6.5) You are preparing the earthmoving specifications for a project that will include a wide range of materials, from loose soil to hard rock. Because of this variation, you plan to use a classified excavation system so the contractor can give two-unit prices, one for common excavation and another for rock. However, to avoid disagreements on classification, you also need to develop clear definitions for each type of excavation. What criteria might you use to differentiate between common and rock excavation?