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The dynamic properties of soil are properties made manifest through movement of the soil. If a block of soil resting on a flat surface is moved, the resultant friction is a dynamic property of the soil; this property cannot be determined until movement occurs. Similarly, as loose soil is compacted, its strength increases; hence, strength is a dynamic property of soil. When soil moves, forces act that cause deformation or actual physical displacement. To describe the relations between the applied forces and resulting deformation, certain basic mathematical equations containing parameters are required. The parameters are measures of the dynamic properties of the soil. The structure or texture of soil is not a dynamic property. The structure may be changed as a result of movement, but it may be measured both before and after the movement. Such is not the case for dynamic properties. Studying dynamic reactions is difficult because the physical measurements must be made during the action. In addition, the insertion of measuring equipment into the soil mass may affect the soil reaction. The equipment may behave differently than the soil, if, for example, it is harder or softer than the soil. In spite of these difficulties, considerable progress has been made both in identifying dynamic properties and in utilizing these properties to describe the reaction of soil to forces (16).