Contributions to understanding the hydromechanical behaviour of pressure tunnels.

Abstract

Tunnels and shafts under high internal water pressure are mainly found in hydroelectric power schemes and in some water supply systems. In addition to the mechanical effect that excavation has on the rock mass and the disturbance that it introduces in the water flow, in pressure tunnels also the contained water can interact from the mechanical and hydraulic points of view with the rock mass. Seepage in fractured rock masses is dominated by the flow that takes place through the joints, and this is highly dependent on their properties, namely aperture and roughness. Joint aperture, in turn, is very sensitive to stress changes. Special attention must therefore be paid to the joints in hydromechanical analyses. This thesis starts with an introduction to the main features of the projects where pressure tunnels are found and an analysis of actual deterioration cases, in order to identify the most important mechanisms that affect serviceability of such tunnels and show the importance of hydromechanical interactions. The next part of the thesis is devoted to study of the geometrical, mechanical and hydromechanical properties of rock joints, with special emphasis on the tests carried out for their determination in the laboratory. After reviewing the models for hydromechanical simulations, a new numerical approach is proposed, and a model using the finite element method is developed. This model allows non-linear hydromechanical quasi-plane analyses of multilaminate media, which consist of a continuum and several joint sets. Discrete joints for simulation of major discontinuities or interfaces between different materials can also be included. A total analysis of pressure tunnels and shafts is presented, which includes relevant rock mass characterization methodologies that allow use of the numerical model, the most important actions on pressure tunnels and shafts, the different types of lining, the most critical design requirements, as well as considerations about observation of their performance. Instances of application of the model to typical situations are presented, in order to assess the influence of several parameters. Applications to the pressure tunnels and shafts of the Alto Lindoso hydroelectric scheme are then described and analysed.