A theory of the internal-response-based rainfall criterion for predicting slope failure is proposed based on artificial rainfall tests for a sandy slope. Key elements in a generic model describing shallow slope failure are identified from the tests with extensive soil moisture and piezometric measurements. The observed retrogressive slope failure was triggered by a toe failure, followed by an abrupt increase in debris discharge. The onset of the failure was preceded by a clear transition point on the soil moisture vs. time curve at the soil-bedrock interface. Both external and internal responses of slopes to rainfall can be expressed using widely adopted I-D (rainfall intensity-duration) plots. Experimental results from the tests indicate close relationships between the internal-based and external-based I-D envelopes for critical slope states. The theory of the internal-based I-D criterion proposed here will be useful for more accurate slope failure predictions.
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