On the Use of Random Material in Dam Cores: Case of the Manso Dam, Brazil

The Manso Dam, in the Brazilian State of Mato Grosso, intended for flood control and electricity generation, was built by a state-owned energy company in partnership with the private sector. In order to reduce costs and minimize the environmental impact of the project, the private consortium proposed changes to the preliminary design, which were reassessed with the participation of the Federal University of Rio de Janeiro. The design and the behavior of the dam are discussed in this paper. The main section of the Manso Dam, 73 m high, has rockfill shells and a core of compacted soil. For the core, colluvial and residual soils derived from the necessary excavations for the concrete structures and dam foundation were chosen in order to reduce both the cost and environmental damage that would result from borrow/disposal. Such composite clay, gravel and cobbles of different sizes and degrees of weathering is generally referred to in the literature as random fill, and because is not typically used in dam cores, laboratory and field tests were then performed. Parameter evaluation and Finite Element (FEM) analyses of seepage and deformation were carried out. The seepage analysis was carried out for the maximum normal reservoir level, and the results were confirmed by the field monitoring data, enabling the establishment of safety levels (alert and emergency conditions). Embankment construction and first reservoir filling were simulated with the FEM, yielding, as expected, stress concentrations in the more rigid materials, such as the filters and transitions. During reservoir filling, seepage forces induced horizontal displacements in the downstream direction and reduction in effective stresses in the upstream region due to submersion. The predicted settlement distribution was compatible with field monitoring data. Construction was completed in 2010 and the dam has been operating normally ever since.