This research aims to study the role of soil suction and clay minerals on volumetric changes of a collapsible soil selected from Hawy Al-Kanissa area in Mosul city. The effect of low and highly plastic clayey soil as an additive to this soil has been investigated. The main results show the optimum clay content for soil to collapse decreases with the increase in the plasticity of soil. Swelling percent and pressure increase with the clay content, plasticity of soil and dry unit weight. 
Soil suction was also studied for collapsible soil with different clay contents. The results show linear relationships for all types of soil. Soil suction also increases with plasticity. Thus soil (CH) gave more suction than soil (CL). However, dry unit weight had little effect on soil suction, which was frequently negligible. Addition to that, statistical models between soil suction with collapse and swelling potential were obtained with an acceptable correlation coefficient.