Relief Drain Hole Cleaning and Dam Safety

Though often neglected, Drain Hole Cleaning is a necessary maintenance item for the prevention of hydrostatic uplift and pore pressure build-up within gravity block dams. Based on published information on dam safety, uplift in the form of hydrostatic forces, acting within a dam and its foundation, is a potential source of dam failure. In most gravity block dams, pore pressure dissipation is accomplished by a curtain of cored, vertical holes that extend into the bedrock along the central core of the dam. However, where dams are constructed within carbonate bedrock, the carbonates tend to precipitate sediment that can fill the drain hole, or crystallize, completely bridging the drain hole within generally high flow zones. If not addressed, the resulting reduction in the effectiveness of the drain holes can permit an increase in the hydrostatic pressures beneath the dam, thereby increasing the possibility of uplift of the concrete structure. In many cases, the high costs associated with the cleaning process, and the lack of a quantitative method for evaluating the effectiveness of the cleaning processes, has caused many dam owners to put off the process year after year, until the effectiveness of the drains may be impaired. Since the year 2000, several methods have been quantitatively evaluated in an effort to provide a viable drain hole cleaning system. The effectiveness of the cleaning processes has been determined through a program of pre- and post-testing that has been customized for each dam and its unique drainage system. Through the evaluation process, improved techniques for drain hole cleaning have been developed. The pre- and post-cleaning evaluation process can be further utilized as a baseline to conduct annual or bi-annual reevaluations. Those drain holes showing a marked reduction in flow during annual pre-cleaning testing can then be scheduled for cleaning. Instead of 100% of the drain holes requiring cleaning, only 5% to 10% of the drain holes may require attention in any given year, resulting in substantial cost savings. 10 pp., various photos and figures, 3 references.