The most common cause of foundation movement is the seasonal shrinking and swelling of expansive soils. The degree of soil movement is determined by the amount of expansive material in that soil and the variation of its moisture content. When the moisture content of expansive soil increases, the soil swells. During the drier months when the moisture decreases, the soil shrinks and achieves its lowest moisture content, allowing foundations to settle. During the wetter months, foundations return to their desired elevation. At times, the soil will swell enough to raise foundations higher than their desired elevation, which is referred to as upheaval. Poor site drainage is a common cause of foundation movement. Surface and subsurface moisture should be diverted away form the home or building in order to preserve the strength and desired elevation of the foundation system. Good site drainage is very important to prolong the life of a structure.