Seepage Analysis

“Evaluate the effects of seepage for all dams. The evaluation must address all potential embankment and foundation seepage related failure modes, including the potential for internal erosion, erosive flow along defects, internal stability, and uplift pressures to damage the embankment, its foundation, and appurtenant structures. The evaluation should be commensurate with the complexity, function, and hazard potential classification of the structure. Seepage control and management must be adequate to accomplish the intended reservoir function, provide a safety operating structure, and prevent damage to downstream property."^[1]

“Seepage analyses should begin with the simplest and most conservative method to analyze the embankment and foundation. Perform progressively more detailed and complex analysis to the extent required, establishing that the design features will safety reduce, intercept, filter, and discharge seepage."^[1]

Purposes of Seepage Analysis

The purposes of completing a seepage analysis for a dam include:

Ensuring that seepage does not exit on the face of the dam. This may indicate a potential for internal erosion from an unfiltered outlet.
Estimating the phreatic surface for various loading conditions to support slope stability analysis.
Estimating seepage quantity to support drain design.

Types of Seepage Analysis

“One or more of the following methods of analysis, listed in order of increasing complexity or other industry standard practices may be required to determine if the structure will safely accommodate seepage:

"Qualitative Methods – For some simple structures with favorable conditions, the seepage performance of the structure can be satisfactorily predicted with limited quantitative analysis. The seepage evaluation for these structures should address the same factors as for more complex analyses. Designers can develop seepage control features for these structures using standard practices based on experience and history of satisfactory performance with similar conditions. The analysis should clearly convey the rational used in the analysis and the justification for the seepage control features incorporated into the design.
"Analytical Methods – Designers may use closed-form solutions or approximate solutions for simpler seepage conditions associated with dams.
"Graphical Methods – Designers may draw flow net analyses by hand or may use adaptations from numerical solutions. Designers may use flow nets to estimate seepage quantities, evaluate the exit gradient for embankments and foundations, and to compute uplift pressures acting on appurtenant concrete structures or soil blankets.
"Numerical Methods – Use computer based numerical methods to analyze seepage in cases where graphical solutions are not practical, there is complex stratigraphy or there is a need to account for saturated and unsaturated or transient flow. Designers can use these methods to estimate seepage quantities and pore water pressures an evaluate the exit gradient for embankments and foundations, as well as uplift pressures acting on appurtenant concrete structures or soil blankets.”^[1]

Guidance for Performing Seepage Analysis

The key inputs to any seepage analysis are:

Other general guidance when completing a seepage analysis includes:

"Existing embankments may not have achieved steady-state conditions. Thus, use the calculated long-term high-level steady-state seepage condition for both new and existing embankments. For existing embankments, compare the computed phreatic surface with field performance and field measurements, when available, to assure that the computed phreatic surface meets or exceeds that which has occurred over the life of the structure.”^[1]
“Include a separate foundation phreatic surface as appropriate at sites with limited foundation seepage cutoff, particularly in sites with confined seepage that results in uplift at the downstream toe.”^[1]
“If steady-state seepage stability is highly dependent on the success of internal drainage features to control the phreatic surface, check stability assuming partially functioning or plugged drainage systems.”^[1]
When possible, monitoring data from piezometers can be used to calibrate seepage models.