Application of equivalent gypsum content to estimate potential subsidence of gypsiferous soils

The application of irrigation water on farmland throughout the arid and semiarid areas poses severe engineering challenges for gypsiferous soils. Subsidence and corrosion also pose severe engineering problems when gypsiferous soils are used for urban development or home sites. Subsidence is attributed to the dissolution and removal of gypsum by water, and it is usually used to estimate potential subsidence of soils. However, a number of other water-soluble minerals may occur in association with gypsum in these soils. Thus, subsidence should be attributed to the dissolution and removal of both gypsum and other water-soluble minerals in soils. The objectives were to (i) develop a laboratory method (equivalent gypsum content, or EGC) to estimate the content of both gypsum and other water-soluble minerals in gypsiferous soils and (ii) apply the EGC method to estimate potential subsidence of these soils. We used the relationship between dissolved minerals and electrical conductivity in soil/water solutions under equilibrium to estimate the EGC for soils. The EGC is defined as the quantity of both gypsum and other water-soluble minerals and expressed as gypsum percentage (by weight) in soils. We measured the EGC for 92 gypsum-rich soil samples collected from different arid and semiarid areas in the United States. A highly significant correlation (r = 0.97**) was found between the EGC and gypsum determined by the standard acetone method. The EGC was greater than gypsum content for almost all soils investigated. The average of EGC was 26.3% compared with 20.2% for gypsum. Using gypsum percentage would underestimate subsidence of these soils. We suggest the application of EGC rather than the gypsum content to estimate potential subsidence. Further, the EGC could be applied to provide a reasonable estimation of gypsum percentage for soils.