Shear Strength and Collapsibility of Gypsifierous Soils

Abstract

Large areas (>20 %) of Iraq are covered with gypsiferous soils which are often used as natural foundation bases for structures. In the last years, many damages were recorded in many strategic projects in Iraq (such as Mosul dam) which were constructed upon partially gypsum stratum or gypsiferous soils. Two site locations were chosen namely Baiji (51.24 % gypsum content) and Tikrit (73.88 % gypsum content), N. Iraq due to foundation problems associated with these sites. Several soil tests physical, chemical, XRD, ESEM, shear strength and collapsibility tests have been carried out. A reduction in shear parameters (∅ and C) after 6 and 24 h soaking with respect to dry condition have been found as 8.64–12.47 and 94.28–91.35 % respectively. The collapse potential increases with increasing stress level showing higher CP for Baiji soil reflecting the decrease in CP with increasing gypsum content. Also CP increases with increasing soaking period at a specific stress level. Macrostructure variations of some soil samples were examined with Environmental Scanning Electronic Microscope (ESEM).The micro structural images of features (gypsum, particles, voids and loading were further discussed). The images show the large voids that have been occurred representing the main cause for high collapsibility in such soils.