ABSTRACT:

Empirical uplift pressure diagrams are still used for stability analyses of concrete structures founded on rock, particularly during preliminary studies. Piezometric measurements on several brazilian dam foundations were collected and analyzed. These measurements were taken at both rock-concrete contact and in descontinuities in the rock foundations, for different types of rocks. To facilitate the analyses the structures were divided in groups with similar characteristics and employing a standard basis of analysis used in other works. The data indicates that the design criterium for normal operating Conditions predicting a 67% efficiency of drainage seems to represent an upper limit for observed uplift pressures, both at rock-concrete contact and in descontinuities. The data also shows that the hypothesis of full reservoir pressure acting under the upstream toe of the structures seems to be satisfactory whereas for descontinuities in the foundation it gives conservative estimates of uplift pressures. The highly beneficial effect of drainage tunnels in the foundations is clearly seen from the measurements.

1 INTRODUCTION

Despite the recent advances in different techniques for analysis of flow problems in rock masses as related to concrete dam foun dations, empirical uplift pressure diagrams are still very used in practice. These are taken as an envelope for measured water pressures for different types of rock foundations, dam structures, drainage conditions, etc. They suffer from obvious limitations but still serve a very useful purpose at least in the initial design stages. In Brazil the most commonly used empirical up lift diagrams are those suggested by the Bureau of Reclamation (USBR, 1976) and by Cruz and Barbosa (1981). A complete review of evolution of criteria for uplift pressures in gravity dams, including empirical, analytical and numerical techniques, is presented by Rocha (1987).

The USBR empirical criterium considers only one drainage line in the rock founda tion and assumes a value 01: pressure at the drainage line equal to the downstream toe pressure plus one third of the difference between tl1e upstream and downstream toe Pressures. This value is connected by Straight lines to the upstream and downstream toe pressures, assumed to be equal to the reservoir and tailwater level pressures respectively. When the dam foundation is not horizontal, the criterium is considered in terms of total head.

The Cruz and Barbosa (1981) empirical criterium for normal operating conditions is shown in figure 1for two drainage lines. In the upstream drainage line the pressure is given by the difference in level between the gallery (under atmospheric pressure) floor and the point of inter section of the slip surface with the drainage line plus one third of the difference between the reservoir water level and the gallery floor. The same consideration is made in relation to the pressure acting in the downstream drainage line, as shown in figure 1. The same authors consider that when a drainage tunnel (under atmospheric pressure) is excavated in the rock foundation and linked to the gallery by drainage holes, zero water pressures should be assumed in the drainage line.

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