1. Levee Seepage: Concerns, Evaluations, and Solutions Pete Nix, P. E
Levee Seepage: Concerns, Evaluations, and Solutions Pete Nix, P.E. Senior Geotechnical Engineer Tetra Tech, Inc.
May 18, 2011

2. Levee Underseepage Evaluations
Current Corps criteria presented in EM , Design and Construction of Levees and ETL , 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

3. TM 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 ).
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

4. Typical Levee Section

5. Seepage Analyses

6. Potential Solutions
May 18, 2011

7. Seepage Cutoff very expensive (generally sheet pile or slurry walls)
generally not effective unless they’re installed into an underlying impervious layer

8. 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

9. 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

10. 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

11. Questions?