Along the bordering of the Belo Monte Hydroelectric Intermediate Reservoir, 28 earth-rockfil Dams have been built, some with up to 60 m height and up to 2,000 m long. In terms of the foundation, while most was executed in a region of gneissic rockmass, locally classified as migmatite, three of them were situated in a sandstone promontory, designated as Graben region. It constitutes by a solid mass of sandstone, with diverse characteristics and usually more permeable and with the possibility of carrying fine sand particles by the water flow. After performing detailed “in situ” investigations, the design incorporated a series of appropriate measures, a detailed internal drained system, as well as a good instrumentation plan. The instrumentation is indicating a good performance, confirming the good quality of the solutions implemented in the project, for the Graben region.