Numerical Analysis of the 210 m-High Nam Ngum 3 CFRD

Abstract

The The Nam Ngum 3 dam, actually under construction in Lao PDR, with a maximum height above foundation of 210 m, will be one of the highest CFRDs. ARTELIA is the Owner’s Engineer of Electricité du Laos (EDL) for this 480 MW HPP built on a major tributary of the Mékong. One of the main challenges in the design of the dam comes up from the tight valley. Actually, based on international feedback, extensive concrete face cracking is expected if specific construction provisions were not adopted. Moreover, the concrete face is constructed immediately upstream of the narrowest part of the valley. This results in high stiffness contrast between the banks where the concrete face lies on the bedrock and the central part where it is supported by the rockfill. These peculiar boundary conditions induce a diagonal bending which needs to be addressed with care. Consequently, ARTELIA’s numerical model aims at checking that the design of the dam and its construction provisions are efficient to avoid a detrimental cracking of the concrete face. The Hardening Soil Model constitutive law is used for the rockfill. The material parameters are calibrated by means of large-scale apparatus laboratory tests. A size effect assessed by means of a rational approach is taken into account. Finally, a creep constitutive law is considered in order to anticipate the effects of delayed deformation of the rockfill on the concrete face behavior. The modeled concrete face includes the compression joints with initial opening for which the efficiency in reducing the compression stress is clearly highlighted. Based on the results of the numerical analysis, this paper describes the supplementary construction provisions proposed in order to secure a safe behavior of the dam and guarantee an acceptable watertightness of the concrete face.