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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 />4 <br />Table 1. Age of Dam at Incident [Adapted from ] <br />Dam Age at Incident No. of Internal Erosion Incidents <br /> ≤ 5 years 36 6-15 years 17 16–25 years 10 <br /> 26-35 years 7 <br /> 36-45 years 8 <br /> 46-55 years 5 56-65 years 4 66-75 years 4 76-85 years 5 > 85 years 2 <br /> Total 98 <br /> <br />Several dam incident and dam failure case histories <br />have been associated with old, deteriorating conduits. <br />Metal conduits, such as corrugated metal pipe (CMP), <br />have a limited life span and are prone to long-term <br />deterioration. Concrete conduits may have a longer <br />life, but also are prone to aging effects such as <br />freeze/thaw deterioration, scour, cracking, <br />reinforcement corrosion, settlement, and deterioration <br />of water stops. Figure 3 shows an example of dam <br />failure after 40 years of operation. <br /> <br /> <br />Figure 3. (a) Corrosion of a 40-year-old outlet pipe and (b) <br />its effects. <br />Outdated Construction Techniques and <br />Materials <br />Nearly 7,000 U.S. dams are more than 100 years old. <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. Technical advances <br />in construction practices specific to dams have resulted <br />in modern structures that are more robust and <br />resilient than their older relatives, and able to better <br />withstand a wide range of loading conditions. Dam <br />failures throughout history were the driving force <br />behind most of the technical advances, but thousands <br />of existing dams—those that haven’t failed yet—were <br />constructed using outdated and inferior construction <br />techniques. This may increase the potential for <br />deterioration and long-term performance issues. This <br />section presents a brief list of some of the key <br />advances in construction practice within the twentieth <br />century. <br />Soil Compaction <br />Although sheepsfoot rollers were first invented around <br />1900, they weren’t widely used to compact earthfill <br />dams until the 1920s.  The sheepsfoot roller’s <br />narrow spikes induce a kneading compaction, which is <br />critical for densifying clayey soils. Self-powered <br />scrapers and compactors were not widespread until <br />the 1930s. Advancements in mechanical compaction <br />improved the in-place density of earthfills, which has in <br />turn improved the soil’s engineering parameters such <br />as permeability, strength, and resistance to surface or <br />internal erosion and limits settlement. <br />Hydraulic Fill Placement <br />The hydraulic fill method is a placement technique that <br />was often used in dam construction prior to about <br />1970. Hydraulic fill materials are transported <br />suspended in water to the embankment where they <br />are placed by sedimentation. Velocity control is used <br />to control the selected deposition of the material. The <br />coarser particles of the slurry settle out along the outer <br />embankment shells, while the finer particles flow <br />toward the center to become the dam core. However, <br />this practice was largely discontinued after some <br />notable failures of hydraulic fill dams (e.g., Fort Peck <br />Dam, Calaveras Dam, and Lower San Fernando Dam). <br />Although those dams failed in the early stages of <br />(a) <br />(b)
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