ETL 1110-2-569
1 May 05
Table 1. Exit Gradient vs. Seepage Condition Trends
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
The above data would appear to indicate that the gradient required to cause sand boils varies
considerably, and sand boils might even occur at an exit gradient of 0.5. However, the 1956
report explains these data as follows:
"...the gradient required to cause sand boils varies considerably at the different
sites, and relatively low gradients were recorded near some sand boil areas. This
may be due to the fact that at sites where sand boils developed previous to the
1950 high water, only fairly low excess head may have been required to
reactivate boils in 1950 and, as a relief of pressure occurs at the boil, readings of
Piezometers near the boil may be somewhat lower than those farther from the
boil."
This explanation has been overlooked in most reports. It is mathematically impossible to have a
first time sand boil in the top stratum with a saturated unit weight above 110 lb per cubic foot
and with an exit gradient less than 0.8.
b. History of Levee Seepage Design Guidance. Detailed design guidance for underseepage
design is presented in EM 1110-2-1913, Design and Construction of Levees. This guidance is
based on methods and equations presented in TM 3-424. These equations assume that the
foundation materials consisted of two soil layers, each of uniform thickness with horizontal
boundaries, a semi-pervious top stratum (blanket of silt or clay) with thickness z and a pervious
substratum (sands and gravels) with thickness d. Also, they assume that flow in the top blanket is
vertical and flow in the pervious substratum is horizontal. These assumptions are reasonable if
the horizontal permeability of the pervious substratum is at least ten times the vertical
permeability of the top blanket.
TM 3-424 presented methods and equations to be used in design of seepage control measures. It
recommended that seepage berms be designed by the following method:
"Seepage berms should have a width and a thickness such that i through the top
stratum and berm at the landside toe of the levee will not exceed 0.5, and at the
berm toe will not exceed 0.75 to 0.80. However, seepage berms need not have a
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