Some Case Histories with Lessons to Learn in Dam Engineering

Abstract

Three case histories of dam engineering related topics are presented as previous knowledge and experience would not have led to predict their impact on dam design. The Lecture honours Prof. Victor de Mello by discussing the Balbina HPP where Prof. de Mello led the saprolite foundation design when ancient termite canaliculae were found, imposing the need to control seepage through the dam's foundation. The remote location of the dam site associated to its meteorological characteristics did not allow for conventional earthmoving or plastic diaphragm solutions to be used, and special pressure grouting was studied and successfully applied. Details of the field studies to allow pressure grouting in saprolite soils as well as to define the grout mixtures are presented. Two other very interesting case histories are presented, one being the Teles Pires HPP. In this project thrust faulting generated unforeseen persistent sub-horizontal joints to depths not associated to the common pattern of stress relief joints in granitic rock masses, with presence of preserved slickensides and estriae. Stability considerations for the Intake Structure lead to the need of excavating shear keys consisting of tunnels filled with concrete, drainage tunnels and installation of passive anchors. Finally, the Chaglla HPP head race tunnel case history is presented. This tunnel 9 km long tunnel excavated through limestones previously investigated did not intersect any karstic feature. During commissioning turbine tests, with the tunnel pressurized, a small magnitude shallow earthquake occurred very close to the site, and muddy water was seen outcropping in a small creek valley in the side slope of the mountain where the tunnel was excavated. Detailed investigation lead to postulate a mode of failure in which the earthquake generated the disarticulation of a very fractured and saturated stratigraphic fault, leading to the retrogressive erosion developing from a distant karstic feature till the vicinity of the tunnel's shotcrete, drastically reducing its passive support, leading to the failure inside out of the tunnel lining and support. The importance of sound geological models is emphasized by these case histories, as always strongly argued by Prof. de Mello.

Stabilization of major soil masses using drainage tunnels

“…The use of drainage tunnels to reduce pore pressures is not restricted to the stabilization of landslides. Drainage galleries are often designed and built to reduce pore pressures and increase safety in the foundations and abutments of dams (de Mello, 2018). Figure 5 presents an example of a typical drainage tunnel (Eberhardt et al, 2007), installed inside the rock mass, below the landslide, with drains upwards drilled into the unstable soil, to reduce pore pressures:…”