CONCRETE GRAVITY DAMS STABILITY: A COMPARATIVE STUDY BETWEEN UNKEYED AND KEYED FOUNDATIONS

Abstract

Concrete gravity dams are mass concrete structures which resist to external loads mainly by their dead weight. The geometrical solution currently considered, which evolved from the reasoned application of mathematical theory to structural engineering, are characterized by right-angled triangular profiles with downstream face slopes of 0.7 to 0.8. In medium to high seismic intensity zones, such as Portugal, other constructive dispositions may be necessary. Under seismic loadings, it is generally accepted that the gravity profile keyed into the foundation at a depth corresponding to 10% of the dam height is a crucial contribution to ensure structural stability conditions. However, this detail is often not considered in stability analyses, which is generally understood as a conservative strategy. In this work, the benefits from considering the keyed depth in stability analyses are evaluated. For that, 100-meter-high hypothetical gravity profiles, keyed at a depth of 10 meters, are considered. To allow the development of rigid-body failure mechanisms, a downstream rock wedge, inclined at a critical angle, is assumed. At first, the analytical expressions that describe the failure mechanisms identified, considering the dam-foundation interface as a dominant failure surface, are deduced and validated through numerical modelling. Afterwards, the frictional properties of the interface, for several loading conditions which result in different total net forces and the corresponding application points, are computed. Lastly, the stability benefit is evaluated by comparing the safety factor obtained with the correspondent of an unkeyed profile. When explicitly considering the keyed depth in stability analyses, higher safety levels are obtained which can be crucial to ensure stability conditions. It was proved that, under the same load conditions, the consideration of unkeyed profiles would demand higher values of the friction angle, up to 12º more than considering the correspondent keyed profile. Moreover, this also ensures stability conditions for more inclined resultant net forces.