Computation of Seepage and Exit Gradient through a Non-Homogeneous Earth Dam without Cut-off Walls by using Geo-Slope (SEEP/W) Software

Abstract

In this study, SEEP/W was used to develop a finite element model of a non-homogenous earth dam and for this purpose Hub dam was selected. Two different cases i.e. (i) with cut-off wall and (ii) without cut-off wall were studied to check the behaviour of the dam in terms of seepage flux and exit gradient respectively. The software was also used to simulate the phreatic line behaviour for both cases. The outcome of the simulated results showed that the dam is safe against piping, at its original design as the installation of a cut-off wall found working effectively in reducing internal pore water pressure within the dam and its foundation. For case (i), the phreatic line showed a normal trend as it is falling into the filter drain after passing the core with overall minimum seepage flux of order 2.1130 x 10^-4 ft³/sec/ft (21.54 LPH) and exit gradient at downstream toe was recorded 0.099 respectively. However, for case (ii) the dam showed an irregular behaviour as the internal pore water pressure at the subsurface region of the dam foundation was continuously increasing due to unavailability of the cut-off wall and the flow vectors move towards toe drain with high velocity and seepage flux. The overall maximum seepage flux and exit gradient was recorded for the maximum pond level without cut-off wall of order 4.6355 x 10^-3 ft³/sec/ft (472.54 LPH) and 0.865 respectively. The comparison results showed that without cut-off walls the seepage flux may increase about (87.314% – 87.493%) and the variation in exit gradient may increase about (48.705% - 63.353%) respectively.