Document Type : Article
Authors
1
Faculty of Engineering & Technology \r\nTarbiat Mo&zwnj
2
Faculty of Engineering University of Zanjan
Abstract
One of the most important factors for the design and site selection of concrete gravity dams is the rock mass foundation. Concrete dam foundations are often rock masses with several discontinuities. The existence of discontinuities in rock mass, and their characteristics, such as aperture, roughness, stiffness, dip angle, spacing, geometric situation relative to dam body, and etc, cause rock mass to have complicated behavior. Furthermore, the hydro-mechanical interaction of jointed rock mass in the foundation of a dam increases its complexity. Also, uplift pressure under the dam body is an important factor in the design of such dams, whose distribution under the dam depends on the water head in the up and downstream of the dam and the joint set characteristics in the foundation. Another problem that hinders the stability of gravity dams is tension crack in the upstream and often in the heel of the dam. Accordingly, ignoring the role of discontinuities in design of dams may cause unrealistic forces on the dam and foundations, or may have adverse effects on the stability and efficiency of the dam. In this research, the stability of a typical 100 m height gravity dam on a jointed rock mass foundation, with two and three regular joint sets, was investigated using the distinct element code, UDEC. In order to provide an initial state in the model for controlling design requirements, dam construction and the first impoundment of the model were considered until the occurrence of steady state seepage flow in the whole dam and foundation system. Owing to the fact that both flow and equilibrium equations were active in all processes of simulations, hydro-mechanical behavior, with the effects of joint set characteristics, was considered. This process was repeated by changing the dip angle of the joint sets and results were compared with each other. All design requirements were controlled in each model and compared with traditional methods of designing gravity dams, in which the dam foundation was considered continuous. This evaluation consists of the overturning of the dam, sliding on the foundation surface, tension crack development at the base of the dam and the global stability of the dam and rock mass foundation. The results show that the number, dip and characteristics of joint sets are very effective parameters in different kinds of stability of gravity dams. On the one hand, the uplift pressure changes dramatically, due to changes in the joint set dip angles, and on the other hand, the pattern of instability in the whole system was controlled through the formation of a specific mechanism in the discontinuous rock foundation. In comparison with traditional methods of gravity dam design, it is noted that in the design of gravity dams on an abutment with continuous media, more caution should be taken into account regarding the discontinuous media.
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