Soft Springtime Roads

Why is it that northern roads which can be dry in fall are almost invariably wet in springtime?

One possible source of moisture is the melting of snow and ice on the road surface, and sometimes runoff water flows onto the roadway. But roads that have no runoff problem and which have been kept clear of snow and ice all winter still can be wet at breakup

The real culprit is upward migration of soil moisture during and after freeze-up. The seriousness of the problem depends upon the composition of the roadbed and the availability of moisture in the road bed material or the soil below. Normally, water exists in the soil in three forms--vapor, liquid water that is free to move through pore spaces in the soil, and liquid water that is relatively tightly adhered to the surfaces of soil particles.

In fine-grained soils (silts and clays) with high water content, the free liquid water in the pore spaces moves toward that part of the soil having the lowest temperature. Since in winter the ground surface is colder than the soil below, the free water moves upward. At the freezing line much of the free water is converted to ice.

Water in vapor form moves upward too. Vapor pressure is highest where the temperature is highest, and water vapor, like any gas, moves from a region of high pressure to low pressure. The resulting upward transport of water vapor is particularly important in sandy or gravely soils. These soils have large pore spaces through which the water vapor can move easily. Where the pore spaces are large, the forces that move liquid water upward are less effective. Being something akin to capillary forces, they operate best when the pore spaces are small.

Even after the roadbed is frozen, there still is upward migration of moisture through the frozen soil toward the cold road surface. Frozen soil is funny stuff since it still contains liquid water. The liquid water is probably mostly that tightly adhered to soil particle perhaps it takes part in the continuing upward migration of moisture.

As winter progresses, more and more water moves upward. The frozen water content can become so high after several months of freezing weather that the soil volume can, in extreme cases, nearly double--the phenomenon called soil heave.

Then comes spring, and anyone having lived through northern breakup knows the sort of quagmire that results from the melting of ice-rich, fine-grained soils.