I am looking for a reference on splitting principle for etale cohomology of simplicial schemes (over arbitrary field k). I found a paper by Schechtman, "On the delooping of chern character and Adams operations", where chern classes are introduced in Grothendieck style, for a given cohomology theory, and the projective bundle formula is stated as a required axiom for the cohomology theory. It will be appreciated if someone can direct me to a reference where the same is explained, possibly with more details.