ETL 1110-2-569
1 May 05
h. USACE levee guidance is under review. Currently EM guidance sets the recommended
acceptable range of a berm's width between 300 and 400 feet [Appendix C-3b (4)]. If design
requirements determine that a berm's width should exceed 300 to 400 feet, current design
guidance recommends that the berm's width be reduced arbitrarily to remain within the range of
300 to 400 feet. This situation results in a hydraulic gradient exceeding 0.8 at the berm toe.
Limiting the width of large berms, to the range of 300 to 400 feet, is based on the assumption
that a sand boil 300 to 400 feet from the levee will not threaten the levee. Consequently, as we
have designed larger berms and then reduced the design to these arbitrary widths, we now have
berms in our levee systems with factors of safety values less than 1.0 at the toe of the berms.
Depending on how much the berm width was reduced, this factor of safety can be much less than
1.0. This means sand boils will form at the toe of the berm during floods. We now know that
when sand boils move material, with repeated floods the cumulative effect of this movement of
material will threaten the levee. Studies are currently being performed to determine an
appropriate factor of safety at the berm toe. In the interim, it is recommended that whenever a
berm's width is reduced, the factor of safety at the toe should be recalculated. Great caution and
good engineering judgment should be used for recommended seepage remediation in these
reaches.
7. Computer Programs to Use for Seepage Analysis [Section 5.4d]. Blanket theory should be
adequate for most all-general levee underseepage analysis, where substrata are known with
reasonable detail and a transformed blanket layer can be developed with reliability. Blanket
theory works fine for a simple model. If the model becomes more complicated and the potential
construction costs justify, the more complicated computer analysis may be the appropriate tool.
A 2-D analysis can be helpful where the substrata system is more complex. A more complicated
3-D type analysis is almost never justified. It is more important to obtain more subsurface data to
better depict the subsurface conditions. When the subsurface conditions are understood, the best
modeling method can be selected.
a. Listed below are a few of the computer programs that can be used to perform these
analyses. (These programs are listed in EM 1110-2-1913; however, additional usage information
has been added.)
(1) If the soil profile can be idealized with a top blanket of uniform thickness overlying a
foundation layer of uniform thickness and seepage flow is assumed to be horizontal in the
foundation and vertical in the blanket, then simple blanket theory equations can be used.
Computer programs like LEVSEEP (Brizendine, Taylor, and Gabr 1995) or LEVEEMSU (Wolff
1989; Gabr, Taylor, Brizendine, and Wolff 1995) could also be used. LEVSEEP would be
simpler to use.
(2) If the soil profile is characterized by a top blanket and two foundation layers of uniform
thickness, and seepage flow is assumed to be horizontal in the foundation, horizontal and vertical
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