The irrigation complex known as the Jaiba Project, administered by the state-owned company CODEVASF, in the north of the state of Minas Gerais, Brazil, includes the construction of a large pumping station EB-l, with a water discharge of 80m3/sec. As such a structure is founded over a karstic formation, a special treatment is required. Several technical solutions for foundations were studied, but after a large in situ test it was concluded that the best solution would be the improvement of the foundation ground by cement-clay grouts with a high friction angle.
Le complexe d'irrigation connu sous le nom de Jaiba Project, administre par la Societe d'Etat CODEVASF, et situe au nord de l'Etat de Minas Gerais, Bresil, comporte une grande station de pompage EB-l, avec un debit de 80 m3/sec. Cette station est situee sur un terrain de formation karstique et a necessite un traitement special pour ses fondations. Plusieurs solutions ont ete etudiees et après un essai in situ, on a decide que la meilleure solution serait d'ameliorer le soubassement par des injections de coulis de ciment-argile à angle de frottement eleve.
Der als Jaiba Projekt bekannte und von der staatlichen Gesellschaft CODEVASF verwaltete Bewasserungskomplex im Norden des Staates Minas Gerais in Brasilien umfaßt den Bau einer großen Pumpstation, EB-l, mit einer Pumpleistung von 80 m3/sec. Da dieses Bauwerk auf einer Karstformation gegruendet ist, wurde dafuer eine Spezialbehandlung benötigt. Verschiedene Gruendungsarten wurden untersucht. Nach Ausfuehrung eines Großversuches kam man jedoch zu der Erkenntnis, daß die Verbesserung des Untergrundes fuer die Gruendung durch die Injektion von Zement-Ton-Gemischen mit großem Reibungswinkel die beste Lösung sein wuerde.
The main pumping station, named EB-l, of the Jaiba irrigation project, which is owned by Companhia de Desen volvimento do Vale do Sāo Francisco - CODEVASF, a branch of the Interior Secretary of Brazil, will provide a water flow of 80m3/sec from the Sāo Francisco river, thus allowing the agricultural use of a 100.000 ha area. The Station occupies an area of 20 × 80m2 and is located on the right bank of the Sāo Francisco river, very near to a village called Mocambinho, in the,state of Minas Gerais, about 250km north of Montes Claros, an important center in the northern Minas Gerais. The elevation of the local terrain is about +444 and the foundation grade at elevation +427m, in need, therefore, of 17m excavation while the normal water level of the São Francisco river is around +443m, that is at about 16m above the foundation grade elevation. The type of foundation for the major hydraulic structure initially planned, as a slab, imparting a 350 kN/m2 (3.5kgf/cm2) pressure on the soil. The special geotechnical problem to be overcome is tied to the nature of the foundation, made up essentially by karstic limestone, whose surface sinkholes (Figure 1) comprised the evidence for its karsticity, in the course of the prospection studies that were performed.
From the many borings, drilled in part by the Instituto de Pesquisas Tecnologicas do Estado de São Paulo - IPT, and later by the Company which elaborated the studies and the soil improvement, Tecnosolo S.A., it became well known the geological configuration of the foundation sub -grade (Figure 2). Between elevations +412m and +443m, which represent the better investigated interval, the sub-grade comprises the following sequence, from bottom upwards: - - Compacted limestone, gray, of the Bambui Group up to elevations +417m to +420m. This limestone still preserves karstic caves, not been assured that such voids aren't present below elevation +4l2m - Highly karstified limestone; whitish, up to an elevation close to EB-1 foundation level, that is. +427m. of the same Bambui Group. Cavities are filled-in by clays, plastic, whitish-yellow; with variable sand fractions and also by yellowish sands. The geotechnical study of this formation, besides its own value for the foundation of an exceptional project, has also a high general interest due to the great extension of similar terrains, assumed to be present in the São Francisco river basin.
Three concepts have been proposed for EB-1 foundation type, taking in account the karstic structure:
Large diameter piles, to be bored mechanically with metallic casing which would confine the piles at the voided portions.
Small diameter piles, again with lost casing, associated by consolidation grouting of the sub-soil formations.
Shallow foundation, set on a 15m thick improved terrain, by means of clay-cement mixture grouting, yielding a compacted slab of ground, strong enough to assure foundation stability, independently of cavity distribution in the limestone skeleton below. Initially, the profession's specialists that were involved preferred solution
a), large-diameter piles, based mainly in the assumption that the sub-grade treatment would mean a difficult process to evaluate, comprising very high, and unforeseen, cement consumptions.
