Internal Erosion and Rehabilitation of an Earth-Rock Dam

Abstract

This paper describes an earth-rock dam whose filters’ characteristics did not comply with the design criterion. In addition, the degree of compaction and the water content in the impervious core consisted of highly plastic residual clay. Although Terzaghi’s filter design criterion was used, the as-built grain size distribution did not comply with such criterion. Several problems arose right after the first filling of the dam, including the following: (1) water leak at a rate of 200 L/s was observed along the downstream slope, in the vicinity of the outlet pipeline; the dark color of the observed water implied that the core material was being eroded; (2) two sinkholes near the crest (20 m apart) were observed along the outlet axis, one on the upstream and the other on the downstream slopes; right after the occurrence of these two sinkholes, the water leakage decreases significantly, noticing a plugged effect of the material that falls down through the sinkholes. To avoid piping, a notch was installed on the spillway to quickly reduce the reservoir level. A site investigation around the affected zone was performed to find the causes of the observed seepage through the dam. The paper presents a detailed description of the site investigation. In addition, the paper presents the instrumentation, laboratory, and field tests employed as part of the site investigation. After analyzing the information produced in the site investigation, the following conclusions were derived: (1) large variation in the water content was used during the compaction of each lift of the impervious core which resulted in a highly heterogeneous core of the dam; (2) stress analysis of the zones around the outlet pipe demonstrated that the hydrostatic pressure in such zones exceeded the sum of the transverse normal and tensile stresses, inducing hydraulic fracturing; and (3) because of the grain size segregation during the placement of the filter material, the upstream and downstream filters did not satisfy the design criterion. Description of the stabilization of the damage zones through a grouting process, together with the construction of a diaphragm wall and an interface grouting, is presented. Finally, this paper reviews existing criteria for designing filters to protect earth and rockfill dams against internal erosion or piping and applies these criteria to the described dam. A special discussion on this topic is also presented.