Artificially prepared, previously frozen specimens of a clayey silt have been subjected to thermal gradients under both closed and open systems. Direct observation of moisture content changes reveals that substantial amounts of water are imbibed by frozen soil under an open system. If an ice lens exists within the system, the increase in water content on the cold side of the lens is small compared with that on the warm side. Heave rate is governed primarily by the frozen fringe of soil between the warmest ice lens and the frozen–unfrozen interface.