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Rock Armoring & Rock retaining walls

<h4><b>Rock armoring </b></h4>
<h4><b>1. Definition & Purpose </b></h4>
<span style=”font-weight: 400;”>Purpose:</span>
<ul>
<li style=”font-weight: 400;” aria-level=”1″><span style=”font-weight: 400;”>makes a trail more robust to handle l</span><span style=”font-weight: 400;”>arger numbers of users </span></li>
<li style=”font-weight: 400;” aria-level=”1″><span style=”font-weight: 400;”>hardening the trail to pragainst erosion</span></li>
<li style=”font-weight: 400;” aria-level=”1″><span style=”font-weight: 400;”>to elevate a trail above soft or wet soils</span></li>
<li style=”font-weight: 400;” aria-level=”1″><span style=”font-weight: 400;”>reinforcing stream crossings</span></li>
<li style=”font-weight: 400;” aria-level=”1″><span style=”font-weight: 400;”>hardening landing areas following jumps or drop-offs</span></li>
</ul>
<span style=”font-weight: 400;”>Where to use:</span>
<ul>
<li style=”font-weight: 400;” aria-level=”1″><span style=”font-weight: 400;”>on fall line trails (20 to 45 percent)</span></li>
<li style=”font-weight: 400;” aria-level=”1″><span style=”font-weight: 400;”>where the soil is poor and easily eroded</span></li>
<li style=”font-weight: 400;” aria-level=”1″><span style=”font-weight: 400;”>on parts of the trails where heavy braking happens and the creation of braking bumps is likely </span></li>
<li style=”font-weight: 400;” aria-level=”1″><span style=”font-weight: 400;”>landing areas following jumps or drop-offs</span></li>
<li style=”font-weight: 400;” aria-level=”1″><span style=”font-weight: 400;”>areas of rainy climates</span></li>
</ul>
<span style=”font-weight: 400;”>What to consider:</span>
<ul>
<li style=”font-weight: 400;” aria-level=”1″><span style=”font-weight: 400;”>difference between user and water caused erosion</span>
<ul>
<li style=”font-weight: 400;” aria-level=”2″><span style=”font-weight: 400;”>Rock armoring isn’t the solution for water management related problems</span></li>
<li style=”font-weight: 400;” aria-level=”2″><span style=”font-weight: 400;”>Water will destroy armoring if it is allowed to flow under the rocks</span></li>
</ul>
</li>
</ul>
<img class=”alignnone size-full wp-image-2224″ src=”https://www.imba-europe.org/wp-content/uploads/2022/03/rock-armoring-being-used-to-manage-water.jpg” alt=”” />

<em>Photo 1: Rock armoring being used to manage water. (Source: Bikeplan)</em>
<h4><b>2. Key Considerations</b></h4>
<h5><b>2.1 Typical Characteristics</b></h5>

Type

Typical use

Pro‘s

Con‘s

Type Stone Pitching

Typical use This is an ancient road building technique where medium-sized rocks are set on edge or pitched up on their side. The stones are hand-fitted tightly together with aggregate packed into the gaps to tighten the structure. It may seem like a tough job, but stone pitching can often be more efficient than flagstone paving, depending on the stone selection.

Pro‘s Hard wearing, can blend into surroundings, exciting to ride, can be creative with curves and features

Con‘s Less experience riders might ride around, expensive, labour intensive,

Type Raised Tread

Typical use Also known as a rock turnpike, the foundation consists of large rocks embedded into the tread. Medium rocks follow and are locked into place. Finally, the tread is capped with aggregate, or crushed stone. Make sure the tread is crowned to shed water and does not form a dam, raising the water level on the sides of your trail.

Pro‘s Hard wearing, keeps the trail flowing

Con‘s Needs maintenance to ensure filler doesn’t come away, expensive

Type Flagstone Paving

Typical use Large, flat-faced stones are placed directly on a mineral soil base or an aggregate base (a mixture of sand, gravel, pebbles and small rocks, with no organic material within or beneath base). Each stone's largest and smoothest face is placed up and at grade to form the tread. This is the most common and simple armoring technique.

Pro‘s Looks good, easier to maintain, can be built by hand

Con‘s Time consuming/expensive, needs a lot of stone

Type Boulder Causeway

Typical use A boulder causeway is basically the same as flagstone paving technique where giant boulders and large rock slabs are used to raise the tread.

Pro‘s Hard wearing, solid, simple

Con‘s Likely to need machines to place boulders, can separate leading to harder trail, disjointed ride

<em>Source: Felton, 2004; Trail Solutions: IMBA’s Guide to Building Sweet Singletrack</em>
<h5><b>2.2 Key Aspects </b></h5>

Aspect

Description

Aspect Trail flow

Description Smooth flow is vital on trails used by mountain bikers. Mountain bikers love the rhythm of a trail where one turn blends into the next, and a trail surface is somewhat predictable. A trail with good flow helps to minimise erosion, user conflict and safety concerns. Strive for a subtle transition into the armoured section.

Aspect Be Safe

Description A hardhat, gloves, eye protection, and steel-toed boots are necessities Don’t hurry Learn the mechanics of lifting without injury, use machines whenever possible Communicate with people around you - consider a crew chief Protect other visitors from rock falls, moreover when working on steep hillsides, consider installing retaining planks on the lower side of the trail Keep hands away from any rock being shifted by pry bars or other means Skid a rock in a controlled manner rather than rolling it. Rolling rocks rarely stop where you want them

Aspect Use Proper Tools

Description At least three heavy-duty rock bars, or pry bars are needed to move rocks Pick mattocks and pulaskis are useful for digging, positioning rocks and light prying A heavy sledgehammer will help to break rocks and final tight fit positioning Rock hammers and chisels are useful in shaping stones.

Aspect Material

Description It’s best to use local stone, when available, so your work will look natural Importing stone may spread invasive plant matter Try to use rocks that are out of sight and uphill from of the trail corridor Don’t move rocks that will damage vegetation or sensitive areas Restore any significant disruption caused by rock quarrying

Aspect Shape and Size

Description If possible, select angular rocks with flat sides and square edges Round rocks are difficult to work with The exact size and shape will depend on the armoring being done.

Aspect Drainage

Description Water will destroy armoring if it is allowed to flow under the rocks. This will undermine the armoring or frost heave the structure out of position.

Aspect Half rule / Grade reversals

Description It is essential to follow the half rule and incorporate grade reversals because the key to long-lasting stone armoring is to prevent water from flowing down or under the structure you just built.

<h4><b>Rock retaining</b> <b>wall</b></h4>
<ul>
<li>Step 1: Choose rocks, not wood</li>
</ul>
Large rocks are the preferred material for retaining walls since they do not rot and their sheer weights lends greather strenght to the structure
<ul>
<li>Step 2: Use appropriate rocks</li>
</ul>
If possible, select angular rocks that have flat sides and square edges as round rocks are difficutl to work with. It is better ot use large rocks (at least 25 kg, ideally 75) than small ones. If you cna lift the rock by yourself, it’s probably not big enough. Try to use local stone so your work will look natural. Importing rock may spread invasive plant matter.

<img class=”alignnone size-full wp-image-2231″ src=”https://www.imba-europe.org/wp-content/uploads/2022/03/rock-retainingwall.jpg” alt=”” />
<ul>
<li>Step 3: Lay a foundation</li>
</ul>
The first rocks in a retaining wall play a crucial role in anchoring everything else in place. Excavate a footing, then place large, well-anchored rocks to form a base layer. The foundation rocks must be immobile once they are set in place.
<ul>
<li>Step 4: Build a Wall</li>
</ul>
Once you have a solid foundation, place more rocks in tiers to form the wall. Ensure that all rocks touch one another and everything is locked in place. This process is a lot like building a jigsaw puzzle. You’ll 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 your wall together.
<ul>
<li>Step 5: Break the Joints</li>
</ul>
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 preding row.
<ul>
<li>Step 6: Use Headers</li>
</ul>
Every 120-160 cm, try to place a header – a large rock used as an anchor – that extends into the bank behind the retaining wall. This helps lock everything together.
<ul>
<li>Setp 7: Inslope the Wall</li>
</ul>
The wall should tilt into the slope. This angle is described as the wall’s batter. Batter should never be hallower than 4:1 (an inward tilt of 30 cm for every 120 cm of height. A 2:1 batter is better.
<h4><img class=”alignnone size-full wp-image-2234″ src=”https://www.imba-europe.org/wp-content/uploads/2022/03/rock-retaning-wall-instruction.jpg” alt=”” /></h4>
<ul>
<li>Step 8: Back Fill</li>
</ul>
After each layer of the retaining wall is built, add a layer of small rock and mineral soil (nothing organic) to fill the space behind the wall. Compact the fill material in layers while it is slightly wet to prevent uneven settling. A mechanical compactor is the best tool for this key step.
<ul>
<li>Step 9: Place Capstones</li>
</ul>
Use lare, flat rocks for the top layer. Their wieght and size will help hold everything together. When you are finished, the top of the retinaing wall should be slightly lower than the surface of the trail to insure proper drainage.
<h4><b>References </b></h4>
<ul>
<li><span style=”font-weight: 400;”>Felton, V. (2004). Trail Solutions: IMBA’s Guide to Building Sweet Singletrack (IMBA (International Mountain Bicycling Association), Ed.). International Mountain Bicycling Association.</span></li>
<li>Havel, R. (2007). Sustainable trail development; A Guide to Designing and Constructing Native-surface Trails. <a style=”font-weight: bold;” href=”http://crgov.com/DocumentCenter/View/1430/Sustainable-Trail-Development-Guidelines-PDF?bidId=”><span style=”font-weight: 400;”>http://crgov.com/DocumentCenter/View/1430/Sustainable-Trail-Development-Guidelines-PDF?bidId=</span></a></li>
</ul>
<h4>Further readings:</h4>
<ul>
<li><a href=”https://www.parks.ca.gov/pages/1324/files/Chapter%2011%20-%20Principles%20of%20Trail%20Construction.FINAL.03.06.19.pdf”>https://www.parks.ca.gov/pages/1324/files/Chapter%2011%20-%20Principles%20of%20Trail%20Construction.FINAL.03.06.19.pdf</a></li>
<li><a href=”https://www.parks.ca.gov/pages/1324/files/Chapter%2013%20-%20Retaining%20Structures.FINAL.03.06.19.pdf”>https://www.parks.ca.gov/pages/1324/files/Chapter%2013%20-%20Retaining%20Structures.FINAL.03.06.19.pdf</a></li>
<li><a href=”https://drive.google.com/file/d/1SXsjjrEmIjy8DeqW3e1XeSS4KCc-dAQg/view?usp=sharing”>https://drive.google.com/file/d/1SXsjjrEmIjy8DeqW3e1XeSS4KCc-dAQg/view?usp=sharing</a></li>
<li><a href=”https://trailism.com/crib-walls-and-retaining-walls/”>https://trailism.com/crib-walls-and-retaining-walls/</a></li>
</ul>