ETL 1110-2-569
1 May 05
width exceeding 300 to 400 feet depending on soil conditions and height of
levee."
The Corps used these design criteria until 1962. In 1962 the now Mississippi Valley Division
office undertook a Staff Study to review design criteria for landside seepage berms for the
Mississippi River levees. USACE Mississippi Valley Division (1962) reported that the reason for
this study was as follows:
"During the May 1962 Mississippi River Commission (MRC) high water
inspection trip, local interests stated that seepage berms constructed in the past
few years are very thin and that larger berms may be required to adequately
protect the levee. The MRC agreed to reconsider the adequacy of its berm design
criteria." Discussions within the Staff Study stated: "Recently constructed
seepage berms are relatively thin and wide and have been designed in a rational
method." It then noted that if the intent was to increase the thickness of the berm,
the increase in thickness could be accomplished by requiring a lower i to be used
at the toe of the levee. (Note: In 1962 USACE design was in accordance with
TM 3-424 that prescribed designing to an i of 0.5.)
Hess and Sills (2004) reported that the 1962 Staff Study recommended that the design
requirements be changed from what was presented in TM 3-424 to what is currently listed in
EM 1110-2-1913 [Appendix C-3b1]. These current requirements apply to all projects, existing
and new construction. If the computed upward gradient through the blanket at the landside toe of
the levee is greater than 0.8, a seepage berm should be designed with an allowable upward
gradient of 0.3 through the blanket and berm at the landside toe of the levee (approximately
equivalent to a factor of safety of 2.8 at the 0.3 gradient). A lower factor of safety could be used
if there were sufficient soil data and past performance information to justify it. The berm width
would be designed to lead to an allowable upward gradient of 0.8 at the berm toe, with a
maximum width of 300 to 400 feet.
If the computed upward gradient through the blanket at the landside toe was between 0.5 and 0.8,
then a minimum berm with a width of 150 feet and a thickness ranging from 5 feet at the levee
toe and 2 feet at the berm crown should be constructed [Sections 3 and Appendix C-3b2]. If the
computed gradient was less than 0.5, but there was a history or an expectation of severe seepage,
then the minimum seepage berm should be constructed (EM 1110-2-1913). It is implied that if
the computed upward gradient through the blanket at the landside toe of the levee was less than
0.5, and there was no history of seepage distress, then no remediation would be required.
6. Recommended Design Guidance. Deviation from the following design guidance is
acceptable when based and documented on sound engineering judgment and experience. This
guidance was developed primarily from the work performed by the USACE Sacramento levee
seepage Task Force. Most of this guidance was included in a report to USACE Sacramento
5
Previous Page
