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Western Dam Engineering <br /> Technical Note <br /> <br /> <br /> May 2017 <br /> <br /> <br />2 <br />Thinking Fast - Emergency <br />Response to Seepage and <br />Internal Erosion <br />Introduction <br />One of the most challenging threats to the safety of an <br />earthen dam is concentrated, uncontrolled seepage <br />that has initiated internal erosion. The progression of <br />such an event may be avoided by inspection, <br />monitoring, maintenance, and, if warranted, <br />rehabilitation. But what if those measures have not <br />been implemented, or in spite of them there is a <br />failure to recognize a progressing problem, and it <br />culminates in an emergency situation? This article <br />focuses on answering that question. <br />The previous Western Dam Tech Note article titled Is <br />Your Embankment Dam under Pressure – <br />Underseepage Impacts describes the mechanics, <br />monitoring, and investigation of seepage through the <br />soil foundations of earth dams in some detail, and <br />provides a good background to the following <br />discussion. <br />Why we care about concentrated seepage <br />As shown in Table 1 more than half of all dam failures <br />worldwide in the modern dam era have been due to <br />seepage-induced piping (i.e., internal erosion). Failures <br />and incidents continue to happen today, incurring <br />significant consequences to the owner and the public. <br />From January 1, 2005 through June 2013, state dam <br />safety programs reported 173 dam failures and 587 <br />"incidents" - episodes that, without intervention, <br />would likely have resulted in dam failure [1]. Examples <br />of incidents involving concentrated seepage that could <br />have led to dam failure without timely, aggressive, and <br />ultimately successful intervention include: <br /> AV Watkins Dam, UT [2,3,4,5] <br /> Lake Needwood Dam, MD [4,6,7] <br /> Washakie Dam, WY [4,8,9] <br /> Powell Dam, MT [4] – see inset this page <br /> Salt Fork Dam, OH [4] – see inset next page <br />Brief case histories of concentrated seepage events at <br />AV Watkins, Lake Needwood and Washakie Dams are <br />presented at the end of this article. <br />Table 1. Seepage-Induced Internal Erosion Dam Failures <br />Worldwide [2]. <br /> <br />Powell Dam Foundation Seepage Blowout <br />Construction of a <br />seepage berm <br />likely <br />concentrated the <br />seepage exit from <br />a gravel layer to <br />the edge of the <br />berm, causing a <br />blowout (see <br />photo to left). Emergency response included the <br />placement of on-hand woven-geotextile fabric covered <br />with stockpiled drain rock (see photo below). A <br />permanent toe drain was later constructed to intercept <br />and drain the <br />gravel layer <br />believed to be <br />the source of the <br />high pressures. <br /> <br /> <br /> <br /> <br />Mode of Failure <br />% Total Failures <br />(where mode of <br />failure was known) <br />% Failures <br />pre 1950 <br />% Failures <br />post 1950 <br />Overtopping 34.2%36.2%32.2% <br />Spillway/gate (appurtenant works)12.8%17.2%8.5% <br />Piping through embankment 32.5%29.3%35.5% <br />Piping from embankment into <br />foundation 1.7%0.0%3.4% <br />Piping through foundation 15.4%15.5%15.3% <br />Downstream slide 3.4%6.9%0.0% <br />Upstream slide 0.9%0.0%1.7% <br />Earthquake 1.7%0.0%3.4% <br />Totals (1)102.6%105.1%100.0% <br />Total overtopping and apurtenant <br />works 47.0%53.4%40.7% <br />Total piping 49.6%44.8%54.2% <br />Total slides 4.3%6.9%1.7% <br />Total no. embankment dam failures <br />(exc. During construction(124 61 63 <br />Total embankment dam years <br />operation (up to 1986)300,400 71,000 229,400 <br />Annual probability of failure 4.1 x 10-4 8.6 x 10-4 2.7 x 10-4 <br />(1) Subtotals and totals do not necessarily sum to 100% as some failures were <br />classified as multiple modes of failure