Berm, Baby, Berm
Our latest Trail Care Crew visit brought us to Greensboro, NC, home to about 40+ miles of singletrack, a strong mountain biking organization and a new berm.
We’ve been building a lot more berms lately with our volunteer groups. They are relatively easy to build, rewarding, fun to ride, compatible with multi-use trail systems, and a good solution for getting from the bottom to the top of a hill that’s no steeper than around 30%.
Building a berm is also a good way to teach people the basic principles of working with dirt. These principles apply to any dirt feature like dirt jumps, pump tracks dirt rollers, earthen igloos, you name it. Any time you need to pile dirt and sculpt something that you’d optimally ride your bike over and also expect to stick around for more than a season, follow these basic guidelines. (In IMBA’s Trail Solutions book, we also refer to bermed turns as insloped turns.)
- First, remember that these are general guidelines, so before busting out a tool, take stock of your local conditions, including soil composition, terrain, user activity, and all that jazz. Think through what you’re building and why. For a bermed turn, it’s optimal to source the dirt from the bench-cut trail construction leading in and out of the turn, providing it’s good dirt, but that’s not always an option so make sure you have good dirt before the build. (Good dirt is a conversation unto itself.)
- After identifying the spot that will soon be home to a museum-quality pile of dirt, move out of the way all of the organic material. Organic material is anything that lives or once lived – stuff that could easily burn, like sticks, leaves, seeds, grasses, or anything with roots. It’s the opposite of mineral soil. You want your dirt to lay atop a purely mineral surface. Mineral soil is cooler, usually lighter than organic soil (some people relate it to baby poop) and nothing really grows in it. If you’re naughty and build on top of an organic layer, this layer will decompose and make your dirt foundation crumble, compromising the structure’s integrity and sustainability.
- You said bye-bye to ALL organics by raking and scraping them off to the side (you’ll need them later). Now bring on the dirt. The key is to build up layers of dirt in the general area where your structure will live. Never dump a ton of dirt where you want a berm to be, and simply compact the top. This is futile and makes us sad. You’re looking to build a pile of rock-solid dirt, and you can only achieve the necessary compaction by tamping in layers. Nor should you worry about what your structure is going to look like just yet. As long as you have a general idea of where to pile the dirt, that’s enough at this point. So, layer 1-3 inches of dirt where you’d like your berm to be, and tamp the heck out of it. Use a McLeod, a tamper, the flat side of a Rogue Hoe, your buddy’s size 16 shoe (preferably with his burly self in it), whatever you need.
- Compact and layer and compact and layer again, until you have your desired elevation of dirt. Remember to scruff the top of each layer so that the layer on top of it grabs on to it. Kind of like patching a flat – you’d rough up the tube before applying the smelly glue.
- When your dirt pile is as high as you think it should be (the apex of your bermed turn or the peak of a roller), this is when you can get artsy. Using a shaping tool (any delicate implement with a flat edge), carve out the feature. Imagine yourself riding it, and take some test runs on foot (the wheelbase on a bike is akin to your stride). If the berm feels too tight, you might need to pull the bottom side out a bit; or if the transition into the turn is too abrupt, maybe you need a longer run-in. Be creative, sculpt your little heart out and continue to tamp and compact loose dirt whenever you shift things.
- With any berm, remember to build a slight reversal in grade before and after the turn to sheet any excess water off of the trail so that you minimize water damage to your turn, and also to allow riders coming in hot to scrub some speed. This will help maintain the flow of the trail as well as the integrity of the turn.
- Also remember to incorporate a wide drainage knick at the bottom of the turn (could be part of your grade reversal) to allow water that uses your berm to travel down the slope a graceful exit off of the trail.
- Take some of the cleared organics and “paint” the exposed dirt that will not see a wheel: cover the back of the berm, the inside of the turn, the sides of the trail leading in and out of the berm with leaves and any other organics. This will help seedlings and other happy creatures to take root, hide your work and make your water quality engineers happy by minimizing sediment runoff.
So, remove organics, compact dirt in layers, then sculpt. These are the basics. It's not the authoritative guide to playing with dirt, but berm know-how is an important arrow in your trailbuilding quiver. We are seeing more and more berms built into trail systems. They’re fun, an efficient and fun use of volunteer resources, allow mountain bikers the opportunity to safely carry speed through turns and if built correctly, cause minimal water and user-based erosion. The nice thing about an earthen berm is that as a rider rolls over the outer edge of the turn, its wheels compact the dirt, so each pass (within skidding of course) contributes to its sturdiness and sustainability.
We left behind a berm and 900’ of new trail at Owl’s Roost in Greensboro. Thanks to the Greensboro Fat Tire Society for hosting our visit and organizing a great, sunny weekend, Greensboro Parks and Recreation Department for their continued partnership with the club and support of trail development, and to the weekend’s sponsors: Sticks and Stone pizza, Proximity Hotel, Earth Fare, REI, VF Corporation and all of our wonderful volunteers!