I am not sure exactly what/or for what purpose you are looking for this information, nor exactly what (or in what context?) you mean by a "conservative" coefficient; nevertheless, I am aware Mssrs. Jeff Boschert of NCPI and Mark Bruce of Can-Clay at least delivered a paper at last year's ASCE Pipeline event in Atlanta where jacking loads to move strings of clay pipes in the ground were discussed (I don't think these gentlemen would be very hard to find in a web search).
