building a retaining wall through a rock formation
Looking for some of your thoughts on easing the process of building through rocks. I've got a 4' wide tread planned for a new trail that needs to be pretty sanitized for beginners and walkers in a city park. One section will go through a mossy rock field around the nose of a small hill. The boulders vary from 2' to 5' wide, with rounded and odd shapes - It's very old sandstone. Some are more likely bedrock than loose. The hillside is steep, 50% slope or greater.
I don't really want to move any large rocks on this slope and would rather put the trail on top of them, building up with a retaining wall, and then knock down any high points that protrude. Anyone have suggestions for how to go about this? Do you cut notches in the existing rock to create a foundation for a rock wall on top? Drive pins in and use boards for the retaining wall?
"...So forget all your duties, oh yeah! Fat bottomed girls, they'll be riding today..." Freddie Mercury
I'll take a crack at it, and let someone else correct if needed.
Originally Posted by knobbyknees
So you need roughly a 2 foot tall wall? It may be closer to 3 feet by time you excavate and set your base layer. You'll need to excavate a foundation for the wall the surface should slope slightly inward towards your hill (as opposed to outslope like the trail should have).
Collect all the rock you think you need before building or at least get a good collection. That way you can pick and choose the right puzzle pieces. Some volunteers make good rock hunters, others will take pride in their stacking. They'll figure it out or assign them a task after watching for a few minutes.
Put the flat rocks on your wall, backfill with the round ones. If you want a 4' tread, you may excavate 4.5 feet out cause the wall needs to slope a little inward as well.
Study a brick wall, note the bricks alternate and overlap for greater strength. Do the same with your wall. Put the larger rocks on the bottom.
When rock is plentiful, don't use wood. Take time with your stacking, make the rocks wobble free. Use small rocks as shims between the larger rock to make things stable.
Buy a rock bar or two. It's amazing what you can move with a little leverage
Last edited by Fattirewilly; 01-04-2009 at 03:12 PM.
What you can't move with rock bars, you can smash with a sledge hammer or better still a powered demolition hammer. We broke up several engine block sized boulders that were in the way of a switch back we were building on an intermediate trail. It just takes time but it is a good way to get your aggressions out. Make sure everyone wears safety goggles.
You can use the excavated boulders from the trail bed to build your crib wall. Make sure you tilt the crib wall towards the trail a couple of inches or more for every foot high you go...for stability. (like the side of a pyramid)
If you fill in the crevices between the rocks in the trail, use crushed stone cause it compacts less and lasts longer but the big rocks will always have a tendency to show their face when the fill material settles or washes away.
You should use rocks from the rock garden itself for the retaining walls.
What i envision you doing is building up the trail tread with gravel and a mix of crushed rock and soil? In that case that "filler" acts like a ballast to the retaining rocks. So if you fit it all together correctly you have the retaining rocks holding in the trail tread, but the trail tread in turn helping the retaining rocks stay in place. Hope that makes sense...
But yah adding spikes and boards may sound like an easier path, but it sounds like more work for something that won't last forever. Working the rock will last forever, and will leave you with a trail that blends completely into the natural surroundings.
It's not easy, but there is a method to it. Having a few experience rock workers providing tips goes a long way into assisting others with a learning curve. But essentially it's like putting a puzzle together. A big heavy puzzle, but a puzzle nontheless... Sandstone makes for a great trail surface so any rounded bedrocks that are exposed should be welcomed when you can swing it.
Hammer Drill and Stainless Spikes
Here in WNC we have a lot of exposed Rock slabs at about a 50% plus to get across. We have been playing around with the idea of drilling holes into the existing rock slab and hammering in 1/2" stainless steel spikes at intervals that allow rock to be stacked in the spaces creating the new rock wall foundation. Our walls will have a much slacker batter than the typical wall to all the pressure on the spikes to be lessened a bit.
We have done this and used locust for the retaining structure, but not rock yet. This year the rock will probably be used on a project or two.
There is a trail called Pilot rock here that was built in about 1900 using a similar technique for the switchbacks and it seems to have held up just fine.
Good luck and post pics when you do it,
If you have deeply embedded boulders that would form a good foundation for a wall but no way to lay the wall on top of them, you might want to consider shaping boulders to fit on top of the existing boulders like the pieces of a jigsaw puzzle. Find boulders that are large enough to fill the gaps and also look like they were meant to fit well on top of the existing boulders. Take some chicken wire and form it around the foundation boulders. Turn the cap boulder over and use a hammer/chisel to shape the bottom of the boulder until the chicken wire form fits. Occasionally place the chicken wire form back on the foundation boulders to make sure it hasn't become distorted. It is a slow process but it will look great and will last for a hundred years.
Awesome ideas everyone, thanks! The beauty of this rock garden is the rounded, shaded, mossy jumble. My goal is to route the trail below many of the mossy rocks so they will be at eye level or thereabouts, and with the slope, almost within reach. I would love to know that whatever we do here will be very durable. I have some experience building a brick wall, and in installing low rock retaining walls for a garden, and in building low staked timber retaining walls for trails. But this is an order of magnitude more complicated.
Originally Posted by bweide
There is more rock nearby and I think we can set up a cable system with a grip hoist to bring them in. I'm trying to anticipate all the challenges. I'd love to hear more ideas, keep 'em comin'!
"...So forget all your duties, oh yeah! Fat bottomed girls, they'll be riding today..." Freddie Mercury
We've built lots of hand stacked walls at home, and one thing I think that's been missed is that the wall needs to angle back a bit. I forget what the number is, but this holds the slope against gravity much better than a straight up and down wall. Skook's advice to use chips for fill is great - we used a 3/4" chip.
If you live in a freeze/thaw area, your backfill and drainage, plus the slope angle of the wall is really important for longevity.
Look into what the CCC did. There is some amazing rock work throughout the west.
This book here,
Is a fabulous guide to building hand and dry stacked walls. It's for stone work what the IMBA Trail Solutions book is for singletrack.
If it is on a 50% slope, I suggest you forget the retaining wall, and instead just move a lot of rocks to create a bench, use rocks of various sizes to fill in the gaps in the bench, use flatter rocks as pavers over top of the bench and fill in the gaps with cursh-run.
If is a prove technique.
This was how I tackled the above project. If you think this technique might work for you I will post a whole series of picture from beginning to end.
Originally Posted by schaarschmidt
There isn't skree in Chat is there? What's all that rock from?
That isn't skree, it is bottom of one of the largest earth damn TVA has ever built. Check out this Google Map, where you can get the idea of just how big it is. That is a man made lake on top of Raccoon Mountain. From the TVA site: "The plant works like a large storage battery. During periods of low demand, water is pumped from Nickajack Reservoir at the base of the mountain to the reservoir built at the top. It takes 28 hours to fill the upper reservoir. When demand is high, water is released via a tunnel drilled through the center of the mountain to drive generators in the mountainís underground power plant."
Anyhow, here are your dam pictures.
In this first picture only about 10 feet of the causeway is completed. You can see the length of the dam running north from the area where we are constructing the causeway. You can see that we had to create the bench for the trail to run on in what varied between about 37 to 50 degree slope. We simply went in and chucked rocks out of the bench, keeping the flatter ones to use as pavers to create a smoother riding surface.
This is a shot down the causeway. If you look hard you can see that I had marked off the route with some kite string, so that we would keep the bend level. It is a lot easier to get messed up and push more or too little and have to go back and modify without the string.
The rough bench is complete and we have started setting in place the larger flatter paver shaped stones were we able to locate on the dam. The flatter stones were difficult to find and we had to move them great distances, including sliding them down the dam from as high as 75 feet up and as far over a 100 feet.
The pavers were a lot heaver then they look once they were set in place. The rounder ones could be held up on end and rolled, odd shaped ones had to be carried.
That's eight pretty stout southern boys carrying one rock. I am lucky that I was the thin (displaced) northern, it gave me the prefect excuse to work the camera on that one. Not that every on hand didn't move their share of rocks. (In fact, I constructed the first 15% to 20% of the causeway all by myself, though I used much smaller rocks.)
Another good size rock being moved into place with some rock bars.
This shot should help you get a perspective on the size the causeway.
The causeway is newly finished and awaits some crush-run to be poured between the pavers to help lock them into place. (I actually went across the whole causeway with small rocks and a mason hammer and pounded them into the crack to lock them in place the best I could before pouring the cursh-run.)
A view of the causeway from the top of the dam.
Open for business.
The whole project took about a month to complete, we met every weekend, both Saturday and Sunday, and had anywhere from 15 to 25 volunteers working 6 hour days. (This doesn't include the time I spent working on the project by myself.)
It took a lot of work but we either had to construct a bridge at the costs of $25,000 to $30,000 to cross a 30 foot deep and 30 foot wide gap at the base of the dam, just into the woods or come up with an alternative. Seeing as how our club is poor and we had to take on this labor intensive project.
We were lucky in that this portion of the trail had more volunteers attend than any other. I attribute it to the fact that it was the last section of the Mega-Watt trail that had to be completed before it was opened. That played a huge part in getting volunteers out. Most never showed up to a previous trial day, and most have never attended one since the causeway's completion, but at least they put in a few hour that year.
I did a preview before posting, for some reason not all the pics are appearing. Here is a link to the folder where they are stored online. http://sorbachattanooga.org/images/Dam%20Pics/
I would play around with it some more, but I have to go to bed. Its 1:30 am and I have to get up early tomorrow.
I hope you found this post helpful.
Edit: Jan. 29, 09 - I figured out why a few of the pictures were not displaying and made corrections. The picture locations are case specific, the difference between "jpg" and "JPG" were the problems.
Last edited by schaarschmidt; 01-29-2009 at 04:24 PM.
Darn near looks paved after all that effort.
We jokingly had a saying "strong backs and weak minds" when we were doing rock projects. It is amazing what a small group of people can accomplish if they are willing to put a little effort forward. Regardless great job.
Nice work and thanks for the pics. That's a very unique project.
I'm sorry to bring this up, but I can't help looking at that and have "TVA, dam failure, and coal slurry" pop into my head! Have you heard any comments along those lines coming from anyone "official"?
We don't communicate with anyone higher up above the site supervisor and that is a rare occurrence. We usually communicate with the site's recreation's supervisor. I suspect that all employees have been advised not to give any comments on the breach. I think the areas were the breaches took place were simply earthen levys, which is much different than the rock dam at Raccoon Mtn.
Originally Posted by FeloniousDunk