Volume 41, N. 3, September-December 2018 | PDF(32 downloads)
In embankment dams and dam foundations, internal erosion is the major cause of structural dam failures and incidents. Internal erosion develops locally along concentrated leaks or in the largest flow velocity zones, where the permeability is high, and at the interface between coarse and fine materials, where the flow velocity in coarse material maybe high compared to the adjacent fine material. The mechanics of the equilibrium of the particle governs the initiation of the internal erosion. Filter and drain systems constitute a first line of defense against the phase of continuation of erosion. It is assumed by several authors that the continuation of internal erosion can be prevented by using adequate granular filters in areas where important hydraulic gradients may develop. This lecture addresses the most important and frequent of those internal erosion mechanisms in dams – induced by concentrated leaks or by backward erosion piping, presenting event trees for estimating the resultant probabilities of failure. To assist in the determination of the range of probabilities of their branches, a set of laboratory tests is described. In complement, some knowledge gaps related to the described mechanisms are identified, where future research efforts are directed at LNEC. Two case studies of dams and one case study of a cofferdam with internal erosion problems are described. The first is an embankment dam, Lapão dam, where due to design and construction problems an internal erosion process was developed. The other is Crestuma dam, a gated structure type. Since the dam started operating, the riverbed near the dam has been subject to frequent monitoring that showed progressive scour of the protective rockfill layer. This lecture presents the results of the studies undertaken in relation to the hydraulic stability of the alluvial foundation of the dam stilling basins and of the downstream rockfill, and the main features of the implemented repairing solutions. The last case is a cofferdam constituted by natural decomposed granite rock mass and a water tightness solution with jet grouting columns and grouting. The problems of internal erosion related to this type of formation are described, as well as the interventions adopted to overcome their effects.