The use of lime to stabilize expansive soils has been the preferred technique for many years. However, heaving and premature pavement failures in lime-treated expansive subgrades containing sulfates led to the search for alternative stabilization techniques. Of the several techniques developed, precompaction mellowing has the potential to be effective in stabilizing sulfate-bearing soils. Yet this method needs experimental evaluation. In the current study, an attempt was made to assess the stabilization effectiveness of precompaction mellowing on high-sulfate soils. For this task, six natural expansive soils from Texas, with sulfate contents varying from 200 to 44,000 ppm, were collected. Soils with low-sulfate contents were spiked with additional sulfates to make them high-sulfate soils. Basic classification and chemical tests were performed to establish the clay mineralogy of the soils. Three mellowing periods (0, 3, and 7 days) were studied. The test soils were treated with lime and allowed to mellow for the specified periods. Following the mellowing, the samples were subjected to three-dimensional tests for volumetric swell, shrinkage, and unconfined compressive strength (UCS). To study the consumptions of alumina and silica during sulfate–soil–lime reactions, reactive alumina and silica measurements were also attempted. The authors observed that shrinkage was of no concern in treated soils because the shrinkage invariably reduced with lime treatment. In four of the six soils, precompaction mellowing reduced sulfate-induced swell to a level below the natural expansive swelling. The UCS strengths of treated soils decreased slightly with mellowing. Reasons for the anomaly in UCS strengths and ineffectiveness of precompaction mellowing in two soils were explained.

Stabilisation of sulfate-bearing soils has proven to be a challenging engineering problem for the pavement and geotechnical engineering community. Pavement distress caused by the heaving of sulfate soils often necessitates pavement rehabilitation and frequent road closures. Although the current literature provides a few reasons for sulfate heaving, there are still remaining issues with respect to normalisation of sulfate levels at which this heave will occur in the field. This paper presents sulfate heave issues, soluble sulfate measurement techniques for soils and various factors that complicate the establishment of threshold sulfate levels. Sulfate heave mechanisms by crystal growth and ettringite hydration are explained. Then an overview is provided of various soluble sulfate measurement techniques; a couple of case histories of sulfate-induced failures are also included. A comprehensive summary is given of various alternative treatments used in an attempt to stabilise sulfate soils. In conclusion, a few other issues related to this heaving and ongoing research developments to address these issues are briefly mentioned.

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