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Western Dam Engineering
<br /> Technical Note
<br />
<br /> May 2016
<br />
<br />27
<br />It is prudent to perform drilling or excavation when the reservoir is at a low pool and to have fill material and equipment nearby in case emergency action is necessary to keep a blowout from progressing
<br />Don’t Rush to Dig In
<br />It may seem obvious at this
<br />point; however, one more
<br />cautionary statement will be
<br />made on the implications of
<br />exacerbating a foundation
<br />seepage condition by careless
<br />activities at the downstream
<br />toe. Drilling or excavating test
<br />trenches or toe drains can
<br />often do more harm than
<br />good.
<br />Installing a toe drain may
<br />seem like a beneficial measure to mitigate the impact
<br />of seepage. However, installing a toe drain when
<br />pressurized conditions are present may be more
<br />detrimental than beneficial as it may not reach the
<br />water-bearing layer and instead may simply weaken
<br />the confining layer. Other options exist to mitigate the
<br />potential for internal erosion through the foundation,
<br />such as a blanket drain and/or berm to filter the exit or
<br />lengthen the seepage path.
<br />Drilling investigations must also consider potential
<br />impacts to dam safety, particularly if the investigations
<br />are performed with a full reservoir and with a
<br />suspected pressurized foundation. Defects can also
<br />develop due to incorrectly constructed or abandoned
<br />monitoring wells. See the previous Western Dam
<br />Engineering Technical Note article: Poking the Bear:
<br />Drilling and Sampling Embankment Dams
<br />Conclusion
<br />Although erosion may initiate, there are several steps
<br />that need to happen to result in breach. The inability
<br />of the foundation layers or the embankment itself to
<br />form a roof, the lack of continuity of natural erodible
<br />deposits, and the likelihood of intervention by staff
<br />(drawing down the reservoir, placing emergency filters,
<br />blankets, etc.) may arrest the potential failure mode
<br />before it fully progresses. Field observations and
<br />engineering judgment need to be applied. As
<br />suggested for other aspects of seepage related issues,
<br />a geotechnical engineer, experienced in embankment
<br />dam design, is an important participant to assist in
<br />making dam safety decisions.
<br />Seeping foundations, particularly of existing dams, are
<br />mysteries that must be solved to make appropriate
<br />decisions on prudent measures. When the evaluations
<br />have provided an understanding of the seepage-
<br />related issues, decisions can be made regarding the
<br />safety of the existing dam and whether remedial
<br />measures are needed.
<br />There are many remedial measures available, and each
<br />seepage situation requires a specific corrective action
<br />to ensure a safe dam. This may range from increased
<br />monitoring to major rehabilitation. Determining
<br />whether a seepage condition is a steady state
<br />condition or one that is progressing is an important
<br />determination in deciding the relative urgency of
<br />action. Signs of a changing or progressed condition
<br />(increasing seepage, sediment pluming, enlarging sand
<br />boils, embankment cracking and deformation, etc.)
<br />should be promptly reported to dam safety regulators.
<br />Remedial measures, emergency responses, and
<br />construction issues will be the subject of a future
<br />Western Dam Engineering Technical Note.
<br />References
<br />[1] Cedergren, H.R. Seepage, Drainage, and Flow Nets. Third Edition. John
<br />Wiley & Sons, Inc., 1989.
<br />[2] Federal Emergency Management Agency (FEMA), Evaluation and
<br />Monitoring of Seepage and Internal Erosion (P-1032), Interagency
<br />Committee on Dam Safety. May 2015.
<br />[3] Geo-Slope International. Geo-Studio 2012, SEEP/W version
<br />8.15.5.1177. August 2015.
<br />[4] International Commission on Large Dams (ICOLD): “Internal Erosion of
<br />Existing Dams, Levees and Dikes, and Their Foundations.” Bulletin 164,
<br />Volume 1: Internal Erosion Processes and Engineering Assessment.
<br />2015.
<br />[5] McCook, D. “A Discussion of Uplift Computations for Embankments
<br />and Levees,” Journal of Dam Safety. Winter 2007.
<br />[6] Pabst, M., Robbins, B., Engemoen, B., Hanneman, D., Redlinger, C.,
<br />and Scott, G. “Heave, Uplift and Piping at the Toe of Embankment
<br />Dams – A New Perspective,” Journal of Dam Safety, Volume 11, Issue
<br />2, 2013.
<br />[7] Schmertmann, J.H. “The no-filter factor of safety against piping through sand.” Judgment and Innovation: The Heritage and Future of the Geotechnical Engineering Profession. Geotechnical Special
<br />Publication No. 111, American Society of Civil Engineers, Reston, VA.
<br />2000
<br />[8] Sellmeijer, J.B., Lopez de la Cruz, H., van Beek, V.M., and Knoeff, J.G. “Fine-tuning of the backward erosion piping model through small-
<br />scale, medium-scale and Ijkdijk experiments.” European Journal of
<br />Environmental and Civil Engineering, 15(8). 2011.
<br />[9] U.S. Department of the Interior, Bureau of Reclamation (Reclamation), Best Practices in Dam and Levee Safety Risk Analysis, Chapter IV-4,
<br />Internal Erosion Risks for Embankments and Foundation. 2015.
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