Armoring - Using Rock to Harden Trails

Armoring is primarily useful in two situations. First, an elevated trail tread can be created above especially soft or wet terrain when no alternate route is available. It is always preferable to create a sustainable re-route around a problem trail section. Second, armoring can be used to harden the trail tread against user-caused erosion. It is important to distinquish between user-caused erosion and water-caused erosion. It would be foolish to spend the time to armor a trail if water drainage issues haven't been effectively addressed. Water will destroy armoring by flowing under the rocks and undermining the foundation or by frost-heaving the rocks out of position.

Examples Where Armoring Can Benefit A Trail

1. Hardening a contour trail in extremely rainy climates. Click for bigger picture.	2. Stabilizing steep sections of contour trail with grades from 20 to 45%. Click for bigger picture.
3. Reinforcing stream crossings. Click for bigger picture.	4. Crossing a low-lying muddy or sandy area when a reroute isn't possible. Click for bigger picture.
5. Hardening landing areas following jumps or drop-offs. Click for bigger picture.	6. Toughening the trail surface on high traffic routes to withstand user-caused erosion. Click for bigger picture.

Five Techniques for Rock Armoring

	1. Flagstone Paving - Large flat-faced stones placed directly on a mineral soil base or an aggregate foundation (a mixture of sand, gravel, pebbles and small stones, devoid of organic material). The stone's largest and smoothest face is placed up, at grade, to form the tread surface. This is the most common and simple armoring technique. Click for bigger picture.
	2. Stone Pitching - An ancient road building technique in which medium sized rocks are set on end, or "pitched" up on their side. The stones are hand-fitted tightly together with aggregate packed into the gaps to tighten the construction. Think of a book in a bookshelf, only the spine is showing and the rest of the book is hidden. Modern trailbuilders in Wales have revived and perfected stone pitching on singletrack. Click for bigger picture 1, 2. Stone Pitching Demo, 11 Photos: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11
	3. Raised Tread Construction - Using rocks to elevate the tread above especially soft or wet terrain. A foundation of large rocks is embedded first. Medium rocks follow, locked into position. The tread is capped by aggregate, or "inch-to-dust" stone material. This is similar to the trailbuilding technique known as turnpike construction but using rocks. Trailbuilders in Wales use the term "raised camber construction" to describe the crowned tread designed to shed water. Make sure your rock turnpike drains and does not function like a dam, raising water levels on one side of your structure. Click for bigger picture 1, 2.
	4. Boulder Causeway - Using giant boulders and rock slabs as the trail tread. Boulder causeways can be described as a super-sizing of one of the above techniques. Click for bigger picture 1, 2.
	5. Natural Rock Outcroppings - (Photos 11) Routing the trail over existing, exposed rock. This is an excellent way to create highly challenging freeride trails because the exposed rock can support fall-line trail sections. Click for bigger picture 1, 2.

Rock Armoring Techniques Illustrated

Nine Tips That Apply To All Armoring Techniques

1. Start at the Bottom. If you're working on a slope, start at the bottom and work upslope. The weight of the rocks will hold your work together.

2. Drop the Anchor. The first "keystone" in an armored trail section plays a crucial role in anchoring everything in place. The anchor rock must be large and immobile once set in place with at least two-thirds buried. Remember that angular rocks are better than round. Place a solid anchor every couple yards.

3. Laying the Tread. Once the anchor is solid, follow by placing more rocks to form the trail tread. Ensure all rocks touch one another and everything is locked in place. Like building a puzzle, you may need to move rocks around to find the best fit. If a rock wobbles under foot, reposition it. Use smaller angular rocks as wedges to fill gaps. Without mortar, friction and gravity must hold the rocks together.

4. Break the Joints. Place each rock so that it spans the gap between the adjacent rocks. Like building a brick wall, you must avoid directly aligning joints because they will weaken the structure. Each course of stone should overlap the cracks in the preceding row. Try to minimize lengthwise gaps that run parallel to the trail that might catch a bicycle wheel.

5. Use Tie Stones. Every four to six feet, place a larger stone that will span most or all of the trail tread. Like the anchor, a tie-stone helps lock everything together.

6. Fill the Gaps. One of the most important steps is to fill the gaps with small rocks, stone dust, gravel or sand. Pack the fill in tightly using hand tools. "Inch-to-dust" is a term used to describe small quarry waste, also known as "crusher fines." This broken stone is the best fill material because it contains natural cements present in the parent stone. Scottish trailbuilders call it "magic dust."

7. Compact. It is best to compact any surfacing material in layers while slightly wet. A mechanical compactor is the best tool for this key step.

8. Corral the Trail. (Photo) Include something to define the sides of the armoring. Large ominous "gargoyle" rocks, logs, trees or other obstacles staggered on either side of the trail serve as physical and visual barriers to keep riders on the armored section of trail. Make sure the narrowing and the rocks flow naturally with the trail. Otherwise people will find it annoying instead of interesting, and may create a new route around it.

9. Consider Trail Flow. (Photo) Smooth flow is vital on trails for cyclists. Mountain bikers love the rhythm of a trail where one turn blends into the next, and the trail surface is somewhat predictable. A trail with good flow helps minimize erosion, user conflict, and safety concerns. Strive for a subtle transition into the armored section.

Construction Steps in an Armoring Project

1. Be Safe. This can't be emphasized enough when it comes to stonework. A hardhat, eye protection, gloves and steel-toed boots are all necessities. Don't be in a hurry around stone. Learn the mechanics of lifting without injury. Communicate with people around you. Keep hands well away from any rock being shifted by pry bars. Skidding a rock is safer than rolling it. Tumbling rocks almost never stop where you want them to - it is best to slide them in a controlled manner.

2. Use Proper Tools. If you plan to make working with rocks a hobby, you'll want to add a couple specialized items to your tool cache. At least three heavy-duty pry bars are essential. Pick mattocks and pulaskis are very useful for digging, prying, and positioning rocks. A heavy sledgehammer is helpful for breaking rocks and coaxing stones into a tight fit. A rock hammer and chisel allow you to shape stones.

3. Locate Material. In mountainous country, you can usually find plenty of stone near the worksite. Pay attention to the terrain and try to locate rocks out of sight of the trail and uphill of your project location. Don't move rocks that will damage vegetation or sensitive areas. Restore any significant disruption caused by rock quarrying. Sometimes you'll need to import rock or transport it long distances. British trailbuilders truck stone from commercial rock quarries and transport huge quantities with power wheelbarrows.

4. Choose Good Rocks. If possible, select angular rocks that have flat sides and square edges. The best are rectangular, shaped like a toaster; round rocks are difficult to work with. The exact size and shape will depend on the armoring technique. It is best to use local stone so your work will look natural. Be careful importing rock that may spread invasive plant matter.

5. Transport Stone. A wheelbarrow is the most common method for moving stone. You can lay it on its side, slide a big stone in, stand it up, and wheel to your worksite. For longer distances and heavier loads, a variety of power wheelbarrows are available. (Photo) The use of ropes, cables, pulleys, and other hardware (rigging) to move materials can be useful when moving large stones. Trained, experienced experts must supervise rigging. Small stones and fill material can be moved in buckets.

6. Remove Organic Material. As with all trailbuilding, it is important to excavate down to mineral soil if practical.

7. Armor the Trail. Incorporate one or multiple techniques described above.

Even though you are using rock to armor a trail tread, all of the principles of sustainable trails still apply. It is essential to follow the half rule and incorporate grade reversals because the key to long-lasting stone armoring is to prevent water flowing down or under that section of trail. Well-built stonework will withstand the pounding of a Roman Army and all who follow, but will fail rapidly if attacked by gravity-powered water. Finally, make sure to get permission from your local land manager before starting any trailwork, especially work with this level of commitment and permanence.