The paper describes the geology, field monitoring and the interpretation of a large rock and soil slide occurring in the border of the basaltic plateau at Malhada, State of Rio Grande do Sul, Brazil. Slope stability analysis with the original topography of 1964, measured laboratory shear strength and field pore water pressures was able to reproduce the initial failure, The total volume of material involved is about 15 × 106 m3. The phenomenon is thought to be an example of some large movements which are part of the basaltic escarpment evolutionary process.
This paper discusses the main geological and geomorphological aspects of Serra Geral mass movements occurring in the south border of the basaltic plateau (scarp of the Serra Geral) in the ms ate of Rio Grande do Sul, Brazil. So much in the south border and east they are frequent the movements of mass of great extension in these natural hillsides. Those mass movements are expansion of concern for engineers, mainly the engineering works associated to the recent expansion of the transport roads and growing urbanization. The areas south and southeast of Brazil for climatic conditions and for extensive ones solid mountainous (denominated Serra do Mar and Serra Geral), they are subject to disasters associated to mass movements in the hillsides. The hillsides of the regions south and south east of Brazil understand three types of geological profile essentially. These are (i) the derived hillsides of the gneiss alteration and granites (that constitute the Serra do Mar) located between the states of Santa Catarina and EspÍrito Santo; (ii) the derived hillsides of the evolution of the basaltic flows (that constitute the Serra Geral), mainly in the states of Rio Grande do Sul and Santa Catarina; and (iii) the derived hillsides of the sedimentary rocks of the Basin of Parana, consisting of sandstones, limestones, claystones and shales in several alteration degrees. In 1995 occurred a large slide of soil and rock in the border of the basaltic scarp at Malhada, near the town of Santa Cruz do Sui, Rio Grande do Sui state. The localization map is shown on Figure 1. The movements had started around 1971 being noticed only by local peasants. There was an increasing activity from 1990. Today there is a large opening with vertical rock walls of 25–30m height, 40m in width and more than 300m long. At the base of the scarp there are clayey siltstones. These are covered by silty sandstones and basaltic flows. The lower part of the slopes are covered by colluviums which had presented movements larger than 10–15m over the last 20 years. The total area showing movements has around 60 hectares and involves around 15 × 106 m3.
Located at the extreme South of Brazil, the state of Rio Grande do Sui presents a stratigraphic sequence similar to that found in other Brazilian states located at the Eastern border of the Parana Basin.