"Moshampa" is one of Iran's large dams under construction. The rock slopes facing the dam diversion tunnels failed during excavation. Site investigations and primary kinematic analyses showed that the mode of instability has been blocky toppling failure. The slope inclination was decreased and the portal was reconstructed with a new design. Since, according to the same investigations, the geological conditions of the rock mass in the portals of the power-house and diversion tunnels are similar, the slopes facing the dam power-house tunnels may also fail during excavation; so, they should be checked against blocky toppling failure. In this paper, a brief review of toppling failures of Moshampa dam has been given first. Then, the stability analyses of the portals of the diversion tunnels and their stabilization method have been checked by Goodman-Bray analytical approach under dry conditions. Finally, the stability of the slope, facing the portals of the power-house tunnels, has been checked against blocky toppling by the method and its stable inclination computed.

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