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Civil engineering projects usually require an improvement on soil properties. In this case, the improvement of soil strength, permeability, and erosion resistance, collapsible or dispersive behaviour might be important. Consequently, engineers are always willing to achieve an optimum way to stabilize/improve soil performance for practical applications. Generally, the presence of dispersive clays in engineering projects is one of the essential problems for civil engineers. Several failures in engineering projects due to the presence of dispersive soils are reported in different parts of the world. In spite of several researches on this subject, there has been little attention to the microstructure of stabilized dispersive soils. In this paper the improvement of properties of a natural dispersive soil is studied with special attention to the solubility of clay mineral peaks in XRD analysis and formation of aluminates and silicates minerals after lime application to dispersive soil. The soil sample of this area is silty clay which has 100% dispersivity potential. Several geotechnical erosions are reported in this area which is attributed to the presence of dispersive soils. Due to the low bearing capacity of soil, and to overcome to its dispersivity behaviour, the application of lime is chosen as soil stabilization means. In this paper, at the first step with application of double hydrometer and cation exchange capacity experiments, the dispersive potential of soil is determined. In addition, the major minerals of soil are addressed by the use of XRD analysis. For the dispersive soil sample, the major geotechnical properties of soil including unconfined compression test, settlement behaviour, and over consolidation stress are measured and reported. Then, the soil sample is mixed with lime and water. After equilibrium and after curing process, several geotechnical and geo-environmental experiments were performed on soil samples to investigate on the dispersivity potential variations of soil. Furthermore, the microstructure of lime treated dispersive soil was monitored by application of XRD analysis on lime treated soil. The results indicate that the dispersivity potential of soil disappears after application of 6% of lime. In this process the dispersive structure of soil changes to the more flocculated structure. Moreover, the results of XRD analysis indicate that following application of lime, parts of clay fraction solubilize, leading to the formation of pozzolanic components. The main conclusion of this paper is the determination of the required percentage of lime for soil stabilization with the application of micro-structural experiments and with the measurement of the quantity of increase in over-consolidation stress. With the performance of several consolidation tests on untreated and lime-treated dispersive soil, it is shown that with application of 6% lime, an increase of 430% in over-consolidation stress is observed. This means for each percent of lime a 50 kN/m 2 increase in over-consolidation stress is achieved.

Keywords: Dispersivity, Solubility of Clay Minerals, lime, Over-Consolidation Stress, XRD

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