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Western Dam Engineering Technical Note August 2016 5 operation, there are numerous existing dams that were constructed using this technique that have developed long-term performance issues associated with seepage, settlement, and arching. This placement method is no longer used due to the lack of compaction and difficulty of quality control. The hydraulic fill method often produced stratified, uncompacted, cohesionless fill that would not adequately relieve pore pressure. This can lead to structure instability in undrained loading conditions (earthquake, end-of-construction, flood) due to the reduced material strength. In addition, the large degree of settlement of hydraulic fills as they slowly drain eventually develops arching in the core and prevents its full consolidation. Miscellaneous Other advances in materials, equipment, and methods continued to improve the quality of dam construction throughout the twentieth century. These include introducing air entraining agents around 1930 to improve concrete's resistance to freeze/thaw damage and corrosion resistant coatings and materials for metal appurtenances. Changes in Flood and Earthquake Load Predictions In addition to static, normal loads, most dams should be designed for unusual and even extreme loading conditions, including those that exceed the historical record of occurrence at the site. Predicting loads induced by these rare events is an exercise centered equally on statistics and engineering. As time passes, the size of the available database increases, which may alter the statistical outcome and at the same time advance the engineering understanding of these phenomena. As a result, predicting rare flood and earthquake loads is an ever-evolving process. Flood Loading Dam flood loading is influenced by the hydrologic characteristics of the watershed as well as local and regional meteorological characteristics and processes. Significant advancements in the understanding of these data and processes have resulted in larger, more refined data sets. With these data collection advancements, more precise modeling has been undertaken. These changes in modeling capability support the finding that older dams are not adequately designed for recently predicted floods. Figure 4. Overtopping of Earth Dam during a Rare Flood Event Earthquake Loading Scientists’ and engineers’ knowledge of earthquake processes and their characteristics, such as ground motions, has increased in parallel with the increase in records of earthquake occurrence. Earthquake databases have significantly improved over the past 50 years, and now represent a more complete distribution of earthquake processes and their characteristics. The increase in the number of seismic recording instruments available to record earthquakes has grown as well. The increase in the number of reliable recording instruments along with the number of events recorded over the last 50 years have provided an enhanced database in which to study earthquake characteristics, improve statistical methods to develop ground motion relationships, and quantify the uncertainty associated with the predictions. Revised Dam Design Guidelines and Practice Most older dams were built with the best construction and engineering standards available at that time, but much has been learned since then. The greatest advance in dam safety practice for earth dams is the implementation of engineered filters. Engineered filters, for all practical purposes, were not implemented in dam design until about the 1980s. Most dams constructed before this era likely do not meet modern filter design criteria to protect against internal erosion.