Hydraulic fracturing (HF) in a dam is the phenomenon of crack propagation after water pressure enters and expands an existing crack. An HF numerical model was tested on an existing dam on a complex foliated rock foundations. The locations and areas of HF could be identified, leading to the development of the hydraulic fracturing index (HFI). The results revealed that the HF area mainly occurred in syncline concave areas on rock foundations. The expansion of HF significantly affected the seepage and the stability of the dam. The HF area on a studied dam mainly started to occur when the reservoir water level (RWL) reached 146.00–156.00 m mean sea level (MSL). These results agreed well with the piezometric monitoring data recorded as 148.00–149.00 m MSL. The findings supported the formulation of the HFI based on the influencing factors of the cross-valley geometry, RWL, dam height, and elastic modulus of the rock foundation. The probability of HF occurrence could be evaluated and categorized for safety evaluation into five conditions: ≤0.14 (very unlikely), 0.15–0.74 (unlikely), 0.75–1.86 (neutral), 1.87–3.10 (likely), and ≥3.10 (very likely). The HFI can be used to predict the likelihood of seepage problems due to HF in an existing earth dam.

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