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This dissertation deals with collapsing soils that are prevalent in Tucson, Arizona. Upon wetting, such soils generally swell under small loads but collapse under large loads. Since the recognition of such collapsing soils in Tucson, before about two decades, more collapsing soils were encountered due to booming construction. Therefore, the main goal of this research was to study in depth the mechanism by which these soils collapse and to investigate the effect of certain mechanical and chemical treatment on that mechanism. The research included studies of undisturbed, compacted, and lime-treated samples. Both mechanical and physicochemical tests were conducted. The mechanical tests included collapse, swell, and unconfined compressive strength. The physicochemical tests involved X-ray diffraction and scanning electron microscopy. Various sites of highly collapsing soils were classified with respect to collapse according to existing criteria and the soil of one site was selected for a detailed investigation. A predictive collapse criterion was developed and used to classify the collapse susceptibility of soils in Tucson. The microstructure of the selected soil was investigated before and after collapse. A physical model was proposed to explain the mechanism of collapse. The effects of initial water content, sequence of loading and wetting, and level of loading on the engineering behavior of the selected soil were investigated. Stabilization by compaction was studied using impact and static methods at seven points on the Standard Compaction Curve. The benefits of hydrated-lime additive and the short-term reactions of lime-treated samples were also studied. The research results indicated that the microstructure of the soil is highly porous due to many interassemblage pores. Fine clay particles were found either clothing or buttressing the larger silt particles. The collapse was due mainly to weakening or failure of the clay connectors between the larger soil particles due to swelling of the expansive clay minerals, reduction of the strength of clay connectors due to wetting, dispersion of the supporting buttresses, and reduction of capillary tension. Compaction by both impact and static methods minimized the collapse but not the swell of the soil. Lime treatment completely suppressed the soil's tendency toward collapse and swell.