Pore Water Pressure Distribution for Use in Stability Analyses of Earth Dams

Abstract

In the assessment of a dam safety with respect to global instability, nominal pore water pressure distributions are often adopted for each stage of the dam life, referring to a hypothetical “expected performance” of the different dam components. The seepage phenomena taking place within the dam may, however, modify substantially the pore water pressure distributions up to make them inconsistent with the nominal ones. With reference to this subject, the present paper represents and interprets the singular pore water pressure distributions measured within an Italian earth dam, pertaining to the typology of zoned earth dam with internal clay core. Pore water pressure interpretation is supported by the representation of the evolution of the measured seepage flows. The work highlights how pore water pressure may differ strongly from the nominal distribution during the first stages of the dam life, when, if nearly undrained conditions take place, pore water pressures are discontinuous as a result of their dependency on total stresses. The work also shows how pore water pressure may assume distributions differing from the expected ones also after some decades of the operational stages, this time as a result of suffusion phenomena induced by the seepage processes within the dam embankment. Measured pore water pressure distribution, suitably interpreted as piezometric heads contours, clearly show how part of the downstream shell contributes to the embankment watertightness.