ABSTRACT:

The Cahora-Bassa dam is located on the river Zambeze, in Mozambique, forming part of a multi-purpose scheme. The general characteristics of the scheme are described, as well as the geology and studies for the geotechnical characterization of the rock mass. The monitoring plan initially planned, and its complement are also presented.

RESUME:

Le barrage de Cahora-Bassa est situe sur le fleuve Zambeze, Mozambique, et il fait partie d'un projet ayant plusieurs fonctions. On decrit les caracteristiques du projet, ainsi que la geologie et les etudes effectuees pour la caracterization geotechnique du massif rocheux. On presente aussi, le systeme d'observation initialement etablie et son complement recent.

ZUSAMMENFASSUNG:

Die Cahora-Bassa Talsperre liegt am Fluβ Zambeze, in Mozambique, als Teil einer Mehrzweckanlage. Die allgemeinen Merkmale der Anlage werden beschrieben, sowie die Geologie und Untersuchungen fuer die geotechnische Kennzeichnung des Gebirges. Das urspruenglich geplante Beobachtungssystem und seine Erganzung werden auch vorgestellt.

1
INTRODUCTION

The Cahora-Bassa hydroelectric scheme is located in Mozambique, in a stretch of the river Zambeze. It comprises a 170m maximum height arch dam and an important underground powerhouse complex and an underground hydraulic circuit. The rock mass consists of granitic gneisses with several lamprophyre and gabbroic veins. The geotechnical characterization of the rock mass was thoroughly analyzed by LNEC, an extensive test programme had been carried out. The Monitoring Plan that was initially established included the observation of the dam and its foundation, the underground structures and the downstream slopes. Subsequently, due to the need of adapting the Monitoring Plan to the current behaviour scenarios and replacing observation equipment that became inoperational, a Complementary Plan was established for the several structures involved. In this paper the general characteristics of the Cahora-Bassa scheme, the aspects related with the geotechnical characterization of the rock mass, and the initial and complementary Monitoring Plan are described.

2
GENERAL CHARACTERISTICS OF THE CAHORABASSA SCHEME
2.1
General considerations

The Cahora-Bassa dam forms part of a multi-purpose project, intended for modulating the river Zambeze flow rates, production of electricity, ensuring the navigability of the river upstream in an area of about 300km up to the border with Zimbabwe, it is also intended for the control of artificial floods caused by the Kariba dam spillways, in Zimbabwe, and for retention of sediments (Figure 1), (HP 1965). The storage reservoir has a 63Nm3 capacity covering a 2. 7x 103km2 area with a length of about 270km and a maximum width of 38km. The studies for the project started in 1956, the construction started in 1971 and was completed in 1974. The first filling of the reservoir was carried out from December, 1974 to June, 1976, the operation of the power scheme started in March, 1977.

2.2
Dam

The dam is set up in a narrow gorge, the cross-section of this gorge presents in its lower part almost vertical slopes up to the proximity of the crest of the dam. It is an arch dam, with the following geometric characteristics (Figure 1): 170m maximum height, 300m crest length, 1.54 chord-height ratio, a 4 and 23m thick central cantilever, respectively at the crest and at the base. There are 8 spillways in the body of the dam, with a 14x103m3/sec total runoff capacity for the maximum flood level and a surface spillway of the "volet" type at the central blocks of the dam.

2.3
Underground structures

The underground structures are located in the right bank of the river at a depth, which ranges, in the case of the powerhouse, from 130m at one end to 230m at the other.

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