stanier:
Use of an E/c ratio of 50 to 100 should get you in the ballpark; but that still leaves a
big concern:
You are
"fine tuning a coarse tuning knob."
Before I continue, let me first say that the effort to develop such a procedure is generally commendable, and I am a firm believer in the use of soil/structure interaction analyses as a part of the design process. Having said that, however...
![[soapbox] [soapbox] [soapbox]](/data/assets/smilies/soapbox.gif)
I'm cynical about
any new design method that uses soil modulus as a design input. The behavior of backfill depends on so many different factors - many of which cannot be known in advance and/or cannot be controlled in the field - that I have to wonder about the wisdom of developing a standard that relies on the Young's modulus, E
Y, or shear modulus, G, of "engineered" backfill as an important design factor. Hell, many (most?) geotechnical engineers don't have a good handle on the range in elastic modulus of the predominant soil / rock in their area! (Which is probably why your geotechnical consultant gave you the answer that brought you to this forum...)
If the procedure that you are using is sensitive to a doubling - or halving - the modulus, then it's a waste of time. Pipelines are linear projects that often extend for tens - even hundreds - of miles; most of the experience with elastic behavior of soil is limited to a localized condition (i.e. one building) that is no more than 300 feet square. The experience extrapolates rather poorly to your type of problem, and gives the impression that one can determine "the" E
Y for soil along a given alignment. You also have two rather large variables: the modulus of the soil around the trench as well as the trench backfill itself. Obtaining a reasonable in situ measure (
not estimate) of
both of these soil moduli will require equipment and procedures that are relatively rare in the geotechnical engineering business: pressuremeter and/or dilatometer. You can achieve similar results with cyclically loaded consolidated-drained triaxial compression tests - which are also expensive, but a little easier to find. And all this for a product that (in much of the country) seldom has a thorough geotechnical study done. Unless there is a failure and lawyers are involved.
[Focht3 now descends from his soapbox]
I hope that your method really works - but be damn careful!
A question:
Your previous message indicated that you have been serving at least eight years on a committee developing this procedure; what are the names of the civil engineers (particularly geotechnical engineers) on the committee? [In my opinion, at least one-third of the committee should have included geotechnical engineers. After all, you can specify pipe parameters within fairly narrow tolerances, but soils - well, they don't work that way!]