In a broad sense, what one attempts to do in rock engineering is to anticipate the motion of a proposed structure under a set of given conditions. The main design objective is to calculate displacements, and as a practical matter, to see whether the displacements are acceptable. Very often restrictions on displacements are implied rather than stated outright. This situation is almost always the case in elastic design where the displacements of the structure of interest are restricted only to the extent that they remain within the range of elastic behavior.