1 May 05
computer models, seepage control measures should be implemented for any levee area that has
previously sustained significant seepage distress.
(3) Seepage control measures should be maintainable to assure that they will perform as
designed. Districts, sponsors, etc., should be committed to regular inspections and sponsors must
adhere to prescribed operation and maintenance of the underseepage control measures,
particularly for relief wells. All responsible groups must control activities and development
within the critical zone of the levee that would impact the effectiveness of the underseepage
(4) For all systems, where observation of levee performance is hindered, and especially for
high intensity use areas (i.e., cornfields next to levees, basements, pools, etc.), the engineer must
consider the implications of flood fighting. A plan to accomplish this flood fighting should be
developed. In these areas, more geotechnical data must be obtained and the flood-fighting plan
must be known in advance. Designing to a lower i in these areas will not completely insure
against failure of the system.
(5) The saturated unit weights of the "in situ" landside blanket soils must be at or above
110 lb per cubic foot.
e. When current design computations require construction of a minimum berm, the
minimum berm width should be changed to 4 X the height of the levee unless engineering
judgment dictates a more conservative berm. If a more conservative berm is used, the width
should not exceed 150 feet. This would be a change from the current USACE recommended
minimum berm width of 150 feet [Appendix C-3b (2) and (3)] regardless of levee height.
The thickness of a berm should only be increased to cover the amount needed for
shrinkage and consolidation of the foundation. This overbuild should be calculated or estimated
based on past performance. (Note: The current requirement to overbuild 25 percent
[Appendix C-3b (1) and (5)] to allow for shrinkage, foundation settlement, and variation in
design factors does not increase the critical factor of safety at that location. The critical factor of
safety is always located at the toe of a design berm. Any variations in design assumptions will
also lower this factor of safety, thereby making any added material to the berm unnecessary.)
g. As stated in TM 3-424, "In order for a semipervious berm to function as intended, it must
have a permeability equal to or greater than that of the underlying top stratum and must not be
appreciably thicker than the computed thickness. On the basis of values of kbl obtained at the
piezometer sites (Table 38), it appears that a berm must be constructed of silty sand or fine sand
to be classified as semipervious." Therefore, care must be taken to insure proper material is used
in the construction of a berm if it is designed using the assumption that the berm is
"semipervious." Berms to be constructed as "semipervious" must be constructed with silty sands
or fine sands and be designed using the semipervious suite of equations [Section 5.4b (2)].