May 18, 2011

Levee Underseepage Evaluations
 Current Corps criteria presented in EM 1110-2-1913,
Design and Construction of Levees and ETL 1110-2-
569, 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
0 to 0.5
0.2 to 0.6
0.4 to 0.7
0.5 to 0.8
Seepage Condition
Light/no seepage
Medium seepage
Heavy seepage
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

May 18, 2011

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

• 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

• 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

• 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