AVALIA O DA SUSCEPTIBILIDADE DE SOLOS GNAISSICOS à EROS O SUBSUPERFICIAL / Evaluation of piping erosion susceptibility in gnaissic soils

The basement rock areas of the Maracujá hydrographic basin show great incidence of hugegullies. This work aims to investigate the influence of the underground erosive processes inthe evolution of these features and to understand how the mineralogical and textural factorscould affect soil erodibility, specially of the saprolites, since the lateric soils of this basin aremore resistant to erosion. So, basic characterization and erodibility essays were carried out in representative soil samples. Soil susceptibility to piping and seepage erosion were investigatedthrough current and modified pin-hole-essays. Laboratory physical models were used in orderto observe these and other possible underground erosion mechanisms, especially the slides.However, piping erosion only occurs in the unsaturated portion of gully slopes, provoked bythe concentration of superficial water, since pipes are not stable under total saturation, whentheir roofs tend to collapse, because of the low cohesion of these saprolites. In the other hand,slumps are very common in the saturated portion of gullies, affecting saprolites even in smalland low declivity slopes.