I'm sure I could get an answer, but not sure I could consistently get the same one. And I am assuming there is not a published standard to refer to for this so engineer and reviewers judgement generally covers this. I was told by my geotech consultant that typical spacing would be 1,000 feet, but FEMA would probably require 500 feet spacing. We have decided to go with 500 foot spacing. We are not submitting anything to FEMA, but would still like to design the levee improvements so that in the future it could be certified.
I'm not convinced that USACE is reviewing new levee certifications at all (except for those structures originally designed and constructed by the USACE or other federal agency). FEMA does provide the criteria and does the review. However, they do reference USACE engineering manuals for design and analysis. See the following which also has the federal regulations for levee cert:
However, the only criteria for spacing I have found is in EM1110-2-1913, Design and Construction of levees which indicates a spacing between 200 and 1,000 feet. It seems to leave it up to the designer. In my case, debate with the geotech was between 1,000 or 500 feet which results in twice the borings and essentially twice the cost for field work. The reasoning was that "FEMA required it".
There is no set spacing. It depends on the conditions, size of levee, what is being protected, etc. I have done levees with 1,000 ft. spacings and I've had borings at 200 ft. spacing.
Levees are definately a case where one size does not fit all. Boring sites should be picked both at random, i.e. at some spacing and based on site condition. Aeiral photos are very helpful for looking for unusual site conditions that need to be explored. Also, borings should be placed not only on the center line but also riverward and landward of the center line.
I would question any geotech who just quotes a standard spacing. Also, geophysical mehtods can be used in some cases along with CPT borings.
Assume that if your client ever needs to certify their levee, that any extra soil borings required by the agency in question can be taken at that time. You and your geotech should agree to perform however many soil borings are needed to properly design/analyze your improvement project. Note that certain areas may warrant borings in a tighter spacing than others (corners, critical section, abutments, areas of soil transitions, etc.).
I also suggest you and your geotech should also review the post-Katrina recommedations for levee design, which are not current in the EM1110-2-1913 publication.
jdonville
that is exactly what we discussed - doing the borings at approx 1,000 feet now and doing the analysis and design to meet FEMA criteria. Then if we (the owner) decides to certify the levee later, and if FEMA asks for more supporting soil data, get additional borings at that time. However, my geotech has settled on approx 500 feet spacing now.
You might also want to do some background reading on Katrina and the New Orleans levees. ASCE published their report earlier this year, and there are two chapters covering to the siting, design and construction of the levees.
