E-prime, or the horizontal soil modulus employed in the "Iowa formula" for deflection of flexible pipes, has been reported by various researchers to involve interaction with, or be in some cases a rather complex function of, the specific soil type and density, pipe/soil depth, and also the ring stiffness of the specific pipe/material involved. Ring stiffness of differing pipe materials in an of itself can be a rather complex subject, as some materials can deflect more over time than others (long-term moduli for example of plastics can be markedly less than that determined from standardized rapid loading rate laboratory tests, due to creep etc.) Also, if the native soil for example in which a pipelaying trench is dug has markedly different support or void characteristics than the pipe backfill zone, this along with the trench width may also have bearing on effective E-prime as well. While I guess it is possible some "geotech" folks understand all of these interactions and might give you a simple answer to your general question, perhaps any simplifications of such complex matters without detailed information need to be carefully considered/conservative.
