Critical Gypsum Content in Leaching of Gypseous Soil

Abstract Gypseous soils cover portions of the arid and semi-arid regions known with their satisfactory engineering behavior at dry-state; however, they are susceptible to progressive deterioration upon water exposure, due to the dissolution of gypsum (CaSO₄·2H₂O). While the consequences of gypsum dissolution - strength loss, increased compressibility, and elevated permeability are well documented in the literature, no prior study have quantified the minimum residual gypsum that persists indefinitely regardless of leaching duration. This investigation introduces the concept of the Critical Gypsum Content (CGC), a residual threshold beyond which no further gypsum dissolution occurs during leaching. Laboratory leaching tests were conducted on four natural gypseous soils designated as (S1- S4) with initial gypsum contents of 14–31% collected as undisturbed soil samples from different regions located at south and south-west of Mosul city-Nineveh-Iraq. Total Soluble Salts (T.S.S.) were monitored at three-day intervals until stabilization, after which direct shear, consolidation, and permeability tests were performed on pre- and post-leaching soil samples. Results demonstrate that gypsum dissolution was rapid during the first 15–18 days and decreased as the CGC was approached, with CGC values ranging from 5.2% to 11.1% and attainment times between 27 and 66 days. Reaching the CGC caused substantial reductions in cohesion (54.8–68.8%), confirming that gypsum cementation is the primary source of soil cohesion, while the internal friction angle decreased only slightly (12.1–14.8%). Leaching significantly increased soil compressibility, with the compression index rising by 2.4–3.5 times, and increased permeability by 5.6–35 times, depending on gypsum content. Higher-plasticity soils exhibited greater resistance to dissolution and permeability growth, indicating a protective role of clay minerals. The results further revealed a positive feedback mechanism between gypsum dissolution and permeability, which accelerates soil deterioration and contributes to the progressive failure of gypseous soil subgrades.