The Bidvaz dam is an earth-rock fill dam with a thin and inclined clay core constructed in the northeast of Iran in a narrow valley. About seven and a half years after the start of the first impounding, a subsidence appeared on the upstream surface of the dam. Evaluation of the dam behavior based on the data recorded by instruments installed inside the dam body and its foundation confirms the occurrence of internal erosion in the lower parts of the core near the left abutment. Hydraulic fracturing phenomenon has been hypothesized as a main cause of initiating internal erosion in this dam. Thus, the main purpose of this research was to evaluate the possibility of this hypothesis. To achieve this aim, first the behavior of the dam during construction and impounding was simulated with a three-dimensional numerical model. Then, the safety factor against the hydraulic fracturing in the upstream face of the clay core was calculated using reliable proposed theoretical and empirical equations. The results showed that being located on a steeply sloped narrow valley and different compressibility properties of materials constituting the dam body have caused significant stress transfer from core near the abutments, especially in their lower regions. This has caused the safety factor of these regions against the hydraulic fracturing to be less than unity. Therefore, the hydraulic fracturing can be a main cause of initiating internal erosion in the core of this dam

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