# Benching and Slope Stability



## null11 (Mar 27, 2008)

We have been working on a trail reroute that traverses a 40-60% side slope at sustainable grades (<10%) and incorporates sustainable drainage features. This trail is to replace an unsustainable trail built on clay/sandy/rocky soil that exceeds 20% grade. The soil type the reroute passes though is mostly clay with small (pea sized) to medium (quarter sized) rocks with 6-8" of loam type soil on top. There are quite a few mature hardwood trees in the area to help hold the hillside together and there are no visible seeps or underground water flows.

The land manager is worried that by constructing full bench trail in the area we will destabilize the hillside because of the large amount of soil removed. The tallest backslope is right at the 3' mark, but the majority is 2' or less. There have been slides in the area in the past, but only where the primary stream has eroded the base of the hill and during times of above average precipitation. The area of concern is not on a hillside touched by the primary stream. 

We have stopped all progress on the trail and it is a real possibility that we will have to abandon the project if we cannot convince the land manager that the hillside is stable. We are about 3/4th the way complete with the trail and it would be large waste of effort to abandon it at this point if there was not a real stability issue.

So, my question is, does this seem like an instance where the trail could cause the hillside to slide? Has anyone ever heard of a bench cut trail triggering a slide under similar circumstances before? Any suggestions on how to convince the LM that we are safe if that is the case?


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## M5Tucker (Aug 8, 2011)

I've seen areas of bench cut slide, but not in a big way, and only when water had destabilized the hill. Most of our soil is pretty loose, sandy & rocky though, not really nice clay. I'm sure guys here with more years under their belts could give better examples. 

With nice clay, no running water through, and good root structures in the hill, I don't think you'll have a problem, but the more you can do to keep the angle of the backslope down (45 degrees) will help minimize the possibility of it destabilizing. If there are any specific short sections the land manager is worried about, would some kind of retaining wall be out of the question?


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## Fattirewilly (Dec 10, 2001)

null11 said:


> We are about 3/4th the way complete with the trail and it would be large waste of effort to abandon it at this point if there was not a real stability issue.


Are you working from the bottom or the top? If you're 1/4 not completed is at the top, not nearly as much hillside weight left to push on it...if you're buying into this.

Does the land manager have something against benching or is there something specific to the geology of the area that's causing concern. Hillside on a neighboring ridge or anything?


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## HypNoTic (Jan 30, 2007)

We machine-built some fulll benchcut on +70% slope without need for artificial stabilizer. We used agressive backslope (smoother and twice as long as usual) which was seeded for fast stabilization but as soon as the local vegetation came up, it wasn't an issue anymore. Also, for 3ft wide thread, you will remove a lot of material so you might want to consider 3/4 bench and merge the excess with the lower slope and/or use it to enhance drainage. Finally, make sure to compact the material as you go. You should be okay with that. If that's not enough, rock retaining wall is another option.

You can always setup sediment barrier below the trail to catch run-off and bring back the LM in a year to show him how your trail is holding up ;-)


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## null11 (Mar 27, 2008)

There have been a few soil movements on neighboring hillsides, but on this particular hillside there has been only one rather small soil movement (25'x25'x2'). The others look like they were caused primarily by the stream and a majority of the larger slides occurred on north facing slopes.

Our tread width is 2' and the backslope is ~60 deg.around the deepest bench mainly to minimize damaging the root systems of the large neighboring trees.

We started in the middle with this trail, where the sideslopes are the greatest, and are working our way towards the ends. It is really only a section that is around 50 yds in length that goes through the 50-60% slopes and the slope levels out to around 30% about 20' higher above the trail. We couldn't route the trail up there because of the topography of the area; I would have preferred that option. Retaining walls are probably not an option at this point.

I don't know if the LM is worried about the upper section of the hillside falling onto the trail or if the dirt removed for the trail will cause the hillside to fracture and go sliding down the hill. On the failures you have seen, how does it usually happen? We have a meeting in a few days with the LM to get all the specific concerns out in the open, but for now I need some information to fall back on.

The LM has not been too friendly in the past with the development of this trail system and I get the feeling this is more about halting our progress than an environmental concern. LM doesn't know a lot about trail building (and doesn't want to learn) so it is difficult for them to understand why we are doing what we are doing (full bench cut). I would really like to have solid evidence to put these concerns to bed and get on with the project. I appreciate your responses.


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## Visicypher (Aug 5, 2004)

*Hope this helps.*

All depends on the soil type. As Willy said, soil moisture saturation will play a factor in the slope stability. Clay soil types you described may be subject to instability if it has a substantial amount of moisture. We the slopes and soil type you described, the saturation would be the biggest factor, then maybe high load on your trail initiate a slide.

Your local soil conservation office should be able to help you with soil stability and soil classification.

You may want to read this so you can educate the LM in question...AND show him that your trail design address the management objectives.


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## Ridnparadise (Dec 14, 2007)

At 60-70%, some sort of slide has to happen eventually. It may be rocks, dirt (maybe snow), or it could go way down to a layer of clay. I have seen a large hillside slide metres deep down on a clay layer loaded with wet above. Grade was only 10-20%, but there was a "bench" above it that loaded like wind loads an avi zone with snow.

At 40-60% in a stable zone it may be fine. But a 60cm tread width sounds impossible - at least until the line is run in by thousands of riders. That superficial trail will fall apart in no time regardless of slides. Honestly, I would not ride a 60cm trail on a 40-60% traverse, because one mistake is going to be seriously, medically ugly. 60% means you will hit your bars on the upslope sooner or later. Also, your LM should not be demanding such a narrow trail because it will lead to damage to the local environment, not to mention the insane decision to close a trail because one bit was made not quite well enough. Why destroy the bush for nothing?

The pics are of a 40-45% grade - don't look it. There is a contour that allowed us more lee-way so it is not like your slope at all really. But, the riding line is on solid base. It is obviously not yet cured in these early photos. The rocks are about as wide (inslope to outslope) as they are high and outside the riding line. All the soil was harvested from the bench. All the rocks were harvested by hand from the trail or next to it - lucky, but oddly terrain supplies the materials it needs for its own trail. It it doesn't, I'd be worried, because it may not be meant to be there (OK it's a hippy approach to trail building, but it works).

This may be a very controversial opinion, but I think you should abandon the single single-track you have described, solidly bench this difficult and vulnerable part of the trail and make it wide enough to handle surface sides and heavy water flow. A border of rock and stone will start to soak up the fine sediment and solidify over time. Without bad luck that will be before it slides and nothing but a deep-to-bedrock event will screw things up. 

Hope you get smiles and miles and a LM who has to admit it can be all good.

Oh - do you like the twisted little tree?


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## Ridnparadise (Dec 14, 2007)

Sorry, my bad - I was using 100% = 90 degrees = vertical. So the trail the OP was discussing just became a lot easier in theory. The pics are of slopes up to 35-45 degrees.


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