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A field test is developed to assess the collapse susceptibility of soils rapidly and inexpensively. The in situ collapse test device measures the vertical deformations which occur in soils when they are subjected to stress and given access to water while under continuous load. Principles of statistics were employed to show that laboratory testing of soil specimens overestimate the magnitude of collapse as measured in the field and that the magnitude of collapse is, in part, a function of the soil moisture content at the time of loading and at the time of collapse. Good correlation was found between the spatial variability of collapsible soils with the location of alluvium terrace deposits and structurally damaged buildings.

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.

Expansive soils contain clay minerals that undergo a change in bulk volume in response to variances in environmental conditions. The ability to predict the occurrence and geotechnical behavior of swelling soils with a known degree of certitude would allow engineers to take measures to limit the damage resulting from these metastable soils. Research was conducted to investigate the regional distribution, mineralogy, and engineering properties of expansive soils in the Tucson Basin. Mineralogic studies employed X-ray diffraction procedures for the identification of clay mineralogy. The compilation of expansion-related soil parameters, from the geotechnical job-files of a local engineering consulting firm, allowed the development of an engineering database. The application of geostatistical analysis for the cartographical representation of mineralogic and geotechnical data permitted a regional characterization of expansive clay soils. Clay mineralogy was found to be directly related to the volumetric stability displayed by native soils, as well as the geology of the Tucson Basin.