Does Caesar predict soil failure (exceedance of soil bearing resistance) at buried bends or are additional calculations required? From what I can tell, it calculates the yield load (fy) from the input soil properties and if it is exceeded the stiffness goes from the high K1 value to the very low K2 value (i.e. soil fails). Does Caesar use the Rankin Passive Pressure equation to calculate this yield load? If not, how is fy calculated

We use a maximim bearing load calculation from Peng's 1987 article. I may be wrong on the year but we quote the reference several places in our buried pipe documentation.

Nope, not passive pressure.

The bi-linear soil load-displacment behavior that characterize the "soil springs" for buried pipeline restraint are approximations of the the actual soil response that is non-linear , much like the shape of a steel material stress-strain curve. At relativly large buried pipeline displacements beyond the ultimate "yield" displacement of the soil, the soil response remains somewhat constant or continues to gradually increase until a loss of soil cover occurs at extremely large displacements. However, at this juncture, there will be something seriously structurally wrong with the pipeline, and the problem has gone way past the small strain thoery, well into plastic starin which CAESAR II cannot account for.

The bottom line is do not worry yourself too much with potential loss of soil restraint at large buried pipeline displacements.