The ter 'active zone' generally refers to the zone of soil that either is contributing to or has the potential to produce heave. In order to predict heave at any particular time, it is necessary to define the zone of soil that has experienced terms can be used for various zones that have in the past been used o define the 'active zone'. This paper discusses the usage of the term 'active zone' and presents a definition that relates it to the movement of water in the soil profile. The nature of water movement in soil, the definition of important zones that influence heave, and the interaction of different foundation elements with the expansive soil are shown to be interrelated. The depth of the wetting front in the soil profile is an important factor in prediction of heave and pier movement. In order to compute the maximum heave that can occur, it is necesarry to assume that the depth of wetting will eventually extend throughout the depth of potential heave. Therefore, this depth represents the maximum depth of the active zone