Sieve Analysis Discrepancy on a Stockpile Sample
A technician reduces a 25 kg field sample of crushed limestone base course by quartering and runs a washed sieve analysis. The No. 200 wash loss is 11.8%, and the gradation plots 6% below the lower limit on the No. 40 sieve and 4% above the upper limit on the No. 200 sieve compared to the specification band. The sample was taken from a conical stockpile, and the technician notes the pile has visible segregation with coarse material concentrated at the base. The lab's last three verification samples from the same source met gradation. The technician is asked whether the failing result should be reported as-is or whether the sample is suspect.
1. What sampling and reduction issues should the technician evaluate before accepting the failing gradation as representative of the source material?
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Two independent issues must be checked. First, sampling location: a conical stockpile segregates by particle size, with coarse particles rolling to the base and fines concentrating near the top or in the core. A single sample from one location cannot represent the pile. The technician should verify whether the sample was a composite from multiple locations and depths, or a single-point grab. Second, reduction method: quartering a 25 kg sample is acceptable only if the material is thoroughly mixed and the sample is split correctly. If the sample was not dried and mixed before quartering, fines can be lost or concentrated. The visible segregation plus the pattern of failure (coarse deficiency on the No. 40 and excess fines on the No. 200) is consistent with sampling from a fines-rich zone rather than a true material change. The prior passing verifications support treating this result as suspect pending resampling.
2. The technician resamples using a composite of at least five increments from different locations and depths. The new result passes. How should the original failing result be handled in the record, and what does this tell you about the source?
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The original result should be retained in the record with a note documenting the sampling deficiency (single-point, segregated location) and the basis for the resample. It should not be silently discarded, because records must show the full testing history. The resample passing does not prove the source is fine everywhere; it shows the original sample was not representative. The correct conclusion is that the source material is acceptable when sampled properly, but the stockpile handling practice is producing segregation. The technician should recommend improved stockpile management (layered construction, avoiding conical drop points) and confirm that future samples are composites. The key judgement is that a single failing result from a segregated pile is not evidence of nonconforming material until representative sampling is confirmed.