from the Tennessee Mountain Bike Alliance:
What is the freeze-thaw process?
During wet winter months, when the trial becomes saturated and temperatures drop, ice forms in soil voids. Through the night temperatures drop and the freezing process pushes soil grains apart reducing particle cohesion and soil strength and making the soil more erodible. During the day temperatures increase and the trail surface thaws.
Even though the surface has thawed, the ground is still frozen just below the surface. To make matters worse, the frozen ground prevents precipitation from sinking in any further. This means the thawed layer at the surface is absolutely saturated with water so it is very intolerant of any disturbance. Come riding along and you’ll cut through the thawed layer right down to the frozen ground. The thawed layer will end up with ruts from the tire tracks, which will persist even when the soil dries out.
from the Macomb Area Cycling Mountain Bike Club:
When soil freezes all the moisture turns to ice crystals between the particles of the soil, and in clay or loam soil they are tightly packed together. As the moisture freezes the crystals expand and tear the clay apart which shatters it, then as it thaws the soil becomes more porous and allows more moisture in. This is repeated with every successive thaw. We can get as many as 100 freeze thaw cycles in a season. Please be very careful when trying to ride the trails in the winter. The best time to ride is when the trail is frozen solid. On days when the temperature goes below freezing at night, the worst thing you can do is wait for the sun to come out and warm up the trail before riding. The trails can be very soft even if it hasn’t rained recently. Even if the temperature stays below freezing, South facing trails can defrost in sunlight and turn to mush. If you are leaving ruts of ½” or greater in the trail where you have ridden please leave the trail. The ruts will allow even more water to remain on the trail and make it even worse. If people see you riding they will want to ride too, so very soon the trail reaches a point of meltdown.
So use some common sense when riding the trails. Don’t use the current temperature as your only guide. Yes we all love riding when it’s 60F out in January, but if it was 20F the night before, the trails will be frozen in the morning and thaw in the afternoon. The best time to ride is first thing in the morning when the trails are still frozen from the low overnight temperatures. If you can’t make it out first thing in the morning, look for trails that drain well (and don’t retain moisture) like Powhite Park.
So far our trails have held up well this winter, but we still have several months ahead of us. So do your part and be smart about riding the trails.