Swelling of sulfate-bearing soil: A case study of A1 highway pavement failure

Lime stabilization is one of the most commonly used methods for improving weak road subgrades, but it becomes very problematic in sulfate-bearing soils. This article presents results of a case study in which the sulfate attack was the most probable reason for a significant deformation of a highway pavement founded on lime-stabilized Doggerian clay. Macroscopic analysis revealed partial degradation of the concrete structure, a larger extent of stabilization than designed, and large gypsum crystals in the native soil. Chemical tests indicated a very high content of sulfates in the soil (> 10,000 mg/kg of SO4) and in the groundwater (> 3,000 mg/l of SO42-). The maximum free swelling strain (εvolmax = 3–6%) and the swelling pressure (pcmax = 56 kPa) of the clay, determined in the Wasiliew apparatus and the oedometer tests, were not large enough to be considered as the only cause of the pavement damage. Sand-cement and clay-lime mixtures, which simulated stabilized soil layers in the road profile, were observed for several months in oedometric tests, under various loading conditions. The swelling of Sa-Cem was negligible, but the Cl-Lim mixtures turned out to be much more expansive than the natural soil (εvolmax up to 21%, pcmax = 230 kPa) with a much longer duration of volume increase. The samples were flooded with distilled water or hard groundwater to clarify whether the constant access of the material to sulfates is important in the swelling process, but the results were unequivocal. X-ray diffraction revealed that the addition of lime resulted in the formation of secondary minerals: butlerite, portlandite, and ettringite. The presence and swelling of ettringite in the stabilized sub-base soil layers was eventually agreed to be the most probable reason for the pavement deformation. These results indicate the need to introduce an obligatory investigation of the level of sulfates in clayey subbase soils subject to lime stabilization.