Borrow Pit Classification and Specification Review
A technician receives a 35 lb bag from a proposed borrow pit for structural fill. The sieve analysis reports 21% retained on the 3/4-in. sieve, 78% passing the No. 4 sieve, and 52% passing the No. 200 sieve. The technician's draft report calls the soil "SC - clayey sand with gravel" and notes that it meets the project's embankment fill requirements because "the sand fraction provides strength." Atterberg testing on the minus No. 40 fraction gives LL = 47 and PL = 23. The project specification requires fill to have LL <= 40, PI <= 20, and no more than 35% passing the No. 200 sieve. The contractor wants to haul tomorrow. You are reviewing the draft before it is issued. A previous sample from the same pit was classified CL, but this sample was taken from a different stockpile and looks sandier.
1. Determine the correct USCS group symbol and explain why the draft classification is inconsistent with the gradation and plasticity data.
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Because 52% passes the No. 200 sieve, the soil is fine-grained, not a sand. That alone rules out SC, which requires sand to be the dominant coarse fraction and fines below 50%. With LL = 47, the first letter is C rather than M because the liquid limit is below 50. PI = 47 - 23 = 24. The A-line at LL 47 is PI = 0.73(47 - 20) = 19.7. Since PI 24 plots above the A-line and LL is between 36 and 50, the fines are lean clay. The correct group symbol is CL; with 22% gravel, the descriptive name is gravelly lean clay or CL with gravel. The draft's "sandy" label overemphasizes the sand fraction and ignores that fines control the classification.
2. Apply the specification limits and state whether the material can be accepted for embankment fill. Include the effect of the gravel fraction.
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The material fails all three stated limits: LL 47 exceeds 40, PI 24 exceeds 20, and 52% passing the No. 200 exceeds 35%. The 22% gravel does not cure those failures. Gravel may improve shear strength and stiffness, but it does not reduce the plasticity or the total fines content, and the clay fines still govern moisture sensitivity and volume change. A passing density test on this material would not make it specification-compliant. The proper disposition is rejection of this stockpile as embankment fill, or approval only after blending with a coarser, non-plastic material and retesting the blended product for gradation, LL, and PI. The contractor should not haul tomorrow based on the draft report.
3. What quality-assurance concern would you raise if the Atterberg specimens had been oven-dried at 110 C before testing, and how would you resolve it?
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Oven drying at 110 C can change the plasticity of some soils, especially organic or heat-sensitive clays, and can lower the liquid limit enough to shift the classification or the specification comparison. The routine classification should be based on a preparation method that does not alter the soil, typically air drying or low-temperature drying. I would check the sample for organic odor, color, or root fibers, rerun LL and PL on an air-dried split, and compare the results with the oven-dried values. If the liquid limit drops significantly after oven drying, the original result is suspect and the soil may need organic-content testing. The report should state the preparation method and use the valid, non-altered results.