# Stability Improvement Method for Embankment Dam with Respect to Conduit Cracks
## 1 Introduction  
Old embankments can have high variability in their filling material because of the low level of compaction and are generally known to exhibit unique heterogeneity [1]. These characteristics result in uncertainty regarding the stability of an aging embankment due to structural defects, which can increase the potential risk in an emergency. One way to describe failure mechanisms of embankment dams that are associated with the uncontrolled flow of water is through internal erosion. Internal erosion can be a threat to the future stability of the embankment due to cracks in the aging conduit. It is highly dependent on the compaction of the embankment zone, the ratio of clay particles, the water content, and the erosion rate, and it is known to be caused by various factors, such as structural defects or construction failures inside the embankment [2; 3; 4; 5]. In particular, the boundaries between a conduit structure constructed of concrete and the embankment zone, the impervious cores, and their shoulders can make the conduit more prone to internal erosion owing to the difference in material properties and differential settlement [6; 7; 8; 9; 10]. A condition in which such internal erosion persists causes piping. Here, piping refers to aphenomenon in which soil and water in the embankment zone are discharged to the slope as erosion gradually develops inside the embankment [11]. A failure due to a conduit crack occurred at the Sandae Reservoir in Korea in April 2013 [12]. The failure, due to internal erosion caused by a conduit crack, resulted in a concave circular shape being 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. The time that elapsed between the leaked water appearing on the downstream slope and the failure occurring was approximately 3 h, which is extremely short. An embankment failure due to conduit cracks is a rare case, but since conduits continue to deteriorate, these failures suggest that measurement strategies to deal with conduit cracks and failure problems will be important in the future. The conduits of small reservoirs are mostly circular in shape and have a small diameter. This type of structure is a poor environment for reinforcing the inside of a conduit because access by personnel and equipment is restricted. Therefore, reinforcement is carried out outside the conduit. Currently, the reinforcement of conduits is based on the results of investigating the leakage using photographic equipment, and a full or partial repair and reinforcement are conducted according to the level of damage [13]. There are various methods for reinforcing a conduit, such as grouting, waterproof mortar, and high-density polyethylene (HDPE) slip lining of the pipe [14], and drilling is required in many cases. However, reinforcement by drilling requires a careful approach, as it can affect the reinforcement effectiveness by introducing new problems due to hydrofracturing, internal erosion, and the contamination of filters and drainage materials in the embankment zone [15,16]. Therefore, in this study, an improvement model was developed as a way to minimize the uncertainty of reinforcement due to drilling and maintain the reinforcement effect in the long term. The improvement model aims to delay internal erosion by applying a drainage layer (geotextile and sand layers) that can distribute the water pressure on the upper and downstream slopes of the conduit without drilling. In order to evaluate the effectiveness of the improvement model, the current model was constructed based on a conduit with a circular shape and a small diameter. For the current model and the improvement model, the erosion characteristics due to cracks occurring in the conduit were evaluated by a large-scale indoor model test and three-dimensional deformation-seepage analysis.