Generally, expansive soils are clays that exhibit a disproportionate affinity for water and, as a result, the pores within the soil fill with water causing mass expansion. This effect is sometimes reported by the testing laboratory as potential vertical rise (PVR). The PVR, which can reach 152 to 305 mm (6 to 12 in.) in some extremes, is a worst-case scenario based on the thickness of the expansive layer, its plasticity, and the availability of water. Keep in mind, however, under normal circumstances, the saturation of the top few inches of clay will swell and seal off the clay below. Therefore, the maximum PVR is seldom realized in practice in pool construction.