Influence of fractures and air bubbles on the pressure distribution embankment dams
Sammanfattning: In some large embankment dams unexpected pore pressure distributions within the core have been observed. As an example, the piezometer pressures in WAC Bennett Dam, Canada, which rose for about four years after the reservoir was filled, were steady for two years and subsequently declined. One peak pressure head was as much as 60 m higher than the expected steady state pressure head of 40 m. However, the pressure head had dropped 55 m from the peak value 25 years later. Four hypotheses have already been proposed to explain the anomalous pore pressures within embankment dams. The objective of this study is to examine two of them, inhomogenities (e.g. fractures) in the core and trapped air bubbles, which can both be examined from a fluid mechanical point of view. The other two mechanisms, settlements and bleeding of fine material, must also be examined from a geotechnical aspect. This examination, based on results from two numerical models, is mainly theoretical. Results from numerical simulations of simplified homogeneous and inhomogeneous embankment dams are compared with analytical solutions and basic experiments. Results from numerical simulations, including the influence of air bubbles, are evaluated using a plug flow analysis and field measurements. A Hele-Shaw cell and a bed of packed glass beads, both a homogeneous and an inhomogeneous experimental set up, were used in the examination of how inhomogenities influence the pressure distribution. In the inhomogeneous case, a horizontal fracture extended from the upstream boundary to a point within the embankment. The fracture was shown to have a significant influence on the pressure distribution, discharge, seepage level, and free surface profile. The numerical model is based on a direct solution of the conservation equations (mass and momentum). In the numerical simulations, the flow resistance is determined from a laminar velocity profile in a slot with smooth walls (Hele-Shaw cell) and from the Forchheimer equation (bed of packed glass beads). The problem is considered to be two-dimensional. Since air bubbles are always initially present in the voids, that air is compressible, and that the amount of air that can go into solution increases with pressure, a mechanism that generates hydraulic blockage in the downstream part of the core can be anticipated. The blockage decreases the hydraulic conductivity in the flow direction resulting in a pressure increase. The numerical model for this case is based on a direct solution of the conservation equations (mass and momentum) and Darcy's, Boyle's, and Henry's law. It is a two-phase problem treated as one-dimensional. The main result of the study is the development of numerical models to simulate how inhomogenities and trapped air bubbles influence the pressure distribution. These models have a solid foundation, i.e. are based on conservation principles, physical laws, and the best available empirical relationships. The models have been validated through comparisons with analytical solutions, basic experiments, and field measurements and thus provide a good starting point in the development of tools that can be used in dam engineering.
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