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Kovács (1981) presented two equations for the critical hydraulic gradient required to cause particle movement on slopes of cohesionless soils. One assumes a sliding failure mechanism due to movements parallel to the slope; the other assumes a heave mechanism due to grain movements normal to the slope. The use of these equations has been proposed for the prediction of piping initiation on slopes. The objective of this investigation was to evaluate through laboratory experiments and numerical analysis the reliability of using Kovács’ equations to predict the gradients required for initiation of backwards erosion piping (BEP). Two sands at three slope angles were tested in a custom-made flume. Results showed that BEP initiation occurs over an incredibly small length. Kovács’ formulation based on sliding was found to be overly sensitive to slope angle and did not match experimental observations. Kovács’ heave-based formulation closely matched the experimental observations, indicating that initiation of BEP is a heave-type failure. While the heave-based equation aligned with the experiments, the length over which the gradient must be examined is too small to be of practical use in field applications and should not be relied upon as a predictive tool for BEP initiation. DISCLAIMER: The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. DESTROY THIS REPORT WHEN NO LONGER NEEDED. DO NOT RETURN IT TO THE ORIGINATOR. ERDC/GSL TR-18-16 iii

Experimental Evaluation of Kovács’ Equations for Estimating Critical Gradients