Erosões internas no aterro da barragem Granjeiro: estudo de caso

Abstract

This report approaches the theoretical knowledge about the principles of the initiation of internal erosion in earth dams and aspects related to the continuity and progression of this erosions, in order to understand the key factors that led to the failures that ocurred in 2012 and 2019 in Granjeiro dam, characterized by large erosions in the embankment, and checks whether the design stage would have the necessary precision to identify these factors. Therefore are taken into account the results of field surveys and numerical analyzes of seepage flow and stability obtained with the Basic Design of Interventios to Control and Minimize the Risk of Failure of Granjeiro Dam, in order to confirm the statement that in the design stage it could be reached the accuracy required to check key factors that could initiate internal erosion in earth dams during their service life. The initial statement was not confirmed for the geological feature of the foundation of the Granjeiro Dam and therefore the steps established in the methodology proposed in that study were applied, supported by the theoretical review, in order to converge to the most feasible hypothesis in relation to what led to the erosions observed in the dam in 2012 and 2019. As the result obtained, it was found that the standart design supported by flow networks, established from the results of infiltration tests, in the foundation layers of Granjeiro, did not provide the necessary precision to discern the flow through the foundation controlled by preferential discontinuities. The assessments point to the persistent discontinuities in the upstream / downstream direction, present in the first 3 (three) meters of the foundation rock, as the key factor responsible for led to the failures that ocurred in 2012 and 2019 in Granjeiro dam, by providing hight flows events with high speeds and gradients, sufficient to initiate the erosive process in contact with the base of the sealing trench of the dam. Erosion continued due to the absence of a filter action on the foundation in downstream of the point where it initiated, but mainly due to the lack of instrumentation and monitoring of the dam, thus, it was not possible for anomalous behavior to be noticed, which could indicate problems related to internal erosion that they were happening in the dam. The progression of erosion happened due to the capacity of the soil of the embankment hold a roof or wall to the pipe or the cavity without collapsing. Finally, as a preventive measure to the effective control of the flow from foundation, it considers as a safer solution the execution of relief wells under the foot drain, drilled so that they cross the entire depth of 3 (three) meters of the superficial rock until the top of the healthy or slightly altered rock and the requirement for instrumentation of the dam with piezometers, flow meters and surface landmarks.