"Investigation and Evaluation of Seepage Conditions and Potential Failure Modes Around Outlet Conduits"

2012 United States Society on Dams Annual Conference, New Orleans, LA
April 24, 2012
By Keith Ferguson, PE, D.WRE

Evaluation of potential seepage failure modes in embankment dams and in particular around outlet works penetrations is a very important aspect of dam safety evaluations. Assessing potential seepage failure modes typically requires designing and evaluating investigation and instrumentation programs. Experience has shown that adverse seepage and piping conditions can develop and remain difficult to detect until the failure mode is in an advanced stage of the continuation phase of the failure mode development process (Appendix O, ER 1110-2-1156). In this paper both the theoretical considerations and practical observations of seepage conditions around conduits will be supported with case history information from two large outlet works conduits through embankment dams on relatively deep alluvial foundations: Lake Darling Dam, North Dakota, and Lake Isabella Auxiliary Dam, California. In both cases, large twin barrel cast-in-place reinforced concrete outlet conduits were constructed with partial cut and cover methods. The conduits were placed on relatively thick alluvial foundation soil deposits and operated for extended periods of time before detailed safety evaluation studies were initiated. These case histories reveal how the identification of small unfiltered defects in and around the conduits is critical to the assessment of the potential for the initiation of seepage and piping failure modes. Once initiation has occurred, the small and insidious nature of erosion pipes that develop during the early stages of the failure mode makes direct detection almost an impossible task. Even with extensive investigation and instrumentation monitoring programs, the assessment of the safety of these structures is a very difficult task and requires extensive experience and keen engineering insight and judgment.