Geotechnical Characteristics of Anhydrite/Gypsum Transformation in the Middle Miocene Evaporites, Red Sea Coast, Egypt

The Middle Miocene evaporites of Abu Dabbab Formation, north of Quseir City along the Red Sea coast of Egypt are composed mainly of thinly laminated, bedded and nodular anhydrite and microcrystalline gypsum especially in its lowermost part. Highly cracked inter-bedded clay, dolomite and stromatolite layers were recorded within the evaporite succession. The cracks in the inter-bedded clays are filled with evaporite materials indicating arid climatic conditions during exposure episodes. Swelling mechanisms in clay-evaporites include mechanical swelling due to hydration of clay minerals and transformation of the anhydrite into gypsum. Field observations as well as powder x-ray diffraction investigations have shown that anhydrite/gypsum modal ratio decreases gradually downward indicating that gypsum has transformed to anhydrite by solar heating. The clay minerals of the inter-bedded clay layers are dominated by smectite (46 %), illite-smectite mixed-layer (18 %), chlorite (13 %), palygorskite (9'%), kaolinite (8 %), and illite (6 %). The liquid limit of the Abu Dabbab Formation (anhydrite and inter-bedded clays) is higher than 65 %, so considered as of very high swelling capabilities. Also, its swelling percentage was found to be (11-14 %), (28-35 %) and (58-65 %) for gypsum, clay and anhydrite, respectively. The swelling pressure was found to be (1.4-1.5 kg/cm(2)), (2.3-3.1 kg/cm(2)) and (4.7-5.1 kg/cm(2)) for gypsum, anhydrite and inter-bedded clay. The swelling pressure of gypsum is moderate and for both anhydrite and clay is high. The highly swelling capability of anhydrite and the inter-bedded clays causes geotechnical problems when got into contact with water. Alternating volume change due to phase transformation and solubility of calcium sulfate adds to the severity of problems associated with the host expansive clay strata.