Textural Class Disagreement Between Field and Lab
A designer submits a soil report classifying the infiltrative zone as a sandy loam based on a ribbon test and feel method. Your own field evaluation of the same horizon yields a noticeably finer feel, with a ribbon about 2 inches long that holds well and feels smooth and slightly sticky when wet. A sieve and hydrometer analysis later reports 52 percent sand, 34 percent silt, and 14 percent clay. The proposed design assumes a loading rate tied to the sandy loam classification. The soil contains noticeable mica and the sample was taken after a dry spell.
1. Reconcile the field observation with the laboratory texture. Which classification governs, and why might the two differ?
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Plot the lab result (52/34/14) on the textural triangle: that lands in loam, not sandy loam. The lab data governs for design because it is quantitative and reproducible, but the discrepancy must be explained, not dismissed.
Likely causes of the finer field feel:
- Mica and smoothness. Micaceous sands feel smoother and more cohesive than their grain-size distribution implies, inflating the apparent silt and clay by feel.
- Dry conditions. Dry samples are harder to ribbon and feel coarser or more friable; the field estimate may have been made on a different moisture state than the lab sample.
- Ribbon interpretation. A 2-inch ribbon indicates more clay than a sandy loam typically produces; the field call was internally inconsistent with the ribbon length.
- Sample location. Confirm the lab sample came from the same horizon and depth as the design's infiltrative zone.
Conclusion: use the loam classification, recompute the loading rate, and note the field/lab discrepancy and its resolution in the report.
2. What should you require before accepting a revised design, and how do you document the classification decision?
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Require:
- A corrected design using the loading rate appropriate to loam, with the calculation shown.
- Confirmation the lab sample is representative - same horizon, depth, and area as the proposed infiltrative surface, with chain-of-custody or sampling notes.
- Verification that the finer texture does not create a new limiting condition - e.g., reduced permeability or a perched zone - by checking the profile description and, if needed, a permeability test in the actual infiltrative zone.
Documentation: record both the field estimate and the lab result in the inspection file, state which governs and why, cite the textural triangle and the code provision tying loading rate to texture, and describe the mica/dryness factors as the explanation for the field discrepancy. The record should let a reviewer see the reasoning, not just the final class.