Freeze-thaw
Friday, February 26, 2010 at 6:58PM The Chattanooga Chapter has a great write up on what freeze-thaw is and why it's so bad. Freeze-thaw is why Blankets is closed this time of year. Please understand that we get more traffic in one day then most trails see in a month so we have to be very careful with the trails right now as opening them too soon will cause months worth of damage to the trails and only force us to keep them close longer.
From SORBA Chattanooga
This photo demonstrates how even foot traffic during the freeze-thaw cycle can impact the trail long after it drys out.Between abundant precipitation and very cold temperatures, off road enthusiasts have been largely kept off their bikes for weeks. After a long cold and wet month many mountain bikers are just itching to ride. The arrival of warmer weather means that many want to head out on to the trails right away.
Most mountain bikers understand riding a trail when wet and muddy can cause rutting, puddling and other forms of trail degradation. However, many of these same users don’t understand that trails are at even greater risk during the freeze-thaw process.
This trail has suffered degradation from foot traffic during the free-thaw process.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.
Reader Comments (10)
I can't listen to this freeze/thaw stuff anymore and not comment. If you want to close the trail because of wet conditions, I can respect that because the ruts in a muddy trail are difficult to repair, but the frostline penetrates maybe an inch if that into the ground here in Georgia. The very fact that the ground is soft makes that point painfully clear. So let's stop this group think explaination and be honest about it, the trails are too sloppy, enough said.
I'm sorry but do you think we just make this stuff up? The trails are not sloppy that is the point. To a normal person the trails look fine it's when you throw a couple hundred bikes on them in one day that they turn into soup. Freeze-thaw is real and is exactly why the trails are closed. I have been working the trails since 2003 and we have opened them when we shouldn't have and paid the price with giant sections of trail that took us months to fix and not to mentioned the damage it caused to the surrounding soil. We have a lot experience with our trail directors and crew leaders who don't just have experience but are also certified. They will open the trails when it's best for them not just so people can ride because they have been stuck inside all winter.
The weather will soon turn into spring and the trails will remain open for months on end and this will all be forgotten about until next seasons freeze-thaw begins again.
My point is not to say the trails should be open, I understand why they are closed. The areas where the trail gets soupy is more to do with the water table and soil composition than frost. You can have the same effect in the middle of summer if there has been alot of rain. A trail can look dry but because of the water saturation of the soil, with traffic the water can be brought to the surface. I appreciate the stewardship issue, really I do, my nit was with the explaination thats all.
As Jeff said the article is a great explanation of freeze-thaw. Maybe you should read it again. It happens exactly as they describe. I saw it this past Friday when I went to check the trails to see if we could open. The trails had vast stretches where they were wet (a light sheen) on the surface where thawing was happening but still frozen underneath. It had not rained for days. As the day goes on more of the trail will become wet looking as it thaws on the surface. It innocently appears as though it had just rained lightly for a few minutes but if used just that one day the trails will become a "Slushy" throughout that does not easily repair itself. Nothing like that happens in the summer. Our Dirty Duathlon race was held in the rain last year and yet a week later you would have never known it. Also most of our trails are pretty hard packed so they tend to run the water off and dry out quickly in the summer. While we have some trouble spots left over it's not like freeze-thaw where it's almost the entire trail that becomes a trouble spot.
What is the rule again for when the trails can be opened? Something like 24 hours without below freezing temps? Or is it 48 hours, or 7 days without freezing temps? I've been told before but don't remember. Thank you!
Tony, there is no set rule when it comes to freeze/thaw. When we have heavy rain without freezing temperatures, the general rule of thumb is wait 24 hours for the trails to adequately dry. However, heavy rain, snow, and freezing temperatures leave no rule of thumb for a set number of hours to open. The trails were a big muddy mess as of Monday, 1/3, and now we have more rain, which hopefully will not be followed by more freezing temperatures. Our volunteers who live near and work on the trails frequently are checking them regularly, so when they are ready to accomodate the high traffic volume, we will reopen them. Thanks for your patience.
When the trails are closed, are they still open for running? thanks.
When the trails are closed, they are closed to all users.
Are they closed to hoof travel too?
Yes they are closed to all traffic.