Numerical Analysis of Seepage and Exit Gradient through a Non-Homogeneous Earth Dam without Impervious Core by using Geo-Slope Software

Abstract

In this study, Geo-Slope software was used to check the behavior of a non-homogenous earth dam (Hub dam) for two different cases i.e. (i) with core and (ii) without core respectively. The simulated results showed that a dam at its original shape and design with core (case - i) is not endangered from an internal erosion and piping, as the central core plays a key role in lowering the phreatic surface within the dam. For each scenario the equipotential lines and stream lines are found normal to each other.  The overall minimum seepage flux of order 2.0291 x 10^-4 ft³/sec/ft and exit gradient at downstream toe 0.091 was recorded respectively. However, when a dam model was run without core (case - ii), an abnormal behavior of the dam was observed as the phreatic surface cuts the downstream slope of the dam for all the scenarios. A very high exit gradient and seepage flux was recorded at different pond levels. The seepage flux was found (12.768 – 41.378%) more, due to the absence of core the hydraulic conductivity of the shell material is not fine enough to resist the seeping water within the shell due to which the internal pore water pressure increased for all scenarios. Likewise, the absence of core increases the exit gradient for about (85.896 – 91.809%) due to which a high exit gradient was recorded at the toe of the downstream slope. In both cases non-linear behavior was observed due to high pore-water pressure in the shell region without core, the exit gradient at the downstream toe abruptly changed during different scenarios. This implies that a core plays a vital role in earth dams to control the phreatic surface by lowering down the positive pore water pressure within the upstream and downstream of the shell, lowering the seepage flux and exit gradient respectively.