3. Assessment of the Dynamic Properties of Soil

Soil is a granular medium that varies in composition from organic peat to gravel and that may contain various amounts of water. The soil physical system is continually being subjected to external forces and is, therefore, dynamic. These external forces may be environmental (climate, plants, animals, and microorganisms) or mechanical (forces applied by man using some type of machine). The specific reaction of the soil to these forces is of interest. The forces provide the means for changing soil from one condition into another and the reaction indicates the kind and degree of change. If one is to be able either to maintain a soil condition or to change it to a more suitable condition, he must first have an understanding of soil behavior; this behavior must eventually be properly described. Soil conditions and properties, widely varying types of forces, and widely varying types of behavior must all be included in any description before the description can be satisfactory. The obvious complexity of such a description requires that broad but realistic classifications of reactions and soil conditions must be made. The conce pt of dynamic properties presented in chapter 2 is an attempt to make such a classification. The first broad classification to consider in simplifying a description of behavior is the reaction of the soil to an applied force system. If the forces cause the soil to yield, the reaction of the soil may be active; if the soil can support the forces, the reaction may be passive. To illustrate, consider the following situations. When a soil is subjected to a dominant force system such as the force applied by a plow, the reaction of the soil is active and movement of the soil is determined largely by the geometry and method of operation of the plow. On the other hand, if the soil is subjected to a transmitted force system such as a low-velocity gas stream, the soil resists the force and the gas flow through the soil is controlled primarily by the geometry and size of the soil pores. Here the reaction of the soil is passive. Thus, depending on the role of the soil, either its dynamic or its static properties become operative to an applied force system. Static and dynamic properties of a soil mass that are operative in soil behavior are not necessarily the same as those of the soil material. The concept of …