LNEC support to the design adaptation of Alto Tâmega dam due to the rockslide on the right bank slope
The dam has a maximum height of 104.50m, measured between the crest at elevation 318.00m and the average lower surface of its foundation at elevation 213.50m. Its geometric definition comprises parabolic arches with increasing thicknesses towards the abutments, based on the central cantilever and reference surface. The total crest length, including the abutments, is 332m, with a chord length between abutments of 210m, resulting in a chord-to-height ratio of 2.01. The dam’s thickness is 5.00m at the crest and 17.46m at the foundation of the central cantilever, reaching a maximum of 22.61m at mid-height near the banks, where the stresses are most significant. The average excavation depths in the valley floor and slopes were 12m and 16m, respectively. The total concrete volume of the dam amounts to approximately 223,000m3
.
The structure is divided into 21 blocks. On the right bank, blocks are numbered evenly from B2 to B22, while on the left bank, they are designated by odd numbers from B1 to B19. The terminal blocks - B17 and B19 on the left bank, and B18, B20, and B22 on the right bank - have gravity-type profiles and house the intake structures for the spillway channels (Figure 3). Contraction joints are oriented perpendicular to the upstream face and foundation, forming arched surfaces. In the lower sections, these joints follow curvatures defined by parabolic functions in the profile based on the reference cylinder.
Figure 3. Alto Tâmega dam planned downstream elevation The dam contains a set of galleries. The perimeter gallery (GP), located upstream, is the drainage gallery.
Horizontal galleries (GO0 to GO4), which are inspection galleries, are near the mid-thickness of the arch at elevations 313.70m, 303.50m, 283.30m, 260.10m, and 238.17m, respectively. These intersect all blocks, extend to the perimeter gallery, and connect to additional galleries in the rock mass foundation (GO2, GRB, GRA, and GRMD). On the left bank, galleries GO2, GRB, and GRA link to an elevator shaft located vertically beneath block B19. On the right bank, only gallery GO4 extends into the rock mass foundation (GRMD). The dam is located within a near-straight 500m stretch of the Tâmega River, oriented approximately N35°E. The valley bed, with a width ranging from 10 to 30m, lies at an elevation of 225.0m. This section of the valley is nearly symmetrical, with the right bank slope averaging an inclination of 34° and the left bank slope averaging 44° up to elevation 247.0m, reducing to 34° above this height. From a lithological perspective, the dam site is predominantly composed of mica schists, intersected by subhorizontal aplite-pegmatite veins on both banks. The rock mass is characterised by two main discontinuity sets[2,4]
. One set is parallel to several subvertical faults, with average orientations of 71°/089° Vol XXXIII Issue 3 | Dam Engineering | 125
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