ETL 1110-2-569
1 May 05
(USACE 2003). The Task Force was chaired by George L. Sills, P.E.; and included Dr. Leslie F.
Harder, Jr., P.E.; Dr. Thomas F. Wolff, P.E.; Christopher B. Groves, P.E.; Dr. Mosaid
Al-Hussaini, P.E.; and Dr. J. Michael Duncan, P.E. John R. Hess, P.E. was the Task Force
Project Manager.
a. A high emphasis should be placed on gathering data from levee performance during
previous floods. This information should be used to plan exploration programs and past
performance data should be used to calibrate the results of seepage models. Past flood
performance should also be used as part of the engineering judgment in the final determination
of the adequacy of existing and future seepage control measures [Sections 2-1 and
Appendix C-2].
b. An extensive subsurface geotechnical investigation along the levee systems should be
conducted and supplemented with geophysical investigation techniques as appropriate. A
minimum target level of subsurface explorations should be a series of explorations
approximately every 1,000 feet, consisting of an exploration at the riverside toe, at the landside
toe, and a deep exploration at the levee crest. It is strongly recommended to use site appropriate
geophysical procedures to interpolate between borings, to guide additional borings in anomalous
areas, and as a basis to avoid unneeded borings [Sections 2-9 and 2-10].
c. Piezometers should be installed to monitor and learn from flood events along selected
portions of the levee system. This information should be used to adjust computer models of the
area where appropriate [Section 2-9].
d. The allowable factor of safety for use in evaluations and/or design of seepage control
measures should correspond to an exit gradient at the toe of the levee of i = 0.5. In general, this
would provide a factor of safety of about 1.6. This change will standardize all levee seepage
requirements to one exit gradient of 0.5. Landside drainage ditches (along the toe of the levee),
seepage berms, and relief wells should all be designed to the same exit gradient of 0.5. However,
to design to a 0.5 gradient, the engineer must also incorporate the following: [Appendix C-3b
(1)].
(1) There must be an adequate amount of subsurface exploration, as outlined above, which
would have at least three borings/soundings (one on the riverside, through the levee, and one
landside) every one thousand feet. This is intended to represent a minimum level of required
effort. In highly urbanized areas where the hazard level is high, or areas of complex geology,
there should be a greater number of explorations in order to minimize uncertainties and overall
risk.
(2) Past flood histories should be reviewed and seepage information used to correct
computer models so that they better predict true behavior. Notwithstanding the results of the
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