In a buried pipe ananlysis, you're trying to simulate a beam on a continuous elastic foundation. The only way to do that with a "3D beam element" program such as CAESAR II is to re-mesh the model such that the resulting element lengths and associated restraint stiffnesses approximate the behavior of the soil (which you define). In such a model, there will be no weight stresses - because the beam in reality is continuously supported. This is why all densities are reset to zero when the model is buried.

A consequence of this is that you can't perform a dynamic analysis (because all the weights are zero).