The Role of Gypsum in Production of Sulfate-Induced Deformation of Lime-Stabilized Soils

Abstract

Gypsum is a major source of sulfate that produces sulfate-induced heave in lime-treated soils beneath roads and other paved structures. This deformation of pavement subgrade is known to result from the growth of the basic hydrous calcium aluminum sulfate mineral, ettringite, or a silica-bearing analog, thaumasite. The problem occurs in soils that have been treated with lime (CaO) for subgrade stabilization. Gypsum is a common sulfate mineral in sedimentary rocks and soils, and in north Texas it is present in soils developed on the montmorillonitic Eagle Ford Group shales (Upper Cretaceous). Because these soils are highly unstable the conventional treatment for road subgrade includes the addition of lime (CaO) or some other cementitious material such as fly ash or Portland cement. The pyrite-bearing Eagle Ford shale contains gypsum (CaSO4 • 2H2O) produced by reaction of calcium carbonate in the shale with acid sulfate from oxidation weathering of pyrite (FeS2). Sulfate movement upward in the soils can occur by capillarity, and it can be carried downward by infiltration, an often incomplete or interrupted event that leaves gypsum stranded as a soil evaporite. Once formed, the moderately soluble gypsum is retained in the clay-rich soils because of their low hydraulic conductivities, which makes them, in essence, reservoirs of gypsum. Experiments performed for this study confirm that gypsum in a lime-treated subgrade soil can supply sulfate for the growth of the expansive mineral ettringite. No sulfate external to the subgrade of the road is necessary for the reaction to occur. Gypsum is widely distributed in soils and surface outcrops in the western U.S. and should be the first mineral suspected where sulfate-induced heave has been diagnosed. Although gypsum is a moderately soluble mineral, it may be abundant in soils in regions with humid climates if its sulfate is derived from pyritic black shales by oxidation during soil formation. Eagle Ford soils do not produce sulfate-induced heave everywhere that road base has been lime-treated, but the problem is observed most frequently where roads follow streams, or run across low-lying areas or hillside slopes. Other studies have documented a correlation between deformation and major precipitation events. Ground water, soil water, and surface drainage regimes appear to control the specific sites at which severe deformation may take place within the stratigraphically-controlled belts of gypsum-bearing soils. Soluble sulfate tests currently in use to identify soils with the potential for sulfate-induced heave are known for inconsistent results. Total sulfate in a soil is a better predictor of the problem, and when gypsum is the sulfate mineral it can be determined quantitatively by existing laboratory methods. Stratigraphy is a first-order guide to where sulfate-induced heave may occur in the north Texas region, thus geologic maps of the Eagle Ford shale outcrop belt are also indicators of areas of possible sulfate-induced deformation.