The safety of a dam can be evaluated by risk analysis techniques, the latter is based on the estimation of the probability of failure from the effects of solicitation and resistance acting on a dam-reservoir. This work focuses on the estimation of the probability of failure at sliding for the case of a gravity dam, according to the different load events and the operating state of the drainage system by the application of two methods of reliability calculation, one based on the Taylor approximation (level II) and by the numerical Monte-Carlo simulations (level III).