Ice Lens

An ice lens or ice lenses are formed when moisture, diffused within soil or rock, accumulates in a localized zone. The ice initially accumulates within small collocated pores or pre-existing crack, and, as long as the conditions remain favorable, continues to collect in the ice layer or ice lens, wedging the soil or rock apart. Ice lenses grow parallel to the surface and several centimeters to several decimeters (inches to feet) deep in the soil or rock. Studies between 1990 and present have demonstrated that rock fracture by ice segregation (i.e., the fracture of intact rock by ice lenses that grow by drawing water from their surroundings during periods of sustained subfreezing temperatures) is a more effective weathering process than the freeze-thaw process which older texts proposed.

Ice lenses play the key role in frost induced heaving of soils and fracture of bedrock, which are fundamental to weathering in cold regions. Frost heaving creates debris and dramatically shapes landscapes into complex patterns. Rock fracture in periglacial regions (alpine, subpolar and polar) has often been attributed to the freezing and volumetric expansion of water trapped within pores and cracks. However the majority of frost heaving and of bedrock fracture results instead from ice segregation in ice lenses in the near-surface frozen regions. Ice segregation results in rock fracture and frost heave.

Frost heave is the process by which the freezing of water-saturated soil causes the deformation and upward thrust of the ground surface. This process can distort and crack pavement, damage the foundations of buildings and displace soil in regular patterns. Moist, fine-grained soil at certain temperatures is most susceptible to frost heaving.

...
Wikipedia