Identification of Mudflow-Prone Areas in Southeastern Brazil

Abstract

Mudflow accidents have been described in Southeastern Brazil, some of them involving huge volumes of organic clay, causing economic losses and casualties. Usually, these mudflows occur in amphitheater-like headwaters of hilly areas of basement rocks. These amphitheaters are filled with layers of soft organic clay, characterized by low values of Nspt and UU shear strength, interfingered with or partially or totally covered by colluvium, with higher Nspt. The organic clay layer can reach 6 m in thickness. It overlies thin pebbly sand layers or silt-rich saprolites. This stratigraphy is a natural consequence of climate oscillations that have prevailed since the Upper Pleistocene/Holocene, causing advances or retreats of the fluvial system and, consequently, of the headwater zones. It is not easy to identify the triggering mechanism of these movements, but a retrogressive rupture model is possible, since these clays present some strength sensitivity. Groundwater uplift pressure on the less permeable organic clay is probably an important triggering mechanism for mudflows and it hinders the clay consolidation throughout the geological time. A detailed geomorphological mapping of a hydrographic basin near Belo Horizonte (State of Minas Gerais) helped identify several amphitheater-like headwaters with similar characteristics to those identified at accident sites. The description of outcrops, auger, cable percussion boring samples and geophysical surveys validated the above-mentioned stratigraphy, proving that this configuration is common in basement areas of southeastern Brazil. With the growth of Brazilian cities towards regions with these characteristics, greater care should be taken to prevent the repetition of mudflow accidents.