Seite - 1198 - in Book of Full Papers - Symposium Hydro Engineering

Combined Seepage and Slope Stability Analysis of Earth Dams Bakenaz. A. Zeidan Faculty of Engineering, Tanta University, Egypt Email: b.zeidan@f-eng.tanta.edu.eg ABSTRACT Earth dams’ failure may occur due to different reasons such as structural instability conditions, hydraulic conditions, seepage through the dam body and/or rapid drawdown. The determination of factor of safety for the dam slope stability, under different cases of operations, is vital to ascertain the dam overall safety. In this work, Finite Element modeling is employed for simulating seepage and stress analysis of earth dam problems via GeoStudio software. Thus, phreatic seepage surface, pore water pressure distribution and total hydraulic head variation of an earth dam are analyzed. The model is verified, then it is employed to analyze seepage and stability of Mandali Dam (Iraq). For three different cases of operation, four major analytical methods are used to verify the stability of the dam side slopes. Benchmark safety regulation criteria (USACE and BDS) are obeyed. The results confirm the safety of Mandali dam against combined seepage and slope instability under all cases of operation. The case of rapid drawdown is the most critical operating case; compared to other cases of operation. Keywords: Earth dams, Seepage, Stability of slopes, Finite element modeling, Mandali Dam. 1. Introduction Dams are built for specific functions such as water supply, irrigation, flood control and hydroelectric power generation. Most of the large dams in the world were built during the middle decades of the twentieth century. There are two types of modern dams, namely: embankment dam and concrete dam. Embankment dams can be classified into two main categories earth-fill dams and rock-fill dams. Embankment dams represent about 85% of dams all over the world. There are several factors to be considered in selecting an earth dam type such as: topography; foundation conditions; environmental impacts, construction facilities and socio-economic studies. A feasible dam should be; built from locally available materials; stable under all operating and loading conditions; watertight enough to control seepage; have appropriate outlet works to crest dam overtopping [1-3]. 2. Literature Review Seepage and slope stability failures are addressed via many authors since Henry Darcy, 1856 [4], who gave the basic law of flow through porous media. Darcy’s law was based on series of experiments conducted in a vertical pipe filled with sand. Fellenius (1936) [5], developed the Ordinary Method of Slices (OMS) known as Felonious or Swedish method. It is assumed that the forces acting on sides of any slice are neglected. Zienkiewiez and Chung, 1967 [6], published the first finite element simulation to solve the Laplace equation for steady ground water seepage., Taylor and Chow, 1976 [7], recorded that the Finite Element Method was used to assess the potential seepage flows and uplift pressure in the foundation rock for Bannett Dam in Canada. Kratutich, 2004 [8], used thermal mode at ANSYS computer code to simulate numerically the case of no stationary free surface in earth dams. Karjani, 2009 [9] used Geostudio computer software to analyze Maroon dam, estimate flow net at passing condition and slope stability factor at overall stability for different operating conditions has been calculated. Zomorodian and Abodollahzadeh, 2010 [10], used Geostudio software to investigate the effect of horizontal drains on upstream slope of earth fill dams during rapid drawdown. Tatewar and Laxman N. Pawade, 2012 [11], used Geostudio software to investigate the slope stability of the 21m high Bhimdi earth dam, by changing different parameters such as changing berm width and changing position of filter drains. Hasani et al., 2013 [12], studied the seepage analysis in Ilam earth dam for four mesh size in order to assess the effect of meshing on results accuracy. 1198