# Bench cut: partial vs. full, lessons



## formica (Jul 4, 2004)

A small group of us did some work today on an urban connector we are working on. It connects a sports complex with a state park. It's a pretty nice deal. The old fall line trail was on an area owned by a cemetery that is being planned for expansion. They gave the city an easement for a reroute.

We've had some work parties, but some guys have been up there working on their own, too. Mostly this is a good thing. Today we spent a whole lot of time converting partial bench cut into full bench cut. The soil is unconsolidated enough that in less that 6 months since it was put in, already the fill was condensing in the middle so that the trail thread was getting concave, plus the outer half of the trail was very loose and not nice at all.

I tried to really pay attention today, so I took a lot of photos in addition to moving a lot of dirt. In some ways it's a shame that this section had to be worked twice and wasn't done properly to begin with. I made some notes in photoshop: maybe this is useful?

How not to do a back slope









Back slope, this is more like it









There was quite a section that had been done partial bench









We ended up moving a huge amount of dirt









The dilineation of the section that had been properly cut and the section that had to be reworked was pronounced









Another look at how much dirt we had to move to make the full bench:









And another









Worth it in the end.


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## Jrkimbrough (Sep 27, 2008)

nice!


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## ebxtreme (Jan 6, 2004)

Finished work looks nice. It's hard to say if the original work was done in vain or not since a lot of the "heavy lifting" seems to have been done with the first pass - even if it wasn't up to standard. In my experience, most people don't go in far enough on the first pass of a bench....maybe they think they're making it too wide, but the vegetation and other organic infill pretty quickly.

The term for how the backslope meets the bench = the "angle of repose". That angle should be shallow...not vertical as the first example shows.

Cheers,
EB


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## formica (Jul 4, 2004)

I suspect the partial cut was done enthusiastically, not intentionally.

Good points, EB. I might re label that first one a bit better. It was actually overhung, the mossy layer holding the gravel together was sturdier than the soil underneath it!

Five semesters of geology, I should've remembered that term "angle of repose".


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## Walt Dizzy (Aug 18, 2003)

Thanks for the photos.

I'm constantly fighting this battle. No matter how many times I demonstrate, I still have to remind people that half bench doesn't work very well. For the exact reasons you show. The difficulty is that the bad stuff doesn't show up right away, and cutting a full bench is a lot of work. Raking the spoil away is even more work.

Walt


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## bweide (Dec 27, 2004)

I think it is helpful to distinguish between accidental and intentional partial bench. 

Intentionally-build partial bench is usually characterized by some sort of a retaining wall to hold the edge of the trail in place and thorough packing of the soil behind the wall.

Accidentally-built partial bench seems to occur when the trail builders lose track of the true edge of the trail due to the disturbed soil that gets packed down by trail building activities. I have seen trained crew leaders convinced they had built full bench trail because the new dirt measured to spec width. One way to quickly check for partial bench is to take a McLeod and try to pull the lower edge of the trail back onto the tread. On full bench trail the McLeod will just slide along the surface but on partial bench the McLeod will cut right through the soil. This is often one of the last tests I perform on a stretch of trail before I release the crew. 

We teach our crew leaders the concept of critical edge which means the edge of the trail should be kept clear of anything that would impede water from flowing off the trail. Crew leaders are instructed to pull soil off the critical edge onto the tread so it can be disposed or to pull the soil well downhill past the critical edge.


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## pinkrobe (Jan 30, 2004)

Our club is running a How To Build day soon, so we can have folks do work outside of formal trail days. This is a great reminder of the importance of doing full bench cuts. We messed up in a few spots last year, and our first trail day this year will be dedicated to fixing the poorly-benched singletrack.


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## zachi (Jul 25, 2006)

Excellent visuals. We go around and around with this issue of partial and full bench. I don't think there is a simple answer and there are many factors to consider. 

I often build with an Excavator and small bladed walk-behind which allows for me to clear organics and rebench good soils. I don't do full bench unless I am on extreme side slopes without alternatives. With soils like depicted in photos there are minimal organics, which is the only thing that is not good building material. Trail building is like road building small scale, you would never see tons of good building soils tossed over the side at a road project. My 4000lb excavator repacks shoulders well and walkbehind sets out slope. I recommend more outslope and frequent grade reversals for sustainable trails. Mechanical wear or center divits will occur equally for both techniques because it happens in the center of the trail and partial bench usually only effects outer 3rd of trail. This is only remedied by maintenance. Your finished trail looks great.

Photos you show looks like most of the work has been done prior. So you don't see the real issue with full bench construction. That is the massive amount of good soil that is tossed over the side of the trail. Not only a waste of good energy but with sedimentation issues you are creating a huge bloom of disturbance that looks really bad. Plants and other organics below the trail are usually killed and recovery takes years in some areas.

I think soil moisture is key, good recompaction will take place if done when soils are moist and prior to rains which will farther restore compaction. Good partial bench construction should also include keying the fill area prior to filling. This creates a unified bench less likely to suffer slippage.

Just some alternative thoughts to consider.


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## Trail Ninja (Sep 25, 2008)

Good point zachi. I usually build very narrow (12" max.) bench by hand so tossing the cut soil is really the only method available. Some of the machine built trails I've seen it would be impossible to "toss" the amount of dirt moved. I just assumed the weight of a mini-ex would pack it enough.


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## formica (Jul 4, 2004)

doing this work, and taking notes on it has been a really good learning experience, and the comments just add layers, thanks.

The amount of soil to be moved is huge. We felt pretty strongly that the soil being broadcast down the hiill should revegetate pretty quickly. And yes, there is a lot of it. I'm glad I've been working out so I could actually move a lot of dirt for:thumbsup: the guys.

Mostly I wanted to share this just as my learning experience. I've been doing trail building with Mike for a long time. He's a great teacher, and this was a really good project to examine the issues with lack of finishing, partial vs full cut and so on. Plus there were only three of us, so lots of good teaching and discussion.


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## BikeWente (Jan 20, 2010)

I agree with Zachi, there are some key situations where a partial bench can be a great option. With my most recent fall trail building project, I have been experimenting with building properly compacted partial bench cut trails. With keeping trails as narrow as possible the outer edge is susceptible of collapsing when riders get to close. Check out some of my solutions.

A jumping jack compactor was used for the tread. A demo hammer with tamper plate was used to compact the downslope. The tarp pictured is coated in vegetable oil on one side and used as a barrier between the demo hammer and soil to prevent the heavy clay soil from sticking to the tamper plate. 

The entire surface is super compact.


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## Trail Ninja (Sep 25, 2008)

Another method I've used when I wasn't able to broadcast the dirt is to do what would look like a partial bench like formica's first picture. Then edge the solid ground on the outside of the trail with logs or rocks (basically right down the middle of the trail in the first picture). Cover the dirt outside the edging with the brush I invariably have to cut. I end up with a narrow full bench with a real pretty log border and no sign of tossed dirt.


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## thefriar (Jan 23, 2008)

Ninja, sounds like fill benching, I've had some success with it but you need to make sure the anchor rocks/logs are tacked down extremely well or else that outside edge become nasty quick.

I think with machine assist it probably a bunch easier to work a partial bench into sustainable trail, but with just hand tools you'll probably have more success going full.

In New England we get enough rain that broadcasting the duff and loan downhill typically doesn't do anything to downhill plants, it just helps the foliage compost quicker! That said, we have a good 3-5'' thick organic layer because of the composting foliage, comes up like carpeting.


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## Trail Ninja (Sep 25, 2008)

thefriar said:


> Ninja, sounds like fill benching, I've had some success with it but you need to make sure the anchor rocks/logs are tacked down extremely well or else that outside edge become nasty quick.
> 
> I think with machine assist it probably a bunch easier to work a partial bench into sustainable trail, but with just hand tools you'll probably have more success going full.
> 
> In New England we get enough rain that broadcasting the duff and loan downhill typically doesn't do anything to downhill plants, it just helps the foliage compost quicker! That said, we have a good 3-5'' thick organic layer because of the composting foliage, comes up like carpeting.


No, it's not fill benching. The logs are to define the outside of the solid ground. I actually lay them well inside the solid area right on top of the ground. the fill goes outside the log but it may look like trail. The log is to stop people from riding on the fill. I usually cover the fill with brush or plant on it if I can.

View attachment 546111


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## Bob W (Jul 6, 2004)

*Issue with Edge of trail*

This goes to Trail Nijas post, how do you acount for gettig water off the trail if you have lined it with logs and/or rock. Seems like this would set up to be problem in the future.

Overall great thread. Would agree with using a machine you can get away with a partial bench, but by hand the extra work you have to do to make the tread stable would equal the amount of work to construct a full bench by hand. That is given a tread width of 2 ft or less.


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## thefriar (Jan 23, 2008)

What Bob said. Seems like the logs will trap water.


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## Trail Ninja (Sep 25, 2008)

The logs are just there to define the edge of the trail, you can use anything. It's a visual deterrent to keep people off the un-compacted fill. Even pin flags would work.

Because they are laid on top the ground (pegged in lightly to keep them from rolling if/when hit) *usually* water will run under the natural spaces. I use grade dips to focus water (if needed) and just dig a scupper or leave a little space between one log and the next. Sometimes it's just a line of stones and not logs at all.

I use this method in areas where tossing the fill isn't practical so I need to just scrape the fill off the edge of the bench cut. Tight switchbacks where the next level is where the fill would end up if you toss it would be one example.


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## sambs827 (Dec 8, 2008)

formica said:


> Five semesters of geology, I should've remembered that term "angle of repose".


YAY ROCKS!!

Yeah..I can be ashamed of myself for forgetting that term as well. Cool photos, good learning tool! I see that in your geology classes they taught you to use scale in photos (McLeod, person, etc.)


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## Fattirewilly (Dec 10, 2001)

Trail Ninja said:


> The logs are just there to define the edge of the trail


+3 on not lining the trail with logs. Even if done to the level suggested, it still requires more future work to keep up, at least where you have leafs and smaller sticks that can clog your drains and create a dam along the "critical" outside edge. Sure grade reversals are key but don't kill your outslope straight after you build it.

When I need to define what the tread is and what the spoils are to trail users, I'll take a log and turn it perpendicular to the trail, not parallel as illustrated by Trail Ninja. One log at 90 degrees to the trail every 20-30 feet is plenty to chock users onto the correct part of the trail Occasional rocks here and there work too, just not a wall of rocks above the outside edge.

Building a full bench is far easier than building a correct 1/2 bench with a retaining wall. I typically build a half bench if I'm worried about the roots of a large nearby tree. If you're using a machine, just put the spoils back on the tread and make rollers/larger grade reversals out of them if your worried about broadcasting the dirt downhill.


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## Trail Ninja (Sep 25, 2008)

Fattirewilly said:


> +3 on not lining the trail with logs. Even if done to the level suggested, it still requires more future work to keep up, at least where you have leafs and smaller sticks that can clog your drains and create a dam along the "critical" outside edge. Sure grade reversals are key but don't kill your outslope straight after you build it.
> 
> When I need to define what the tread is and what the spoils are to trail users, I'll take a log and turn it perpendicular to the trail, not parallel as illustrated by Trail Ninja. One log at 90 degrees to the trail every 20-30 feet is plenty to chock users onto the correct part of the trail Occasional rocks here and there work too, just not a wall of rocks above the outside edge.


Absolutely right. I didn't mean it to sound like I was lining the trail with logs or rocks. Quite often nothing is required. Just replant or cover the fill with brush.

If you knew the terrain I work in, you would know why I don't worry about run-off. I had 4 feet of snow melt in a week. No run-off, and one day later, no puddles.


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## BigJay (Aug 15, 2004)

Don't line the path to grandma's house!

All you need is to cover up the edge of the trail and revegetate the side of the trail. Hide the work as much as possible...


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## Woodman (Mar 12, 2006)

I will agree with my fellow PTBA member Zachi.

We like to use the term "fully compacted bench" instead of "full bench". With machine building, we can get a high level of compaction using the machines. The inner edge of the trail will be full bench and the outer edge will be compacted bench. Machine build tends to have a wider finished bench than is really needed, so if some of the outer edge creeps that is OK and yields a narrower active tread that is certainly stable. We have a plate compactor that goes on several of our machines (DitchWitch) and we use it often to get a really high level of compaction across the full width. This helps a lot fight off the outside edge berm effect due to user compaction in the center of the tread (esp. true with horses on trails).

Other key factors are soil moisture content (dirt does not compact if too wet or too dry) and soil content. Dry western soils simply will not compact, not matter what you do. I have run a plate compactor in very dry soils and was only making dust. Soils need some clay content or other binder to compact.

Hand built trails need to be full bench. Simply put, hand tools and human power can not compact soil enough for that to be used as trail tread.


Woody Keen
President- Trail Dynamics LLC
President- Professional Trailbuilders Association
Certified NC Clear Water Contractor


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## Skookum (Jan 17, 2005)

.



Woodman said:


> Hand built trails need to be full bench. Simply put, hand tools and human power can not compact soil enough for that to be used as trail tread.


That's going a bit far... i mean really if this were true, dirt jumps simply wouldn't exist.

If we are talking about the virtues of benching trail, it's good to recommend better practices, but yah i'm not going to buy into the premise that all tread has to be hard pan.


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