Phreatic line and pore pressure stresses in zoned rockfill dam

The position of the phreatic surface influences the stability of the earth dam because of potential piping due to excessive exit gradient and sloughing due to the softening and weakening of the soil mass as it touches the downstream slope or intersects it. The phreatic surface should be kept at or below the downstream toe. In this study, finite element software, ABAQUS, is used to determine the phreatic line within the Jebba dam (zoned rockfill dam) with a drainage system in steady-state condition. The dam model was meshed with a 6-node quadratic plane strain triangle elements (CPE6MP). A total of 401 elements and 872 nodes were generated. This study shows that the drain has restrained the phreatic line almost in upstream side of the dam and the downstream side of the dam is free of pore pressure. Following the finite element modelling, the deformations and stress distributions were determined. From the analysis result, it is observed that the central section of the embankment is free from tensile stresses; however, part of the upstream and the region around the berm of the dam body possess some amount of tensile stresses with no drainage system. These are zones of crack propagation. However, these tensile stresses are reduced and eliminated in some region with the introduction of the drainage system. This implies that positive pore water pressure (i.e. higher phreatic line) increases the tensile stresses in the embankment.