The designed steepness and proposed treatment of the final slopes of the mined land shall take into consideration the physical properties of the slope material, its probable maximum water content, landscaping requirements, and other factors. For example, the maximum stable slope angle may range from 90 degrees in a sound limestone, igneous rock, or similar hard rock to less than 20 degrees in highly expansive clay. The mining and reclamation plan shall specify slope angles flatter than the critical gradient for the type of material involved. Whenever final slopes approach the critical gradient for the type of material involved, the Department may require an engineering analysis of the slope stability. The Department may require additional measures if public safety or adjacent property may be affected.