the ideal flow trail dirt mix- Mtbr.com
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  1. #1
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    the ideal flow trail dirt mix

    I have a unique (for me) opportunity to build ~3km of trail at a location where my crew can import dirt from multiple truck access points. Our natural substrate is granite bedrock slab, bouldery glacial till, and deep organics -- instead of working with that mess, for once we can use 'dirt to order'!

    The trail will be a green circle flow trail, with mild berms and rollers mixed with our natural granite slab in places. It will be an entrance trail with a high volume of users.

    Our climate is maritime temperate, so quite a bit of rain and freeze-thaw. Most of the alignment is a mild side slope. I'm pretty confident we can control drainage using trail grade and clear-stone+geotextile french drains where needed.

    What dirt mix would you order? Obviously we want a lot of cohesion. I can get pure clay, and have it blended with other material. My instinct is a 70% clay, 30% sand mix, but I am a hand-build-natural-singletrack guy and don't have much relevant experience.

    Thoughts?

  2. #2
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    Loam. Roughly equal fractions clay, silt and sand. 0 organics.

    The organic broken-down material that everyone in the mountain bike culture calls “loam” is properly referred to as duff and/or litter.

  3. #3
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    One stretch of my trail has some clay/sand mix. When it’s wet, it’s like moldable gold. The sand when bonded with clay basically turns into grippy concrete. Hard to beat that surface for a flow trail!
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  4. #4
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    Quote Originally Posted by tbmaddux View Post
    Loam. Roughly equal fractions clay, silt and sand. 0 organics.

    The organic broken-down material that everyone in the mountain bike culture calls “loam” is properly referred to as duff and/or litter.
    Thanks. I also prefer to use the soil science, not Instagrammer, definition of "loam"!

    The recommendation for loam seems common (eg. the IMBA books). I guess I'm pushing back, or probing, that recommendation a bit --at least for my application of flow trail berms and rollers on which I can use mechanical compaction.

    Have you tried sandy clay or clay loam, for well-compacted berms and rollers?

    One reason I'm interested in a higher clay fraction is that my local pump track builder uses a mix that he says is mostly clay because it holds up better than the more equal ratios he used in the past. I've also ridden on trails in areas with soils that seem naturally clay-rich, and once that dirt is shaped and compacted properly the berms seem pretty bomb-proof and shed water beautifully because they are so impermeable. I understand that working with clay-rich material can be problematic ("pudding" that is slow to dry to the H2O content at which one can compact it, for example). However, I'm willing to work through those issues if the end results are turns and rollers that are fast and durable.

    Maybe I'm overthinking this, or just wrong. I feel like there is probably a lot of knowledge about this amongst bike park builders, but that's not my normal realm.

  5. #5
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    One thing to consider is adding limestone fine aggregate. The best surface I have ever rode is Sunnega flow trail in Zermatt. That are videos of its construction on YouTube (but in German). That area is loaded with limestone and I rode sunnega in between some rain showers. That trail surface was frickin amazing. Limestone slightly dissolved in rain and the runoff is very rich in calcium which I would guess is a pretty good bonding agent. Calcium carbonate being the basis of concrete and all that.

    So maybe clay 60 and 20/20 sand and crushed limestone? Never tried it but I bet it would be stellar.
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  6. #6
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    Thanks! I'll look into that.

  7. #7
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    Clay mixed with sand does sound like a good soil for rollers and berms.

    I build in a park which has clay with no sand (under an organic layer of course). The clay shapes and compacts well. Like any other fine material, it erodes away quickly under any substantial water flow. So water control features are an essential part of the trail design.

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