The authors have proposed a modified intensity–duration (I-D) method that incorporates field measurements of tensiometer to improve the accuracy of predicting the risk of slope failure. This method uses an indicator that considers the relationship of the duration from the time point at which the saturated zone is assumed to have formed to the average rainfall intensity during that period. The usefulness of this method has been verified, but its applicability to different slopes has not yet been investigated. Here, the authors collected long-term observations on a natural slope in Minamiashigara City, Kanagawa Prefecture, and examined the Modified I-D method using data on slope failures in the surrounding area. The authors also compared the results with plots of previous rainfall index (soil water index–accumulated rainfall in 60 min, effective rainfall amount with a half-life of 72 h–effective rainfall amount with a half-life of 1.5 h, and accumulated rainfall–accumulated rainfall in 60 min). The snake curves for rainfall events during slope failure and non-failure were clearly separated. The accuracy was high, confirming the applicability of the modified I-D method.

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