2nd Berm Scott Community College

[NCoulter]

Hey Folks:

I wanted to hear from the court of public opinion, in case I am missing something. Mods please advise if I need to make changes to the post.

On Sunday, I went out to remove a metal fence post, which had been plaguing toes and tires on the Scott Community College trail, for years. It also had me terrified of landing on it face first, since it was mid trail on a corner and jagged metal.


Post In.jpg

Post Out.jpg
I figured, while I was out there, I would fix the 2nd berm, which had a very shallow slope to it (maybe 45 degrees). In this form, you can't really use it as a berm because if you hit it fast and leaned over, you would fly right off the top of it, since it has little slope and no lip.

I cut more slope into it and extended it about 5' because it fell away where you would still want to counter the g forces of the turn.

Well... I went out there today, and someone had packed dirt back in to remove the slope I put into it.

Can whoever did this explain why they changed it back. I assumed everyone would appreciate what I did and agree with me that it makes the corner safer. I am genuinely confused about why anyone would want the berm to be in this shape.

I recognize that it looked bumpier after I cut the slope in, but that is because I was cutting it in from packed dirt and I couldn't smooth it because the dirt was too dry and crumbly. I was going to smooth it after it had been ridden and the dirt was moist again.

For everyone's reference-

My berm:
My Berm.jpg
Berm today:
Today Berm.jpg

[rohnerj78]

I get an error 404 url not found on every photo link.

[NCoulter]

Sorry... Can you see them in the post now?

[bleedfororbea]

do you work with scott community trail crew?
if not you may want to get with them.
if you do not work with them. have not contacted them. then technically you are changing trail features without approval.
if you contact them im sure they would like the help maintaining and improving the trails! im sure you guys could come to an agreement on how certain berms should be built and where.
i think most of them work at Healthy habits bike shop .

[NCoulter]

Fyi- I just met Alex out there and we discussed it. All good and understood.

To answer your question Orbea... It's true that I did not get approval. However, I do quite a bit of small maintenance out there without approval, and I think everyone should be regularly doing things like filling braking bumps, trimming back brush hanging in the trail, rebuilding collapsed or eroded sections, etc... without involving the trail steward. Going back and forth trying to describe it in detail over email would be too cumbersome, when the solution is obvious and takes little time.

In this situation, I didn't understand why there was a problem, so that is why I made this post.

Regardless, it is all sorted now.

Additionally, I want to say that Alex and Healthy Habits have created an excellent trail for people like me to ride to straight from our front doors. I appreciate the work they put into it.

[DH001]

Opinion from a long time steward/builder/maintainer: people doing little maintenance things like clearing small branches, fixing small erosion areas, trimming back vegetation, etc. is great and saves us a bunch of time. However, rebuilding a berm is something I'd want to know about if it was happening at a trail I'm overseeing. Sending the steward a note takes seconds and they might have relevant input about that project/feature. I've seen many fixes from people trying to be helpful that end up causing more work for the trail crew. There is a science to trail-building that may not be obvious to the average trail user. Or, the trail crews may have plans to rework the issue/feature completely in which case it would be better to put effort into a different project. Communication is king.

Hard to tell from the photos, but it looks like there isn't enough "meat" to support a tire up high on that berm and it doesn't look well compacted? Rule #1 of dirtwork: don't do it when the soil can't be compacted. If we get a heavy rain that loose dirt will be washed away... The reason I don't build berms with a steep face is because they are a high maintenance item. Local soils will only hold a slope of 30-35 degrees reliably (research "angle of repose" for soils if you want to get into the weeds). Drainage towards the exit of the berm may have also been a factor, but again, hard to tell from photos. Many downhill berms require a drain at the exit and extending them too far will block water from flowing off the trail and create a mud pit.

[NCoulter]

Thanks for your reply DH001. To clarify, I didn't rebuild the berm, I cut into the slope (at most 3 inches down low) where I figured the berm had settled flat, so I didn't remove any support from the backside of the berm. The stuff on top is what I had taken away. It definitely needed more rain, to be packed in hard, but the berm face below it has years of compaction. The line you see in the photo after I cut into it is actually a root line, from vegetation. As for drainage, I realize it is difficult to tell from the pictures, but this is a downward sloping berm. The drainage point is further down the hill, and I did cut a small channel for drain exit, at the low point, just because I was there with a shovel.

However, I am interested in this statement: "The reason I don't build berms with a steep face is because they are a high maintenance item." Can you explain this further? Additionally, I am wondering where you build berms geographically and what you consider steep.

[DH001]

Dirt berms naturally erode and the soil needs to be put back where it came from eventually (ie rebuilt/maintained). If the face angle of a berm is steeper than 30ish degrees this erosion happens much faster because the "equilibrium" angle of the soil has been exceeded. Like I mentioned, read up on angle of repose for further clarification. The critical state is when soils are very dry and easily pulverized by bike tires. Local soils are very non-cohesive when dry and break down into fine powder.

I've built dozens of berms at Illiniwek, Dorrance, as well as other local trail systems. Soil types are similar across the QC area so the same guidelines will apply; >30-35 degrees is steep for local soils and will result in accelerated erosion. When maintaining many berms across multiple trail systems with limited volunteer power it is best to create features which minimize the amount of maintenance required.

If a berm needs a face angle steeper than 30 deg, I'd start looking into using different materials (pavers, rock, wood) or increasing the turn radius/trail routing. Formulas below can be used to estimate the required berm radius/face angle. With a 30 deg face angle, a radius of 20-25ft is sufficient for up to 15mph which is a pretty quick pace for most local MTB trails. No need to go steeper unless the berm is constrained to a very small space.


Find turn radius given rider speed & berm angle:

r = v^2 / ( tan(a) * g )

Find berm face angle given rider speed & turn radius:

a = atan( v^2 / (r * g ))

Variables:
a = Berm Face Angle (degrees)
r = Turn Radius (ft or m)
v = Rider Velocity (ft/s or m/s) [1 mph = 1.46 f/s]
g = Gravitational Constant (32.2 ft/s^2 or 9.81 m/s^2 )