A general presentation of the geological and geotechnical conditions present at the site of the Baixo Sabor main dam (NE Portugal) is made, and the rock mechanics tests carried out are described in more detail. A jointing study, borehole dilatometer (BHD) tests, large flat jack (LFJ) tests, stress tensor tube (STT) tests (for the powerhouse), as well as different laboratory tests were performed. Some comment


The Sabor river, located in the NE of Portugal, is a right bank tributary of the Douro river. The Baixo Sabor Hydroelectric Development is owned by the EDP (the Portuguese Electricity Company), and will have two schemes, equipped with reversible generating units, the main one situated nearly 12.6 km from the Sabor river mouth, and the downstream one at about 3 km. The main (upstream) scheme productivity will be of 181 GWh/year [1], not considering the one obtained by pumping. The upstream reservoir (Fig. 1) has a gross capacity of about 1,090 hm3 (630 hm3 for hydroelectric purposes) that will be created by a 123 m high concrete arch dam, with a volume of about 700,000 m3. Located in the dam body, there will be a controlled spillway, with a capacity of 5,000 m3/s, discharging in a stilling basin, and a bottom outlet with a maximum discharge capacity of 220 m3/s [1]. A shaft type powerhouse, at the right bank, will be equipped with two reversible units, located in independent wells, with a diameter of 11.5 m, a height of 79 m, and topped by an unloading and erection building. The nominal peak load of each unit is 70 MW, under a 93 m head [1]. The pressure tunnels, also independent, have a 5.7 m diameter and approximately parallel layouts. Fig. 1. Upstream scheme transversal section [1]. The diversion works will consist of two cofferdams and a 400 m long tunnel, with a diameter of 7 m.

Geology and Geotechnique

The upstream dam site is located in a NE-SW, approximately straight, 1 km long valley segment, with a deep (~ 350 m), V-shaped and slightly asymmetrical transversal profile, which is 30 m wide at the valley bottom, and approximately 430 m at the dam crest level. The observed geological units are [2, 3]:

  • Heterometric (generally sand and gravel) alluvial deposits, 1 to 5 m thick;

  • Granitic residual soils (< 1 m thick) and colluvium, at the base of the slopes, with variable thickness, but, more frequently, less than 5 m; 286

  • Medium to coarse-grained, porphyroid, two- - mica granite, showing orientation of the micaceous minerals, due to te micaceous minerals, due to tectonic compression. At the left bank (LB), the granitic rock mass observed in the trenches, is more often W3 to W2, and F2 to F4 (Fig. 2).

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