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8/31/2016 1:25:16 PM
WESTERN DAM ENGINEERING NEWSLETTER, VOLUME 4, ISSUE 2 AUGUST 2016
RISKS OF AGING DAMS, HYDROLOGIC INADEQUACIES, INTERNAL EROSION
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Western Dam Engineering <br /> Technical Note <br /> <br /> August 2016 <br /> <br />5 <br />operation, there are numerous existing dams that <br />were constructed using this technique that have <br />developed long-term performance issues associated <br />with seepage, settlement, and arching. This placement <br />method is no longer used due to the lack of <br />compaction and difficulty of quality control. The <br />hydraulic fill method often produced stratified, <br />uncompacted, cohesionless fill that would not <br />adequately relieve pore pressure. This can lead to <br />structure instability in undrained loading conditions <br />(earthquake, end-of-construction, flood) due to the <br />reduced material strength. In addition, the large <br />degree of settlement of hydraulic fills as they slowly <br />drain eventually develops arching in the core and <br />prevents its full consolidation. <br />Miscellaneous <br />Other advances in materials, equipment, and methods <br />continued to improve the quality of dam construction <br />throughout the twentieth century. These include <br />introducing air entraining agents around 1930 to <br />improve concrete's resistance to freeze/thaw damage <br />and corrosion resistant coatings and materials for <br />metal appurtenances. <br />Changes in Flood and Earthquake Load Predictions <br />In addition to static, normal loads, most dams should <br />be designed for unusual and even extreme loading <br />conditions, including those that exceed the historical <br />record of occurrence at the site. Predicting loads <br />induced by these rare events is an exercise centered <br />equally on statistics and engineering. As time passes, <br />the size of the available database increases, which may <br />alter the statistical outcome and at the same time <br />advance the engineering understanding of these <br />phenomena. As a result, predicting rare flood and <br />earthquake loads is an ever-evolving process. <br />Flood Loading <br />Dam flood loading is influenced by the hydrologic <br />characteristics of the watershed as well as local and <br />regional meteorological characteristics and processes. <br />Significant advancements in the understanding of <br />these data and processes have resulted in larger, more <br />refined data sets. <br />With these data collection advancements, more <br />precise modeling has been undertaken. These changes <br />in modeling capability support the finding that older <br />dams are not adequately designed for recently <br />predicted floods. <br /> <br />Figure 4. Overtopping of Earth Dam during a Rare Flood Event <br />Earthquake Loading <br />Scientists’ and engineers’ knowledge of earthquake <br />processes and their characteristics, such as ground <br />motions, has increased in parallel with the increase in <br />records of earthquake occurrence. Earthquake <br />databases have significantly improved over the past 50 <br />years, and now represent a more complete distribution <br />of earthquake processes and their characteristics. The <br />increase in the number of seismic recording <br />instruments available to record earthquakes has grown <br />as well. The increase in the number of reliable <br />recording instruments along with the number of <br />events recorded over the last 50 years have provided <br />an enhanced database in which to study earthquake <br />characteristics, improve statistical methods to develop <br />ground motion relationships, and quantify the <br />uncertainty associated with the predictions. <br />Revised Dam Design Guidelines and Practice <br />Most older dams were built with the best construction <br />and engineering standards available at that time, but <br />much has been learned since then. The greatest <br />advance in dam safety practice for earth dams is the <br />implementation of engineered filters. Engineered <br />filters, for all practical purposes, were not <br />implemented in dam design until about the 1980s. <br />Most dams constructed before this era likely do not <br />meet modern filter design criteria to protect against <br />internal erosion.
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