Investigation into the Internal Erosion and Local Settlement of Esfarayen Earth-Fill Dam

A considerable sudden settlement was discovered on the Esfarayen Earth-fill Dam surface near to the left abutment when the reservoir level rose to 51 m from the riverbed in April 2013. As a result, a detection process was prepared to comprehend the problem and relevant challenges required to consider devising an appropriate remedial work. For this purpose, the work was divided into three main parts: (1) analyzing the information gathered from the monitoring and surveillance plan, (2) performing a geotechnical site investigation to test the subsurface condition and the extent of the problem, and (3) finding the cause of the problem initiation by considering three aspects—the bedrock geological condition, material susceptibility, and three-dimensional numerical modeling. The outcomes from the different parts of the study categorized the problem as an internal erosion incident. The existence of an inadequate sealed crushed zone of a fault passing through the reservoir on the contact surface with the clay core was the major cause of the internal erosion initiation. The water, appearing through this narrow crushed zone, applied additional hydrostatic pressure on the clay core, which depended on the reservoir level and had no influence on the stress distribution inside the dam. With the water pressure exceeding the minor principal total stress, a crack formed and extended with time, turning to a large void. Because of the high quality of the clay core material and its well-designed downstream filter, the internal erosion progression was concentrated on the core and directed to the upstream. When it reached the upstream filter, the filter entered into the core, and the material from the overlying part of the dam repeatedly dropped, resulting in a final settlement on the dam surface. The numerical modeling was successful in the determination of the approximate time of the internal erosion initiation by considering the past loading history of the dam and the influence of the existing defect.