Page - 1202 - in Book of Full Papers - Symposium Hydro Engineering

7.2 Seepage Analysis of Mandali Dam The Finite Element Mesh of the model can be seen in Figure (7). While figures (8) and (9) present the water head variation and pore water pressure through the dam body, respectively. Figure (7) shows that the phreatic line has been lowered down effectively by the central clay core. Figure (8) confirms that the pore water pressure in the internal surface of downstream are far away from downstream which ensures stability of the downstream against seepage failure. Figures (7) and (8) show that the variations of pore water pressure as well as energy lines follow (BDS) safety regulations. Moreover, the flow lines are diverge from the downstream toe of the dam and the drainage filter minimize excess developed pore water pressure, which ensures the stability of the downstream slope. 8. Analysis of Mandali Dam and Results To investigate the stability of dam slopes, the dam was simulated, using Geostudio software, three different cases of operation are considered, as follows: Case (1): Just after construction; where reservoir is empty and significant pore pressure development is expected either in the embankment or foundation during construction of the embankment. Case (2): Steady-state seepage; where reservoir is full and the long-term phreatic surface within the embankment has been established. Case (3): Rapid drawdown; in the reservoir is drawn down faster than the pore pressures can dissipate within the embankment after the establishment of steady-state seepage conditions. The considered analytical methods in the present study are Ordinary, Bishop’s Simplified, and Janbu’s Simplified and Morgenstern-Price methods. The obtained factors of safety (F.S.) are compared to the Limits of USACE (2003) [18] and BDS (1994) [19]. Figures (10 - 14) present F.S. for the dam slopes under different cases of operation using the four analytical tools. Table (4) summarizes the determined upstream and downstream factor of safety (F.S.) for all cases of operation and used analytical tools. Table (4): Results of Mandali dam stability analysis by Geostudio with Limit of USACE (2003) and BDS (1994). Rapid drawdown Steady state End of construction Critical Stability Condition Downstream Upstream Downstream Upstream Downstream Upstream USACE (2003) 1.2 1.5 1.3 (1.3-1.2) (1.5-1.3) (1.5-1.3) BDS (1994) 2.008 1.958 2.008 2.343 2.394 1.985 Ordinary 2.154 2.061 2.154 2.488 2.427 2.082 Bishop's 1.962 1.895 1.962 2.205 2.357 1.922 Janbu's 2.187 2.103 2.187 2.543 2.444 2.12 Morgenstern-Price Stable Stable Stable Stable Stable Stable Remark For steady state condition, the maximum water level was 182.5 m [AMSL], where the dam crest level is 184 m [AMSL] (1.50 m free board). While for rapid drawdown case, the water level was lowered from the estimated maximum flood level of 182.5 m [AMSL] to the minimum operating level of 170 m [AMSL]. As can be seen in the presented figures and table, the factor of safety (F.S.) values ranges from 1.895 to 2.543 which satisfies the minimum limits of factor of safety (F.S.) in USACE (2003) and BDS (1994). The factor of safety (F.S.) values reflects stable slopes of the dam for all operating cases. The least values of factor of safety (F.S.) are given by Janbu method. This is mainly due to the moment equilibrium equations that are not satisfied by Janbu’s method. While the largest factor of safety (F.S.) values are given by Morgenstern-price method, as Morgenstern-price method relates the shear and normal force [13]. 9. Conclusions Finite element modeling was employed in this study to analyze the combined seepage and slope stability of Mandali earth dam (Iraq). Three different cases of operation were considered, and four different analytical tools were used to analyze the slope stability. The dam safety check was based on the minimum required F.S stated in USACE and BDS. The results showed that the geometric design of the dam is acceptable according to BDS criteria and the seepage through the dam is within the code recommendations. The factor of safety (F.S.) values of upstream and downstream slopes stability satisfy the minimum limits for all cases of operation. Rapid drawdown 1202