ABSTRACT

ABSTRACT

The effect of flow/deformation coupling in the analysis of the jointed-rock foundation of a dam is studied using the distinct element method for modeling deformations and a discrete network of conduits for modeling flow. The aim is twofold: to determine the design implications of including or ignoring coupling and to get a better insight into the performance of real dams. From the results, one can see that coupling produces uplift-force and corresponding overturning-moment estimates that are 30-35% higher than equal-apertures modeling, and 15-25% higher than deformed/uncoupled modeling. The analyses also show how flow, uplift pressure and deformation of the dam foundation change with drainage and grout curtains. Of particular interest is the fact that opening of fractures on the upstream side of dams, which have been observed in reality, can be predicted.

1 INTRODUCTION

The flow rate and the water pressure field of seepage through the foundation and abutments significantly affect the safety and economy of dams. This paper deals with the narrower topic of seepage through jointed rock under a concrete dam. Uplift pressures strongly influence safety against sliding and overturning, and, by implication, cost. For instance, reducing the uplift pressure by 50% can be translated into a 25% reduction of the width to height ratio of a gravity dam with triangular cross-section. Moreover, water pressures open fractures in the rock foundation and abutments on the upstream side of the dam. Countermeasures such as drain and grout curtains aimed at reducing uplift pressures and the flow rate add complexity to the problem. Figure 1 shows selected dam cross-sections with grout and drain curtains along with design-and measured water pressures, taken from Stuart (1963). The variety of rock conditions and designs and the disparities in water pressures are striking. Given this situation the designer-would like to have the possibility to do two things: 1. To explore the foundation and abutment conditions as accurately as possible, and 2. To analyze the effect of water flow and pressure in the jointed rock in dam foundations and abutments.

Exploration of rock mass conditions and in particular of joints has somewhat improved over the past few years but still leaves much to be desired. Fully recognizing this need for additional work, this paper will, nevertheless, concentrate on the analytical aspects. Analysis of flow effects in rock masses can be done in a number of ways, employing equivalent-continuum or explicit representations of the joints in modeling flow and deformation, and considering flow and deformation separately or coupling them.

2 RECENT WORK ON COUPLED FLOW/DEFORMATION ANALYSIS

The pioneering work on coupling of flow and deformation in rock mechanics was that reported by Brekke, et al. (1972). They coupled a fractured-rock deformation model (analyzed using finite elements) and a discrete-conduit flow model to study water pressures under a dam and inside a slope. The deformation model assumes that intact rock is linear-elastic and accounts for the behavior of the joints through special joint elements incorporating normal and shear stiffness parameters and a friction angle.

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