## 1 Introduction  
Despite numerous technological advancements in the field of dams, the demands for further cost reductions and eco-friendly technology developments persist. A hardfill dam is a relatively new type of dam developed in response to such demands. A hardfill dam is constructed by evenly spreading and roller compacting a material that is continuously mixed with water and a small amount of cement (hardfill) in a simple, temporary facility. The construction process uses materials (such as bed gravel and soil) readily available near the dam construction site with minimal processing. The hardfill dams that broaden the choices of material sources for dam construction are advantageous from both economic and environmental perspectives because they can reduce the damage to nature caused by the development of quarries while recycling submerged resources [1; 2; 3; 4]. Hardfill dams are often compared with roller-compacted concrete (RCC) dams constructed by the roller compaction of concrete, such as rockfill dams. RCC dams allow for faster construction relative to block-type concrete gravity dams; for example, the latter have drawbacks concerning the various processes required for controlling the hydration heat caused by pouring block-shaped mass concrete, resulting in an increased construction time (decreased construction speed) [5; 6]. In other words, hardfill and RCC dams are similar in that the main material sources are aggregates, water, and cement, and the construction speeds are faster than those of concrete dams owing to the roller compaction [7; 8]. However, hardfill dams have further advantages relative to RCCs. In particular, an RCC dam follows a mixing design method for concrete during manufacturing. Thus, the range of material sources (and, correspondingly, the particle size distribution of the aggregates, cement content, and unit water content) is regulated according to the design goal. Accordingly, complex mechanical equipment is often required during manufacturing. In contrast, a hardfill dam uses the on-site aggregate at the planned dam construction site as-is without requiring any additional treatment other than the removal of particles exceeding the maximum particle size (80 mm in general). Therefore, the dams can be manufactured with simpler equipment after determining the cement content and unit water content according to the characteristics of the aggregate, thereby simplifying the construction. Furthermore, in terms of structural stability, hardfill dams with a trapezoidal cross-section have a larger dam body volume than RCC dams with a right-triangle cross-section but nevertheless provide high stability against overturning and sliding [3,9]. Despite the above advantages, however, since hardfill dams use almost the same riverbed materials as the construction materials, the range of variations in the mix design can be large according to the particle size distribution (PSD) of the aggregate; correspondingly, the range of changes in the strength of the hardfill dam body can also be large [9,10]. This makes it difficult to manage the quality of the material(s). Therefore, in terms of the stability of the dam body and quality control, the differences in behavior according to the characteristics of the materials should be thoroughly examined during the design phase. Many construction cases of hardfill dams have been reported from several countries since the first development of a hardfill dam in the early 1990s [11, 12, 13, 14, 15, 16]. Moreover, meanwhile, a number of studies have been made on the mechanical properties of hardfill material and the stability of hardfill dams through experimental or numerical investigations by many researchers [15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28].