# Failure Behavior Attributed to Internal Erosion Caused by Conduit Cracks in Homogeneous Embarment
###### Abstract  
This study determines the effects of leakage attributed to conduit defects on the failure behavior on the downstream slope of a homogeneous reservoir. The outcomes of model experiments and three-dimensional deformation-seepage analysis, which were conducted based on the failure shape, pore-water pressure, hydraulic gradient, and settlement, were compared with the changes in the water storage capacity of a failed reservoir owing to conduit defects. embankment; failure behavior; internal erosion; conduit defect; leakage; monitoring system; settlement; model experiments +
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because it occurs over a long period of time inside the embankment, which fails gradually [8; 9]. As the number of reservoirs that have recently reached their embankment life limits increases, the likelihood of internal erosion as a major cause of embankment failure increases. In Korea, the lifetime of a reservoir is estimated to be approximately 50-70 years. The relatively short lifetime of these reservoirs compared with those in other countries is attributed to the poor construction of the reservoirs, which were built under Japanese colonial rule, during the Korean civil war, and during industrialization in the 1940s and 1980s. In addition, reservoirs are frequently damaged on a yearly basis owing to heavy rains due to the fact of seasonal disturbances in Korea. These complex environments make it difficult to predict potential embankment failure. Embankment failure due to the fact of internal erosion is highly dependent on the physical properties of the soil materials that make up the embankment [5; 10]. In particular, the structural defects of the conduit or the material anisotropy with the filling zone at the boundary of the conduit can increase the probability of failure in terms of aggravation of internal erosion. Internal erosion has a time-dependent relationship with soil weakening. Therefore, a cautious approach is needed, because if a conduit cracks, the internal erosion can be accelerated further by an increase in water pressure. In this study, the following three situations were considered for the path of internal erosion:  
* Long-term internal erosion owing to material anisotropy occurring at the conduit boundary and fill zone (e.g., the Sandae Reservoir) [11];
* (Conduit cracks are known to cause structural defects (such as corrosion) in weak areas of the conduit owing to soil and water loads, thus resulting in internal erosion [12; 13; 14];
* The outer part of the conduit may be surrounded by a clay layer for the purpose of reducing the water pressure applied to the conduit. However, at the soil-structure interface, an increase in the degree of compaction and in the clay content can lead to a more severe failure due to the increase in the critical hydraulic gradient [11]. The commonality of these three assumptions indicates that internal erosion extends downstream of the embankment, and the downstream slope can become a hazardous area. Figure 1 illustrates an example of failure caused by conduit leakage at Korea's Sandae Reservoir in April 2013 [15]. The Sandae Reservoir is a homogeneous embankment composed of low-plasticity clay. The permeability coefficients in 2003 and 2013 were 2.50 \(\times\) 10\({}^{-8}\) and 3.57 \(\times\) 10\({}^{-7}\) m/s, respectively, which is equivalent to a 14.3 times increase. Failure due to the fact of internal erosion resulted in a circular concave shape dug out around the conduit on the downstream slope, which gradually expanded to the top of the embankment. The failure width of the embankment, which started on the slope around the conduit, was approximately 12.2 m. Furthermore, the time that elapsed from the moment at which the leaked water appeared on the downstream slope until failure occurred was approximately 3 h, which is extremely short.