Expansive soils are a worldwide problem especially in the regions where climate is arid or semi arid. These soils swell when they are exposed to water and shrink when they dry. Cyclic swelling and shrinkage of clays and associated movements of foundations may result in cracking of structures. Several methods are used to decrease or prevent the swelling potential of such soils like prewetting, surcharge loading, chemical stabilization etc. Among these, one of the most widely used method is using chemical admixtures (chemical stabilization). Cyclic wetting and drying affects the swell &ndash / shrink behaviour of expansive soils. In this research, the effect of cyclic swell &ndash / shrink on swell percentage of a chemically stabilized expansive soil is investigated. Class C Fly Ash is used as an additive for stabilization of an expansive soil that is prepared in the laboratory environment by mixing kaolinite and bentonite. Fly ash was added to expansive soil with a predetermined percentage changing between 0 to 20 percent. Hydrated lime with percentages changing between 0 to 5 percent and sand with 5 percent were also used instead of fly ash for comparison. Firstly, consistency limits, grain size distributions and swell percentages of mixtures were determined. Then to see the effect of cyclic swell &ndash / shrink on the swelling behavior of the mixtures, swell &ndash / shrink cycles applied to samples and swell percentages were determined. Swell percentage decreased as the proportion of the fly ash increased. Cyclic swell-shrink affected the swell percentage of fly ash stabilized samples positively.

[pt] No presente trabalho, pretende-se avaliar os efeitos de ciclos de umedecimento e secagem na resistência ao cisalhamento do solo. Este estudo é de grande importância, particularmente,para os solos brasileiros, já que se encontram em uma região tropical, onde verifica-se a ocorrência de períodos bem definidos de chuvas e estiagem. Sabe-se que, longos períodos de chuvas podem saturar o solo, assim como durante a estiagem,pode ocorrer uma maior evaporação do que infiltração de água, o que leva o solo a uma condição não saturada, provocando o surgimento de uma nova componente de resistência, a coesão aparente. Na natureza, estes dois períodos não ocorrem de maneira uniforme, fazendo com que o solo atinja valores de sucção diferentes, dependendo do seu umedecimento e secagem. Assim sendo, ciclos de umedecimento e secagem e o nível de sucção atingido correspondente, podem afetar o comportamento mecânico do solo. Afim de verificar a influência de tais ciclos de umedecimento e secagem, determinou-se a resistência ao cisalhamento do solo, por meio de ensaios de cisalhamento direto com sucção controlada no equipamento desenvolvido na PUC-Rio. O umedecimento e a secagem do solo,foram simulados, em laboratório, através da diminuição e aumento da sucção, respectivamente. Utilizou-se o solo maduro do campo experimental II da PUC-Rio, peneirado e compactado em laboratório.Foram realizadas várias séries de ensaios, em que se modificavam os procedimentos realizados em relação aos níveis de sucção aplicados. A partir dos resultados obtidos,verificou-se a influência dos ciclos de umedecimento e secagem e dos níveis de sucção atingidos em cada caso, na resistência ao cisalhamento do solo, bem como na variação de volume total e de água nas amostras, durante o cisalhamento. / [en] This work intends to evaluate the effects of wetting and drying cycles on soil shear strength. It is a study of great importance, specially for brazilian soils, since they are found in a tropical area, where there are periods well defined of raining and dryness.It is well known that long raining periods can saturate the soil, as well as during dryness it can occur a bigger evaporation than infiltration of water, what brings soil to an unsaturated state. Unsaturated soils have an additional strength component: the apparent cohesion. In nature, raining and dryness do not occur uniformly, because of that, the soil suction can reach different values, depending on its wetting and drying, thus being, wetting and drying cycles and the corresponding suction levels can affect the mechanical behaviour of soil.Aiming at verify the influence of such cycles of wetting and drying, soil shear strength was obtained through laboratory tests of direct shear with controlled suction, using PUC-Rio s developed equipment. Soil wetting and drying had been simulated in laboratory, reducing and increasing suction. Mature soil brought from the experimental field II of PUC-Rio, sifted and compact in laboratory, was used.Many series of laboratory tests had been carried out, modifying procedures related to applied suction levels. From the gotten results the influence, of wetting and drying cycles and reached levels of suction, was verified. Such influence also occurred, on total and water volume changes, during the shear.