# Survey terminology question: What is a "grade offset mark" and what is its purpose?



## Walt Dizzy (Aug 18, 2003)

*Survey terminology question: What is a "grade offset mark" and what is its purpose?*

I am in the planning stages for a trail reroute. The reroute has to travel over a fairly steep hillside. I've measured the slope in one area at approximately 36%, and it's steeper elsewhere. The usual limitation applies in that there isn't enough land to do a simple traverse. I will need to build at least two, and probably three turns on the steep areas. I'm convinced that my usual climbing turns (as defined by IMBA) won't hold up on a slope this steep.

I have found this resource on the net for switchback theory and layout:

http://www.scn.org/sbtp/swbktheory.html#grade

The author sounds to me like he knows his stuff. Unfortunately for me however, some of the explanation is less than perfectly clear.

The author makes use of stakes for establishing the grade of the turn. The stakes are to be driven into ground a little bit below the outer edge of the proposed trail. This allows the trail to be dug without disturbing the stakes. I *think* the grade mark is there to show how to establish where the trail is (because the stakes don't actually mark the edge of the trail). This makes sense so far.

But then the author goes on to talk about "grade offset marks". It's not at all clear what the function of grade offset marks are.

Is it the y-axis distance between the stake and the outside edge of the trail? Doesn't seem likely because the grade mark is supposed to determine the trail edge, if I understand his process correctly.

He states that the grade offset mark is needed because the grade may be below the ground surface. I have two problems interpreting that statement. First, the grade is almost always below the surface where the trail is being dug. The only exception I can think of is a place where a low spot would be filled. But then an offset mark wouldn't really be needed, would it? Second, if the stakes are driven lower on the slope than the trail, how could the grade mark for the trail be below the surface where the stake is located?

I thought I had it figured out last night. Perhaps the offset is there to allow workers to figure out where the grade is when they shovel dirt over the edge of the trail and partially bury the stakes. I felt pretty good about it until I read in a different location that you might need to mark a negative offset. That statement makes me think I really don't understand grade offset marks after all.

Anyone out there have experience with this concept and are able to explain it clearly? Or do you have contact information for the author, J. Johnson?

I'd also appreciate links to other resources explaining switchback layout.

Walt


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## JDM (May 2, 2008)

Thanks for the link. 

The grade offset seems to mean the elevation offset from where a mark is drawn on a stake do to where the final tread will be. You use an offset because the final tread height is usually below the initial surface.


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## Walt Dizzy (Aug 18, 2003)

JDM said:


> Thanks for the link.
> 
> The grade offset seems to mean the elevation offset from where a mark is drawn on a stake do to where the final tread will be. You use an offset because the final tread height is usually below the initial surface.


Yeah, that's probably it.

I guess that what's making me miss the obvious is that I'm thinking about building trail on a steep hillside where the grade mark (remember it's on a stake driven down hill from the trail edge) would probably not be below the surface at the stake. But that would still make it below the surface of the not-yet-built trail and difficult to read for a worker standing above it. The author doesn't state it this way, probably because the article is aimed at the person doing the layout work, not the digger of trail.

Thanks,
Walt


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## bweide (Dec 27, 2004)

One of the challenges of trail layout is how to stake trail where some on the trail will be deep underground and some will be hanging in the air. I don't understand what grade offset means but I am guessing it is a way to help visualize the three dimensional trail. That is a very interesting website on switchbacks but I disagree with some of his conclusions, in particular that the distinction between switchbacks and climbing turns being irrelevant. 

One trail layout suggestion regarding switchbacks on a slope is to wander all over the hillside looking for the flat spots (as flat as possible) or at least spots that require less construction effort. Considering the amount of work that goes into properly building a switchback, you can justify building a lot of extra contouring trail to tie existing flat spots together.


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## Walt Dizzy (Aug 18, 2003)

bweide said:


> One of the challenges of trail layout is how to stake trail where some on the trail will be deep underground and some will be hanging in the air. I don't understand what grade offset means but I am guessing it is a way to help visualize the three dimensional trail. That is a very interesting website on switchbacks but I disagree with some of his conclusions, in particular that the distinction between switchbacks and climbing turns being irrelevant.
> 
> One trail layout suggestion regarding switchbacks on a slope is to wander all over the hillside looking for the flat spots (as flat as possible) or at least spots that require less construction effort. Considering the amount of work that goes into properly building a switchback, you can justify building a lot of extra contouring trail to tie existing flat spots together.


Your point about finding a relatively flat area for the turn is a good one. I've used it!

I think you are right about the offsets.

My understanding of switchback vs. climbing turn comes out of the IMBA literature. It may be similar to yours in that climbing turns are limited, if you want them to last, to places where the fall line slope is equal to or less than the maximum slope that the local conditions permit. In contrast, a switchback (per IMBA) has a level, or nearly level area at the apex of the turn. And so is more suitable for steeper slopes.

I think the author of the linked article is trying to point out that there isn't a distinct point where a climbing turn suddenly becomes a switchback turn. In his view, building a good turn isn't about getting a flat platform, it's about managing the grade of the trail at all points around the turn.

My disagreements with the SBTP approach, so far, are:

-He's obsessive about grade. In my experience his point that the soil and other local conditions probably dictate a maximum sustainable grade is true. His corollary point that building only to average grade can lead to local, unsustainable micro-slopes is probably also true. However, I want to build trails, not highways! A much better way to handle this IMO is to keep the overall trail grade far enough below the maximum so you can have things like grade reversals, dips, and other things that make riding interesting without exceeding your maximum grade. In places where you have to max the grade to hit a certain point on a hillside, obsessiveness makes more sense, but I don't get that distinction from the article.

-My current understanding of his system of turn layout results in all the ramp swing-out going to the upper leg. (This is explained better in the article, but the short version is that the vertex where the upper and lower leg of the trail join to make a turn doesn't allow for the width of the trail. One or both sections of trail have to be "pivoted" away from the vertex to pass the tread around the vertex.) This will entail a lot of digging to ramp the upper leg correctly. That may be the better trade off compared to building a large retaining wall for the lower leg, but if so, he doesn't explain why.

In the attached picture (property of J. Johnson), V marks the intersection of the flag lines. CT is the center of the turn, the red dashed lines are the flag lines. In this layout, the outer radius of the turn ends up on point BT, very close to the lower flag line. I would think the turn center should be closer to BTI. Why not?

Walt


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## Fattirewilly (Dec 10, 2001)

Walt Dizzy said:


> I am in the planning stages for a trail reroute. The reroute has to travel over a fairly steep hillside. I've measured the slope in one area at approximately 36%, and it's steeper elsewhere. The usual limitation applies in that there isn't enough land to do a simple traverse. I will need to build at least two, and probably three turns on the steep areas. I'm convinced that my usual climbing turns (as defined by IMBA) won't hold up on a slope this steep.
> 
> I have found this resource on the net for switchback theory and layout:
> 
> ...


The author doesn't make use of a clinometer measuring grade. You already have 36% where you want the switchback. Going from memory, IMBA doesn't recommend a climbing turn on any slope above 7-8%.

I think IMBA recommends 1 foot of retaining wall for each 12% of grade, so you're looking at about a 3 foot retaining wall. If you don't have flat spots, find a spot with a large rock that would already be in place for your retaining wall.

The "rolling crown switchback" is insloped on the upper leg. the middle of the platform is sorta like a pitcher's mound that sheds water all directions. Unfortunately IMBA has deleted all their links in the website overhaul. Found this...

http://www.poedunk.com/wp-content/uploads/2007/06/switchback.pdf


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## radair (Dec 19, 2002)

Offset stakes in construction are used outside the area of disturbance so the stakes don't get wiped out. Generally they're put a distance from the centerline as noted on each stake. Common in road work; you've probably seen them just off the edge of the roadway when they're doing final grading and just prior to paving.

Some info here


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## Walt Dizzy (Aug 18, 2003)

Fattirewilly said:


> The author doesn't make use of a clinometer measuring grade. You already have 36% where you want the switchback. Going from memory, IMBA doesn't recommend a climbing turn on any slope above 7-8%.
> 
> IMBA recommends 1 foot of retaining wall for each 8-10% of grade, so you're looking at about a 4 foot retaining wall. If you don't have flat spots, find a spot with a large rock that would already be in place for your retaining wall.
> 
> ...


Thanks for the link.

The specification of 5% grade ( or less) leading into and out of the turn makes sense to me. Or it's at least a good target to shoot for.

The point about having a foot of wall per 8-10% grade is troubling. A 4 ft wall is huge, and I have to build three of them. Your point about finding a big rock (or a platform) to build on is a good one, but my preliminary survey isn't encouraging. I'm seeing steep hillside, lots of small to medium size rock, and not much else.

I'm starting to wonder if the SBTP author's way of laying out the turn makes more sense: by swinging the upper leg of the turn into the hillside I might end up having to dig deeper, but as a result the retaining wall on the lower leg would be shorter. Something to ponder over the winter.

From SBTP, Geometrical Derivation of a Switchback:

"Even if there is a sufficiency of rock, the effort and time of building a proper wall may exceed that of doing a full excavation, so in the end it may just come down to aesthetics and personal preference."

The difference for me is the dirt could be thrown downhill, and the rock has to be hauled up from the valley below. Granted, it's only a hundred foot climb at most, but that's a lot of work if tons of rock are involved.

Walt


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## Fattirewilly (Dec 10, 2001)

Walt Dizzy said:


> Thanks for the link.
> 
> The specification of 5% grade ( or less) leading into and out of the turn makes sense to me. Or it's at least a good target to shoot for.
> 
> ...


If you have hikers, it may be beneficial to put more "real estate" between the switchback legs, particularly if the switchback lacks a strong anchor or visual focal point that draws walkers to the platform, discouraging short cutting. You'll add real estate obviously by coming in at angles greater than 5%. If you have a great view or a cactus for a barrier, the 5% makes sense.

I've lent out my IMBA trail solutions book...I think the foot of wall per 8-10% is wrong and should be about 12%. I've edited my post above. Think about it like this, a level 8 foot wide turning platform (as shown in the illustrated link) will have drop of 8 x 36% or 2.88 feet on a 36% slope. 36%/2.88=12.5%. For a 10 foot platform the math is 10%.

I've certainly taken some (up to half?) of that 2.88 feet out of the slope on the upper leg and pushed it behind the wall on the lower leg.


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## Walt Dizzy (Aug 18, 2003)

Fattirewilly said:


> If you have hikers, it may be beneficial to put more "real estate" between the switchback legs, particularly if the switchback lacks a strong anchor or visual focal point that draws walkers to the platform, discouraging short cutting. You'll add real estate obviously by coming in at angles greater than 5%. If you have a great view or a cactus for a barrier, the 5% makes sense.
> 
> I've lent out my IMBA trail solutions book...I think the foot of wall per 8-10% is wrong and should be about 12%. I've edited my post above. Think about it like this, a level 8 foot wide turning platform (as shown in the illustrated link) will have drop of 8 x 36% or 2.88 feet on a 36% slope. 36%/2.88=12.5%. For a 10 foot platform the math is 10%.
> 
> I've certainly taken some (up to half?) of that 2.88 feet out of the slope on the upper leg and pushed it behind the wall on the lower leg.


Your math seems correct, but my (limited) experience seems to be that the need to cut into the slope to seat the first row of rocks, and the need to lean the wall back into the slope results in the wall always ending up more work than I'd estimated. Thanks for your confirmation that it's feasible to move grade between the legs.

We're definitely short of cacti for shortcut prevention here in the Great Lakes region!

The issue of preventing shortcutting the switchback is interesting. Actually, this has completely ruined my first serious attempt at building one.

The IMBA design shows what is essentially a zero radius turn resulting in minimal separation between the legs. In the side view, a retaining wall is constructed supporting the upper leg and is extended upward forming a berm. This would probably be effective in preventing shortcutting the switchback (less and less so toward the vertex), but I don't have any actual experience. Comments anyone?

The SBTP design, in contrast, emphasizes using a radius at the inside of the turn. This separates the upper and lower legs. It might be easier to put a barrier to shortcutting between the legs with this design. Come to think of it, the climbing turns I've built have no problems with shortcutting, possibly for this very reason.

I'm guessing the trade off is with a wider turn radius, there is more elevation difference across the turn, resulting in either a taller retaining wall for the lower leg, more digging for the upper leg or some combination of both.

Again, I'd appreciate the benefit of your experience.

Walt


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## langford (May 7, 2004)

Another thing about offset stakes is you can use a system of batter boards to make sure you have the right crossfall and slope on a road being constructed. I don't think you'd need that kind of detail on most trails, though.


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## Trail Ninja (Sep 25, 2008)

Walt, a simple rail fence works well to discourage shortcuts. It doesn't have to be much. 3' high and one rail will do it.


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## slocaus (Jul 21, 2005)

Agreed, we use them where the legs of a switchback cannot be space very far apart in certain terrain, usually when it is not steep enough, but a switchback is the only choice. You can see the down leg of the this one just to the right of the post.



There is a view bench and tools stashed behind me. I lock the bike to the rail and to trail maintenance from here.


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## zachi (Jul 25, 2006)

I do a lot of trail building near Downieville California. Major steep.

We have developed some sweet switch back techniques. Attempting to assimilate all the techno babble on J Johnsons page made me head hurt.

We use mechanized equipment (mini ex & skid steer with adj blade)

Here are a couple key elements I recommend considering...
*Biggest wear on turns is speed. Design approach grade so that people do not skid into turn.
*Place grade reversals on both legs 50ft from turn.
*I consider turns on side slopes, minor control points...meaning I anchor my alignment to them when doing recon. I look for less steep area, significant element to pivot around, good soils & good building material. 
*My turning bench is almost level and utilizes a 4-9ft radius. Flat near center, banked on the outside.
*I excavate the up hill side DRAMATICALLY. Sometimes I start digging down (increase uphill sidewall height) 50ft prior to the turn. This creates a channel (walls on both sides) that allows me to push soil down the trail to the bottom of the turn without spilling downhill. I with decent moisture, I can build up the lower part of the trail in 6" lifts with good compaction. (prior to lower turn build up, all organics must be removed and bench leveled to key in new build up)
*On extreme slopes, I excavate the center to maximize available resources to build lower berm of turn. Excavated area drains along uphill side of lower turn to the grade reversal.









Pretty minor slope here but best I had handy.

Some ideas to chew on...

zachi


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## J. Johnson (Feb 20, 2011)

Glad you all found the Switchback Theory and Principles pages (http://www.scn.org/sbtp/). Ask the author if you have any questions. (There is contact info on the page, but some study may be required.) There was some revision around Nov./Dec. that might clarify the confusion about grade offsets, but possibly more work is needed?

As to prior questions: I don't believe anyone else has studied or analyzed switchbacks. And if there is anything else similar on the Web it is very well hidden.


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## Walt Dizzy (Aug 18, 2003)

J. Johnson said:


> Glad you all found the Switchback Theory and Principles pages (http://www.scn.org/sbtp/). Ask the author if you have any questions. (There is contact info on the page, but some study may be required.) There was some revision around Nov./Dec. that might clarify the confusion about grade offsets, but possibly more work is needed?
> 
> As to prior questions: I don't believe anyone else has studied or analyzed switchbacks. And if there is anything else similar on the Web it is very well hidden.


Your replies to my questions were timely and most informative. Thanks again.

In my opinion, J. Johnson's write up is well worth the effort to understand.

Walt


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