Luiz Antonio Bressani

Anisotropic shear strength of a residual soil of sandstone

This paper discusses results of laboratory tests carried out with a residual soil originated from the weathering of eolian sandstone from southern Brazil. Parent rock features, like microfabric and particle bonding, are remarkably well preserved within this residual soil. Stiffness and shear strength properties were evaluated with consolidated drained (CID) and consolidated undrained (CIU) triaxial compression tests. Undisturbed specimens were tested with two different orientations between the specimen axis and bedding surfaces (i.e., parallel (δ = 0°) or perpendicular (δ = 90°)) to investigate the effect of anisotropy. When CID triaxial tests were performed with δ = 0°, the yield surface associated with the structure was much larger than when tests were performed with δ = 90°. Coincidently, CIU tests with δ = 0° showed peak shear strengths much greater than for δ = 90° at comparable test conditions. Once the peak shear strength was surpassed, CIU tests followed collapse-type effective stress paths not sh...

Rational Approach to the Evaluation of Soils for Low-Volume Roads

Given the paramount importance of soils in unsurfaced or thinly surfaced roads, it seems worthwhile to evaluate soils on the basis of both pavement and soil mechanics. Such a rational approach is used to discuss the mechanical behavior of a residual soil found in southern Brazil. The experimental program included tests traditionally used in pavement engineering, such as grain size distribution, Atterberg limits, compaction, California bearing ratio, resilient modulus, and permanent deformation under repeated loading. The influence of water content and compaction degree on soil strength and stiffness was quantified. In addition, direct shear tests were performed to define shear strength parameters that were used to verify layer safety against failure. Water retention curves, which relate soil suction to saturation degree, were used to analyze permanent deformation evolution under repeated loads. The interpretation of test results confirmed the suitability of the suggested approach for evaluating soils for low-volume road construction.

Soil susceptibility to compaction under use conditions in southern Brazil

O grau de compactacao do solo e intensificado pelo seu inadequado manejo, sendo a compactacao depente do tipo de solo, uma vez que mesmo sob condicoes identicas de uso, diferentes solos tem diferentes habilidades para suportar a carga. O objetivo deste estudo foi avaliar a susceptibilidade a compactacao de diferentes classes de solos sob sistema plantio direto (PD) em relacao ao estado original. Assim, i) solos com o mesmo material de origem apresentam distinta resistencia a compressao com o aumento do tempo adopcao do PD; ii) os indicadores mais sensiveis desta mudanca sao os da relacao massa: volume e volume: volume e; iii) existe uma relacao entre resistencia e suscetibilidade a compactacao com a quantidade e o tipo de oxido. As amostras de solo foram coletadas em areas sob PD e sob condicao natural (CN), a fim de avaliar o impacto imposta pelo PD sobre os atributos densidade e porosidade, tensao de pre-compressao e indice de compressibilidade e relaciona-los com o tipo de oxido e conteudo. Os resultados mostram que a densidade e a macroporosidade foram os mais afetados pelo uso agricola em PD, independentemente do tipo de solo, ou seja, a sua dinâmica esta mais relacionada com o uso do solo e menos com as caracteristicas mineralogicas. A resistencia do solo e suscetibilidade a compactacao foram maiores no solo desenvolvido em basalto, seguido por aqueles desenvolvidos em arenito e granito. Tanto o teor de materia orgânica e tipo e concentracao de oxidos de ferro foram relacionados com a resistencia do solo e susceptibilidade a compactacao.