Permafrost by permafrost. Of this area slightly more than half is underlain by continuous permafrost, around 20 percent by discontinuous permafrost, and a little less than 30 percent by sporadic permafrost. Most of this area is found in Siberia, northern Canada, Alaska and Greenland. Beneath the active layer annual temperature swings of permafrost become smaller with depth. The deepest depth of permafrost occurs where geothermal heat maintains a temperature above freezing. Above that bottom limit there may be permafrost, whose temperature doesn't change annually—"isothermal permafrost". Permafrost typically forms in any climate where the mean annual air temperature is less than the
Permafrost construction on permafrost owing to the change of soil properties of the ground on which structures are placed and the biological processes in permafrost, e.g. the preservation of organisms frozen "in situ". Building on permafrost is difficult because the heat of the building (or pipeline) can thaw the permafrost and destabilize the structure. Three common solutions include: using foundations on wood piles; building on a thick gravel pad (usually 1–2 metres/3.3–6.6 feet thick); or using anhydrous ammonia heat pipes. The Trans-Alaska Pipeline System uses heat pipes built into vertical supports to prevent the pipeline from sinking and the Qingzang railway