Turnpike or French Drain?
I led a work day today and had the crew perform some repair work. The trail section we worked on is in a relatively flat area with poor drainage. The current trail is sinking into the black muck and forms the local drainage channel.
We dug out the soft wet stuff, and filled with sand/gravel mix until the tread was above the surrounding dirt by several inches.
My thinking is that the water will tend to spill off to the sides of the trail now that it's higher than the surrounding mud. At worst, I may have to lower the tread and armor a short section to let water cross. (There is a very tiny side slope across the trail, but it's much to shallow to bench cut.)
However, one of my co-workers thinks the gravel bed will act more as a French Drain and channel water through itself. I guess if the water moves through slowly enough, it may still work, but I'm worried that I may have made the problem worse. Any opinions?
My opinion is that time, and some rain, will provide the answer.
Fill the topside where it puddles with organic loam and throw in some flora.
If your organics aren't keeping up with the water and the entire upperside is ponding then you're dealing with a higher level of saturation. Then you should have approached the turnpike a bit different.
Instead of sand/gravel mix as your build up, you should have instead added cobble rock as your base 3 to 4 inch, then throw in your gravel, top it with sand/mineral mix. Retain it if you rise high enough to justify it.
This turnpike is not a drain per-se but will allow for seepage.
All this being said even if you have water spilling over your tread surface, as long as it's sheeting, and as your compaction creates a hard surface with as much rock as you used, you should be fine.
In a similar type of area, mostly flat with seeping water, we built up the tread with drainage rock (fist sized and larger) and then capped it with sand/gravel mixture. We used the largest rocks on the sides of the tread to hold it all together. That has held up well after a couple of years.
Another trick for durability is the revegetate the "side slope" of the raised thread and make sure you have a 2:1 to 3:1 ratio on that slope.
Water will percolate tru your material, but a real french drain is based on 3/4 clean crushed stone for a reason, to allow water to flow. Bigger rocks that remain clean is key to proper "under-drainage".
A trailbuilder from the north
Sorry for being unclear on this. I don't want to make a drain. I don't think there's enough slope to make it work.
I'm aiming for filling the tread with something with more strength than the black silt that currently forms the trail surface, something that will be solid in wet weather so the hikers who won't stay off the trail in the spring don't churn the trail into mud soup. My guess is my best bet is to encourage the water to not infiltrate the gravel, but to run off the top and hopefully away from the trail.
My purpose in posting was to air my co-worker's criticism. I'm hoping he's wrong, but I don't have a lot of experience in building turnpikes. BTW, I was hoping to partially fill with rocks, but the nearest source is ~150 yds away across churned-up black-silt mud. I can't get across with any kind of wheeled vehicle. I could go farther away, but have limited rock hauling capability.
We've fixed a trail that went tru flat, undrainable land with only local material -- blue clay. We used a mini-X to raise the turnpike about 3ft above ground, and 5-6ft wide. Most of it sink back, but after 2 years, the thread is about 12" above ground, about 3-4ft wide and the shoulder are very solid with good vegetation that grew back.
Obviously, establishing a rock-based foundation is a much more durable solution but almost anything can do the trick given sufficient time and effort.
Have you considered building a bridge instead of hauling rocks? Not as durable, but way easier to get volunteers to help with.
A trailbuilder from the north
Hi Walt. I'll ty to get you some pics of the devices we use to transport rock/soil over difficult terrain - very simple and cheap and can be one or 2 man operated. We leave a couple out bush, but the simplest device can be carried in your backpack.
Thanks for sharing your experience. This leads me to believe the gravel fill might work. We're not making as big a structure as you describe, but if it improves the awful mess we have now, I will have an example to show workers to convince them digging the mud out fixes the problem.
Originally Posted by HypNoTic
I am dead certain that we could get twice as many people to volunteer building a bridge as are showing up to build the turnpike. I don't want to go that way though because I believe a well constructed turnpike will out last a bridge many times over. Also, my club doesn't have the financial resources to build 200 yards of bridge at the current location, another 200 yards of bridge at the next location, and several hundred yards more later. The park manager gives us the gravel.
Walt, here's some dodgy pics of simple tools for moving earth and rock. No trail needed to move stuff with these - you can walk across country. It is simply shade cloth, or roller-blind cloth if that is what you call it in the US. Any blind company has off cuts and will give it away free. Some simple stitching, or with the plain sheet, no stitching at all, just fold up and off you go.
The white one is meant for soil, but here we have stone in it. A larger black one has the handles oriented lengthwise like a medical stretcher and is really meant for rock, but we didn't use it here. The tubes are 40mm Al. The simple black sheet is for one or 2 person quick jobs. This one is a bit narrow and long. It can be carried over the back like Santa Claus or by twisting the sheet corners, 2 people can carry a heavier load. The bag is for gravel collection and could have handles, but is easy enough to carry like a baby. The stretchers carry up to 100kg I guess.
When we dig soil from trail we put it straight on one of these to avoid mess and then take it where we want it - today into a large hole some goose dug in a successful attempt to turn a rocky and stable trail into a bog.
I don't think you have to worry nor was it criticism. I have a bizarre mix of materials where the pump track platform was filled and the gravel holds weight when wet and the silt does not.
Originally Posted by Walt Dizzy
Originally Posted by bitflogger
For the clarification. I'd have more confidence if we'd ever done this before. I'm really hoping this makes things better. You'd think I'd know more after doing this for 10 years. Every day's an adventure out in the woods!
I finally remembered to post an image. This screen grab from a video shows the section where we built up the tread where it had always been wet from a seep coming down the hill.
We used the largest rocks on the sides of the tread to contain it, then built up the middle with drainage rock capped with sandy gravel.
This is about three years old and as you can see it has held up just fine. The issue now is that the water is flowing down the tread and pooling further down the trail -- we neglected to put in a drain or culvert to get the water across to the other side!
Here is a before and after of a turnpike we built just a few weeks ago at a county park. Each winter, the section of trail on the left would turn into a lake with 12" of water becoming almost impassable.
We built a turnpike by laying down and compacting 10 yards of 1-3" gravel and then another 7 yards of 5/8 minus over the top.
All the gravel was moved from about 1/4 mile away using wheelbarrows. The local Jr. High and High schools have a community service hours requirement and I was able to tap into this to get 30 kids out who used this session to fufill their requirement. Took us about four hours to complete. Since we built the turnpike two weeks ago, it's compacted nice and hard and is appropriate for peds, bikes and horses to use.
This picture does not show it, but we installed a 8" pipe for drainage.
There is some merit to what you coworker said. The permeability of the gravel and sand mixture will be much higher than the surrounding silt soil and will act as a drain to collect water out of the surrounding soil. Will it be a problem? Probably not as long as there are enough gravel and larger rock fragments to keep the sand from becoming liquefied when wet. Over time, silt particles from the surrounding soil will migrate into the gravel/sand core, reduce its permeability, and then you may start to see problems with the elevated trail acting as a dam for water flowing off of the slope. Wrapping the gravel/sand core in a geofabric material will keep the silt out and reduce the chances of forming a dam.