Active Layer

What does Active Layer Mean?

Definition of Active Layer in ConstructionThe active layer is the layer of material, sand, earth, gravel, silt, etc. that freezes and thaws in a cold climate above a permanently frozen material. An active layer must exist over, what is referenced as permafrost, which is the earth constantly frozen. The active layer is the layer of material that is influenced by temperatures that are above freezing. Dependent upon the climate, the active layer can be extremely shallow, a few feet, or very deep. The active layer is totally dependent upon the temperature of the air above it, as well as the length of time that the air remains above the freezing mark. In addition, the location of the active layer makes a large difference on the depth of the active layer. If the active layer is being measured out in the open sun, and without shade of any kind, then the active layer could be considerably deep. If the active layer is being measured under a rock overhang or on the north side of a hillside, then the active layer might be only a few feet thick. Construction on the active layer must be cognizant of the insulating value of this active layer. The level of the top of the permafrost is a factor of the thickness of the active layer. If the active layer is excavated, the depth of the insulating layer is reduced and additional heat finds its way to the top of the permafrost, resulting in thawing of the permafrost. This is then compounded by the deeper thawing and the deeper penetration of heat into the permafrost. In theory, the erosion can burrow itself deeper and deeper until a point of equilibrium is reached. Construction on an active layer with permafrost beneath is a difficult endeavor and must be carefully engineered and managed.