test is short. Gradation: 1,800/1,000 = 1.8, so 2 gradations are required; only 1 was run, so one gradation is short. Proctor verification: 1,800/1,000 rounds to 2, but the requirement is a verification per lot; at minimum one current verification is owed, and none was run. So the lot is short one density test, one gradation, and a current Proctor verification. The lot cannot be closed on the existing testing.
2. The 96.5% result is below the 98% requirement. How should the technician handle this result relative to the lot and the shortfall?
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A single failing density test means the lot does not meet the density requirement at that location, regardless of the other three passing results. The failing location must be reworked (additional compaction or moisture adjustment) and retested. The shortfall in test count is a separate deficiency: even the passing results cannot support lot acceptance because the required sampling frequency was not met, so the lot's coverage is incomplete. The technician should not average the four results to declare the lot acceptable; acceptance is by location against the specified minimum. The correct path is to rework and retest the failing location, add the missing density test, run the missing gradation, and run a current Proctor verification before requesting lot closure.
3. Why does the 14-month-old Proctor matter, and what should the technician recommend about the target density?
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The Proctor defines the maximum dry density against which every field result is judged. A 14-month-old curve may no longer represent the source if material has changed, and the specification requires periodic verification. Without a current verification, all four density percentages rest on an unverified target, so even the passing values are not defensible. The technician should recommend running a current Proctor (or verification) on the source, confirming the maximum dry density and optimum moisture, and recomputing percent compaction for the lot against the verified value. If the verified maximum differs, the 96.5% result and the passing results must be re-evaluated. The lot should not be closed until the target is current and the failing location is corrected.