U.S. Fish & Wildlife Service Low Hazard Dams - Association of State ...

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U.S. Fish & Wildlife Service Low Hazard Dams - Association of State ...

STANDING OPERATING PROCEDURES6.2.2 Structural FailureStructural failures can occur in either the embankment or the appurtenances such asspillways or low-level outlets. Structural failure of a spillway, reservoir drain or otherappurtenance may lead to failure of the embankment. Cracking, settlement, instabilityand slides are the most common signs of structural failure of embankments. Large cracksin either an appurtenance or the embankment, major settlement, or sliding requireemergency measures to ensure safety, especially if these problems occur suddenly.Figure 6-2: Structural Failure of a Concrete Spillway from Uncontrolled Seepageand Uplift Pressures Developing Under the Structure6.2.3 Uncontrolled Seepage and PipingAll earth dams have seepage resulting from water percolating slowly through the damand its foundation. Seepage must, however, be controlled in both velocity and quantity.If uncontrolled, it can progressively erode soil from the embankment or its foundation,resulting in potential rapid failure of the dam. Erosion of the soil begins at thedownstream side of the embankment, either in the dam proper or the foundation,progressively works toward the reservoir, and eventually develops a “pipe” or directconduit to the reservoir. This phenomenon is known as “piping”. Piping action can berecognized by an increased seepage flow rate, the discharge of muddy or discoloredwater, sinkholes at or near the embankment, and a whirlpool in the reservoir. Once awhirlpool (eddy) is observed on the reservoir surface, complete failure of the dam willprobably follow. As with overtopping, fully developed piping is difficult to control andwill likely cause failure if not arrested.Seepage can cause slope failure by creating high pressures in the soil pores or bysaturating the slope. When soil saturation occurs, the slope loses stability and cannotsupport its own weight. A slope which becomes saturated and develops slides may beshowing signs of excess seepage pressure. Slope failures have occurred during prolongedperiods of high water or heavy rainfall, and can lead to serious problems. The pressure ofseepage within an embankment is difficult to determine without proper instrumentation.6-3Revision No. 0October 2008

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