LNEC support to the design adaptation of Alto Tâmega dam due to the rockslide on the right bank slope

Abstract

During the excavations for the Alto Tâmega dam, a rockslide occurred on the right bank slope immediately downstream from the dam insertion surface, involving a volume of about 23,000 m3, which led to the complete stop of the works. To resume construction, design adaptations had to be introduced to stabilize the downstream slope and ensure adequate safety conditions during dam construction, first filling and operation. The main measures adopted were the following: i) construction of a concrete structure anchored to the rock mass, to replace the volume of rock mass that was affected by the landslide; ii) adjustment of the insertion surface by deepening and widening the excavation of several dam blocks; iii) extension of a gallery in the foundation to monitor two faults and to increase the drainage of the rock mass; and iv) relocating, symmetrically, the dam’s bottom discharge from the right side of the river bed to its left side. Additionally, a series of procedures related to the monitoring plan aimed at guaranteeing adequate safety conditions for the work execution and, mainly, to ensure that the project adaptations yielded the intended effects were implemented. This paper presents a brief description of the events preceding the slope failure, a description of the countermeasures that were implemented to ensure dam safety and some results of the monitoring of the right bank during dam construction and the first filling of the reservoir.