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A newly built roller-compacted concrete gravity dam is located on the Yalongjiang River in southwest China. Complex geological conditions greatly puzzled the designers of the project. There are abundant joints within the riverbed that form the double- inclined wedge foundation. This is a typical mode of instability of concrete gravity dams. The equal-K method, which falls into the category of conventional limit-equilibrium methods, is provided in the design codes of gravity dams to evaluate the stability of a dam against sliding along deep failure surfaces. The factor of safety calculated by the equal-K method is determined by the angle of the force acting on the interface of the two inclined wedges, denoted by φ, which is unspecified in the design codes. On the basis of Sarma's assumption, this study proposes an algorithm for the determination of ∅, which needs only slight improvement of the algorithm in the existing codes. The justification of the proposed procedure is demonstrated by showing that the results from the proposed procedure are very close to those from a rigorous limit-equilibrium method in which all equilibrium conditions are satisfied. [ABSTRACT FROM AUTHOR]