This is kind of another vote for Terzaghi; he devised an equation for pull out resistance (which the NJs certainly had a high factor of safety against).
so here's what I don't get. My wife and I discussed the use of a 5-year contraceptive. It is supposed to be more efficient the snip snip or even tying the tubes. In fact, it was explained to me that there is a 1/1000 failure chance for both the tubes and snip, where as a 1/2000 failure chance for the contraceptive.