Lime Modification of Expansive Clay With a Sulfate Question
Subgrade for a new single-story medical office is a highly plastic clay: liquid limit 52, plastic limit 14, 88% passing the No. 200 sieve. The specification requires the upper 3 ft of the building pad to be select fill with LL of 35 or less and PI of 15 or less, or lime-modified on-site soil meeting those same limits. The contractor proposes treating the existing clay in place with 5% hydrated lime by dry weight, mellowing overnight, then compacting to 95% of ASTM D698 maximum. Only one chemical test exists for the site: a water-soluble sulfate result of 2,800 ppm from a single boring. In the field you watch the spreader truck apply lime with no calibration sheet, and the superintendent says the crew will mix and compact the same afternoon because 'the lime dries it out.' The clay is currently at 26% moisture; the untreated optimum is 18%.
1. Is 5% hydrated lime by dry weight adequate, and how would you verify it before production?
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Not on this evidence. Lime dosage comes from a dosage series, not a rule of thumb. The standard path: run ASTM D6276 (Eades-Green pH) to find the lime fixation point, meaning the lowest dose that holds pH near 12.4, then confirm with Atterberg limits and unconfined compressive strength on mellowed mixtures at, for example, 3%, 5%, and 7%.
For a clay with LL 52 and PI 38, 5% is plausible but unverified. Also verify the lime itself (available CaO and fineness) and verify the placement rate by weighing lime spread over a measured area or by calibrating the spreader before production; a visual 'white enough' judgment is not acceptance. Check mixing depth and uniformity by exposing a cut section, and retest the treated material for LL and PI to demonstrate it actually meets the 35/15 limits in the specification.
2. How does the 2,800 ppm sulfate result change your approach?
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2,800 ppm water-soluble sulfate is above the range where calcium-based stabilization is treated as low risk. Lime plus sulfate plus alumina from the clay forms ettringite and related minerals that expand during and after curing, heaving and cracking the treated layer.
One sample is not a site characterization. Before full-scale work, sample several borings across the footprint, test water-soluble sulfate by ASTM D516 or the local DOT method, and map any high-sulfate zones. Then let the geotechnical engineer select the stabilizer: a reduced calcium dose, a cement or slag-cement blend evaluated the same way, or removal and replacement with imported low-sulfate select fill. Do not let the contractor commit the entire pad to lime until the sulfate distribution and the dosage are settled in writing.
3. What do you do about the same-day compaction and the 26% moisture?
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Three objections. First, 26% against an 18% optimum is 8 points wet. Lime hydration consumes some water but will not dry the layer 8 points, and compacting wet clay yields low dry density and high strain regardless of lime content. The material must be aerated, or placed in thinner lifts and dried, before compaction.
Second, mellowing. Lime modification needs a mellowing period, commonly 24-48 h and longer for high-plasticity clay, before compaction so the plasticity actually changes. Same-day mixing and compaction defeats the purpose, and there is also a compaction-delay limit after which the mix becomes unworkable.
Third, verification. After mellowing, retest LL and PI on the treated material, then density and moisture by the specified methods at a defined frequency per lot or per acre. Hold the work until dosage, moisture conditioning, and mellowing are documented in an approved plan.