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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 />3 <br />consequences (economic, environmental, or loss of <br />life) of the failure. Therefore, dam deterioration that <br />occurs over time, coupled with the increasing <br />development downstream of dams, results in <br />increased risk. This article discusses these time-related <br />phenomena, how they influence the risk dams pose, <br />and the path forward for owners and engineers to <br />maintain safe and reliable structures. <br />How a Dam’s Age Influences Risk <br />Deterioration <br />The structural integrity and operational effectiveness <br />of dams often deteriorate with age. Deterioration of <br />dam structures refers to time-related changes in the <br />properties of the materials of which the structure and <br />its foundation are composed. The International <br />Commission on Large Dams (ICOLD), Committee on <br />Dam Ageing, studied the various aging phenomena of <br />concrete and embankment dams and appurtenant <br />works. The committee identified features of <br />deteriorated structures and the processes of <br />deterioration. Methods by which the deterioration <br />may be controlled and perhaps prevented were also <br />identified. These findings were later presented in a <br />white paper published by the United States Society on <br />Dams (USSD) : THE AGING OF EMBANKMENT DAMS. The <br />highlights from this study are summarized at a high <br />level here. The reader is encouraged to read the <br />referenced paper for more details. <br /> <br />Foundation: The causes of deterioration of the dam <br />foundation were identified as: <br />• Deformation leading to cracking <br />• Internal erosion (see Figure 2) <br />• Loss of strength or increase in the permeability <br />due to slaking, dispersion, solutioning, and <br />thermal and chemical processes <br />Embankment: The causes of deterioration in the dam <br />body were identified as: <br />• Deformation and settlement leading to <br />embankment cracking or loss of freeboard <br />• Loss of strength due to improperly compacted <br />fill or cycles of wetting and drying. <br />• Long-term elevation of pore pressure due to <br />cracking and seepage <br />• Internal erosion (see Figure 2) <br />• Surface erosion <br />Miscellaneous: Other causes of deterioration of dams <br />were identified as: <br />• Deterioration of conduits due to freeze/thaw, <br />corrosion, or long-term settlement induced <br />cracking (see Figure 3) <br />• Clogging of internal drainage systems <br />• Seepage through concrete faced rockfill dams <br />• Loss of bond between concrete structures and <br />embankment <br />• Deterioration and/or clogging of geosynthetic <br />material <br />• Deterioration of asphalt facing <br />• Deterioration of soil-cement or RCC armoring <br />• Vegetation and animal activity <br />All of these can lead to dam safety incidents and <br />failures after years or even decades of successful <br />performance, as presented in Figure 2 and Table 1. <br />The Bureau of Reclamation studied the ages of dams at <br />the time of internal erosion incidents within their <br />inventory of dams. It found about one-third of the <br />incidents occurred within the first five years of <br />operation. However, incidents continue to occur <br />beyond 20 years, with no dramatic decline in rate of <br />incidents after 20 years. <br /> <br />Figure 2. Internal Erosion Incident Timeframe [Adapted <br />from ] <br /> <br />Year Constructed
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