Gradation Shift on a Stockpile After Rain
A plant produces 1 in. nominal maximum size base course. Morning gradation tests on belt-cut samples meet the master gradation. After 1.5 in. of rain, the plant pulls samples from the stockpile face instead of the belt. Results show the percent passing the 200 sieve jumped from 6% to 13%, exceeding the 10% maximum, while the percent passing the 1 in. sieve dropped from 98% to 91%. The stockpile has segregation (coarse at the edges, fines in the center). The technician used a sample split on the stockpile face with a shovel and did not dry the sample before sieving. The specification limit for percent passing 200 is 4–10%.
1. Identify the sampling and preparation errors that could explain the out-of-spec results.
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Several errors compound: (1) Stockpile-face sampling without sampling the full vertical profile and multiple locations is biased; fines concentrate in the center and at the base, coarse at edges. (2) Wet sieving without drying causes fines to agglomerate or adhere to coarse particles, distorting the 200-sieve result; the sample must be dried to constant mass before sieving, and washing over the 200 sieve is required for accurate fines determination. (3) A shovel grab is not a proper sampling tool for a segregated stockpile. The 1 in. passing drop (98% to 91%) is consistent with sampling coarse-rich edge material, while the fines jump suggests sampling a fines-rich zone-mixed bias.
2. What resampling and testing protocol would you specify to get a defensible result?
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Resample from the conveyor belt (preferred, full-stream cut) or, if stockpile sampling is required, sample the full vertical profile at multiple points around the pile using a proper sampling thief or by taking a full-section sample. Reduce with a riffle splitter sized for 1 in. material. Dry to constant mass, then wash over the 200 sieve, dry again, and sieve. Run duplicate splits. Compare to the master gradation and specification (4–10% passing 200). Do not report the wet, biased result as a compliance failure without confirming with a valid sample.
3. If the valid resample still shows 11% passing 200, what does that mean for acceptance?
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It is an out-of-specification result (limit 4–10%). The material fails the master gradation and should be reported as nonconforming. Investigate cause (plant screening, crusher settings, dust control, stockpile handling). Do not average with the earlier in-spec belt results to force compliance; report each result and its sampling basis. The supplier may need to reprocess or blend, and the accepting authority decides disposition per the specification's acceptance provisions.