Recent major upstream raised tailings dam failures have led to a reopening of the discussion of the validity of some of the existing routine practices within the profession. Despite its many shortcomings, deterministic slope stability limit equilibrium analysis is and will continue to be, at least for some time ahead, an important tool for tailings dams' safety assessment. Within this context, this paper presents a contribution to the postulation of minimum factors of safety required for tailings dams' slope stability analysis. A recent review and discussion of limit equilibrium analysis and the guidelines of international standards and current trends, with focus on tailings dams, are presented. Based on this review, and the authors' academic and professional experience, minimum required factors of safety recommendations are proposed. The framework of the recommendations strives to conciliate, in a simple manner, the deterministic minimum required factors of safety with concepts of consequence, uncertainties, risk and characteristics of loose tailings behaviour as a material.

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The destabilizing effect of groundwater is one of the major causes for landslides, which represent a major hazard to human life and the environment. Groundwater lowering is often the most efficient way to stabilize large unstable ground masses. Among groundwater lowering measures, drainage tunnels have several advantages, although construction costs may be proportionally high. This paper presents the concepts involved in the design and construction of drainage tunnels. Three case histories, two in Brazil and one in Argelia are presented, including geological background and monitoring results, where large landslides were stabilized using deep drainage through tunneling solutions.

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.

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