ABSTRACT

The paper presents a description of the Picote II site, the objectives of the study that was carried out to assess the in situ state of stress in the rock mass, details of the testing programme and the methodology used for its interpretation. An overcoring testing programme was performed at the design stage, and a specific methodology was developed for the global interpretation of the test results, making use of a numerical model and considering the effect of the existing canyon on the stress field. This enabled to make recommendations regarding the initial stress field to consider in the design of the powerhouse cavern.

Owing to the scarce information obtained at the design stage and to the high values of the calculated horizontal stresses, it was decided to perform additional stress measurement tests, using small flat jacks, once construction started and excavation of the adits reached the proximity of the underground powerhouse. Numerical simulations were also done for its interpretation and they confirmed the stress levels obtained at the design stage. The monitoring scheme implemented in the powerhouse cavern includes instrumentation to measure stress changes, which will occur during the excavation. Finally, the significance of the state of stress assessment to the powerhouse design and to the interpretation of monitoring data collected during construction is discussed.

1 INTRODUCTION

The Picote hydroelectric scheme, built 50 years ago, is located in the North-East of Portugal, in a canyon excavated by the Douro River. The Picote II project, which is now under construction, corresponds to repowering the Picote scheme and includes a new hydraulic circuit and a new underground powerhouse.

The designer included in the site investigation tests for determination of the in situ state of stress, to be used as input to the numerical models of the underground structures. At the design stage, six STT (strain tensor tube) overcoring tests were carried out in 2 boreholes. A specific methodology was used for global analysis of the test results, in order to obtain the most likely stress field.

Owing to the relatively scarce information obtained at the design phase and to the high horizontal stresses that were determined, the designer decided to perform additional stress measurement tests, using small flat jacks, once construction started and excavation of the adits reached the proximity of the underground powerhouse. Having in mind the influence of high in situ stresses on the behaviour of the powerhouse cavern, STT cells were also included in its monitoring scheme.

2 THE PICOTE II HYDROELECTRIC SCHEME

The hydroelectric potential of the international stretch of the Douro River exploited by Portugal is made by three power plants in cascade: Miranda, Picote, and Bemposta, (Figure 1), all built in the 50 and 60 decades of the 20th century (EDP, 2005). At this location the Douro valley is narrow and deep (Figure 2). The reservoirs have small capacities and are unable to regulate the incoming flows.

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