Levee Seepage

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Levee Seepage:
Concerns, Evaluations, and
Solutions
Pete Nix, P.E.
Senior Geotechnical Engineer
Tetra Tech, Inc.
May 18, 2011
Levee Underseepage Evaluations
 Current Corps criteria presented in EM 1110-2-1913,
Design and Construction of Levees and ETL 1110-2569, Design Guidance for Levee Underseepage
 Based on exit gradient, i = Δh/L, where Δh is the change
in head over a given distance L at the seepage exit point
 High exit gradients near the landside levee toe can
cause sand boils to form which can lead to piping
(internal erosion) in the foundation soils; piping often
results in the failure of the levee
TM 3-424 - Investigation of Underseepage and Its Control, Lower
Mississippi River Levees (1950 Flood)
Exit Gradient, i
Seepage Condition
0 to 0.5
Light/no seepage
0.2 to 0.6
Medium seepage
0.4 to 0.7
Heavy seepage
0.5 to 0.8
Sand boils
Mathematically impossible to have a first time sand boil in the top stratum with
a saturated unit weight above 110 pcf and with an exit gradient less than 0.8
(ETL 1110-2-569).
• exit gradient of 0.8 or higher required to form new sand boils
• exit gradient of 0.5 to 0.8 required to reactivate old sand boils
• for seepage evaluations of existing levees, we consider that exit
gradients must be less than or equal to 0.5
Typical Levee Section
Seepage Analyses
Potential Solutions
May 18, 2011
Seepage Cutoff
• very expensive (generally sheet pile or slurry walls)
• generally not effective unless they’re installed into an underlying impervious layer
Seepage Berm
• may still have high exit gradients but they’re further way from the toe of the levee
• relatively inexpensive
• requires extensive space on the landside of the levee
Toe Drain
• relatively inexpensive; cost could vary depending on need to provide pumping
• good where thin, surface clay layers can be easily penetrated
• requires sufficient space on the landside of the levee for a large excavation
• may need to pump water from wells to upstream side to prevent interior ponding
• if space is available, locate toe drain away from levee and use excavated material for berm
Relief Wells
• relatively expensive and must be maintained
• good where clay layers cannot be easily penetrated by a toe drain excavation
• good in areas where space is limited at landside toe
• may need to pump water from wells to upstream side to prevent interior ponding
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