Switchback Theory and Principles:

Flagging and staking a turn.

The key princple in building switchbacks is to respect grade, and the the fourth rule of respecting grade is: Measure it! But measurements are evanescent (i.e., just so much marsh gas) if they are not marked somehow, such as with flagging and staking. Some aspects of this were covered in the section on Measurement. Here I would like to cover some other aspects, and then demonstrate how to stake a reversing ("switchback") turn.

We must first distinguish between reconnaissance flagging and construction flagging. The former is the marking of possible routes, and it is generally adequate to hang flagging every 50 feet or so from any convenient branch. Some attention should be paid to overall grade, and to possible challenges (e.g., exposures of bedrock), but as long as the parameters are not pushed too far, and there is sufficient margin to handle any problems that might be encountered, then it is reasonable to be somewhat casual about grade, and even the exact location of the tread. (But do not forget: only if the parameters are not pushed too far.)

Construction flagging is the marking of where to construct¹ tread or structures; on steep slopes or near any kind of special construction (such as a reversing turn) it needs to be a lot more precise. The random branch is no longer adequate, especially if it is likely to be cut. For construction flagging it is better to use "pin" flags,² generally placed at intervals of 100 inches. Where careful vertical control of grade or associated structures is needed stakes should be used.

Placement.

There is some debate regarding the best placement of the flags/flagging/staking relative to the trail. The options are:

top of the cut-slope.
bottom of the cut-slope (inner edge of the bench).
center line of the trail.
outer edge of the bench.
bottom of the fill-slope.

Standard surveying practice for highways, railways, etc., is all of these. That would be rather burdensome for trail builders, so we usually do only one. But which one?

The choice is constrained by two principal considerations:

effective vertical control (elevation).
avoiding disturbance of the flagging and staking, which would seriously impair the previous consideration.

Vertical control is more critical than horizontal control. Vertical location (elevation) directly affects rise, and thus grade, whereas horizontal location affects grade only to the extent that shifting a location sideways will move it up or down the slope, thus changing elevation indirectly. For purposes of grade it is best to design and build with careful attention to elevation (vertical control), and allow some flexibility in horizontal location in case the actual surface is not quite as expected.³

The Forest Service recommends flagging along the center line. That is okay for reconnaissance flagging on level ground, but I find it problematical on slopes, on two counts. First, the point on the slope over the center line of the future trail is not at the intended elevation of the tread, and the relationship between these two elevations (flagged and intended) depends on both the angle of the slope and the width of the tread (or bench). To depend on center-line flagging for vertical control means that the person or persons doing the flagging have to correctly apply a two-factor conversion (for slope and width), which the constructors then have to correctly reverse. With such a loose and round-about procedure one cannot reasonably expect any consistency or accuracy of vertical control.

The second objection to center-line flagging is that it is right where you are working, and it is a certainty that such flags will be disturbed.

Flagging the inside edge of the tread (the bottom of the cut-slope) is even worse, on both counts. And while flagging at the top of the cut-slope or bottom of the fill-slope is least likely to be disturbed, it is ludicrous to expect either of these to provide an accurate guide to the intended elevation.⁴

For traverses – that is, trail crossing slopes, and aside from turns, which as will be seen below are a bit more complicated – I strongly recommend flagging on the outside edge of the bench on which the tread will be constructed. This is not entirely secure from being disturbed (the second consideration)⁵ but that is rather outweighed by the advantage of effective vertical control, which is the primary consideration. (One of the big advantages of flagging the actual intended elevation, and not some other location that is indirectly related to it, is that one can simply look up or down the line of flags and observe directly if any are anomalously high or low.)

Diagram of how to flag elevation. [vlocation.gif 4 KB]

Placing pin flags horizontally would provide definite specification of elevation. But due to brush and low-angle slopes they would be hard to see and easily trashed. So we place them vertically, with the understanding that the point on the surface intercepted by the vertical line of the pin flag also locates the horizontal line of the elevation. But there is a problem – in the real world the topographic "surface" is often an indistinct layering of vegetation, duff, and roots. The proverbial "mineral soil" on which the trail is to be built often has a reasonably distinct surface, but this is hidden below the topographical surface, and often irregular. So it is necessary to make a best guess of the nominal surface to be used for flagging, which might not be quite the same guess made when the trail is constructed. This does not seem to be too great of a problem. (And if it might be a problem, use stakes and explicitly mark the elevation with grade and offset marks.) The main thing is to understand that the flag marks a point on the "surface" which is to be extended horizontally into the slope until solid ground is encountered. (And a little more if there is to be provision for a margin.) Presumably grade in such case is not too critical (else stakes would have been used). If any irregularities develop it is adequate to simply even them out up and down the grade as the tread is constructed, using a string if necessary.

For traverses generally, where elevation is not critical, pin flags generally suffice. At turns, steps, or other locations where there needs to be tighter control of elevation, stakes should be used. As these are more susceptible to damage, it is useful to set them six to ten inches out from the expected outside edge. As was described previously a grade mark (a zero with a horizontal bar through it) is used to specify the elevation of the grade; it is the constructors' responsibility to extend this horizontally into the slope. As this grade mark is often in the vegetation, or even below the original surface, it is useful to provide an offset mark. This is simply a horizontal bar at a certain distance above the grade mark, with the height of the offset being marked above the bar (with a plus sign if the offset is above the grade, or minus if below).

For an eye-ball verification of the grade (and a convenient way of checking the elevation as the bench is excavated) it is useful to bend the end of a pin flag in a manner so it can be clipped onto the stake at the offset mark and hung horizontally over the bench. These will get disturbed, but they are easily restored using the pencil level.

Turns.

All of the previous examples have assumed a fairly straight traverse. When going around a turn several points need to be kept in mind. First and foremost of these is: grade around turns should always be measured on the inside! Second: tight curves need stakes placed closer together; every 25 inches works pretty good. Third: defend the inner radius! On a traverse the stakes are set off a little bit to avoid being disturbed by construction. However, builders (as well as hikers) have a tendency to "widen" a turn inwards. So the stakes on the inside radius should be put at the radius, or even a couple of inches outward. Finally, find the center of the turn and stake it. That stake should have "CT" (Center of Turn) written at the top, and the radius written just below it. This will greatly facilitate restoration of any stakes that are disturbed. (It may also have a grade mark, but more on that later.)

Walls.

One other detail. Construction of a wall or emplacement of a curb log not only has to be done precisely, but also tends to wreak havoc on any stakes in the way. Once the overall staking has been done and the elevations worked out and marked, I find it useful to put a stake just beyond each end of each intended curb log or structure, carefully marked with the stationing and the height of the log or structure. Any intervening stakes can then be removed. I usually tie a bit of blue ribbon on the top of these stakes to indicate that they are special, and are not on the usual stationing interval.⁶

These are also useful when you have large curb logs that you want to fit right the first time you set them in. Measure the diameter of each end, then mark on the stakes where the bottom of the log should be. This will guide you in setting the rock bolsters to the proper height, and with any luck at all the log will fit perfectly the first time.

Staking a turn.

Okay, let's walk through the staking of a reversing turn, something similar to that shown in View 12. Note that we are taking on a simple case with no complications, as should you until you have had some experience with this.⁷

We start on the presumption that a line has been flagged across and up a slope, which then reverses and crosses the slope in the other direction. Where the flag line reverses is the vertex of the flag line, and the location of the turn to stake out.

The first step is to take a close, careful look in the vicinity of the flag line for possible problems, such as bad soil, large boulders, bedrock, large tree roots, etc., which might impair construction. Also take in the bigger picture: does the slope suggest that there may be water flows during the rainy season? Or that this is a landslide zone, debris flow, or otherwise unstable area that may be shifting in the coming years? (Not that trail should not be built in such areas, but any decision to do so should be fully informed as to possible future problems.) This is the time to stop, take a deep breath, and consider if this is the route you want to live with, as this is the time when it is easiest to make any changes.

So the route looks good. Put a temporary stake at the vertex of the turn, and check the grade of the flag line. If you have any fixed control points nearby (such as bedrock), you will want to check the grade from there. If the rest of the traverse is flexible measure the grade back 25 or 50 feet, and set a stake with a grade mark for future reference. If the grade is little tight, your first option is to push the vertex out a little further. If not, can the control points be adjusted? (I.e., can you start the grade a little sooner?) If by good fortune (or diligent search of the topography) you are on a bit of a platform you might be able to get some extra run with a large radius. If none of that suffices to get the grade down to spec, then you need to plan where you will install steps.

At this point the grade of the flag line looks good. But as was shown in Geometrical Development, a turn will not fit directly on the flag line, and usually requires some extra run. This should not be problem here, as the turn being staked here will have some radius, which effectively increases the run. However, to be on the safe side (in case problems pop up, or the construction is a little off) you should, if at all possible, move the vertex out an extra foot or so. (If you do not have room for at least two feet of radius, you should extend the vertex at least two or even three feet.) To do, set a second stake at the proper point, then use the pencil level to transfer the grade mark. (If stake is removed or disturbed before the mark is transferred you should re-measure from above and below to ensure you have the proper elevation.)

And here comes the tricky part. At this point the vertex is fixed only vertically (elevation) and along the slope (length); how far out of the slope is still indeterminate. Scrape off the duff and loose soil on a line going down past the stake to expose the surface of the solid ground. The horizontal extension the grade mark to this surface marks the actual vertex of the flag line. What you need to figure out now is how far out from the slope to come with the outer edge of the ramp. One option is that this is already determined by other considerations (and you will be building a wall to fit). In that case come out from the vertex the specified distance and set a stake.

A second option is that you are coming out from the slope only as far as can be handled by a curb log. You have located the biggest log you will be able to drag in and measured its diameter (probably only ten inches), which will be the height of your "wall". Come down the slope until you are that far below the grade mark; set a stake there.

Either way, make sure this stake is set solidly, mark it "BT" (Begin Turn), tie some bright ribbon on it, and transfer the grade mark. This is the key stake from which everything else will be measured. (As this stake will inevitably be disturbed, you might want to set another stake just down slope marked with the grade and the horizontal offset to BT. Mark this stake "BTO".) This takes us to the situation shown in the first "Turn Staking" diagram (right). Note that the relative position of BT with the vertex (V) might not be exactly as shown in the diagram.

Time to check for possible hidden problems. With the BT stake fixing the location of the turn, and a width and radius fixing the size of turn, it extent of excavation can be determined. Let us assume the tread width is to be 32", and the radius of the turn will be 16". This makes the width of the entire turn 98" (32+16+16+32) across, or just over eight feet. (Even more if you include room for margins or ditches.) From point BT go up the slope until you are nearly above the far side of the turn (in this case about eight feet in), scrape off the vegetation, and examine the ground carefully. Also scrutinize the adjacent area, as sometimes a problem that is hidden at one spot will be manifest at another. If there is any chance of hidden boulders, bedrock, large tree roots, etc., you may want to check by probing with rebar. If a problem is found you will need to reconsider how to do the turn, or even to relocate the turn.

Let us assume everything is looking good. From BT measure into the slope the distance of the width, plus any outside margin. Three points to note:

The outside margin (if there is any) might well be on the layer of roots, so point BT may already be inside of it.
Don't use an inside margin on a turn, as that only reduces the radius.
Strictly speaking the width should be measured perpendicular to the ramp (tread), not perpendicular to the contour. But in the real, physical world these details tend to blur. As long as you provided enough run to ensure a good grade it does not matter too much whether you measure perpendicular to the ramp or perpendicular to the contour.

At this point set a stake marked "BTI" (BT Inside). This stake marks the inside of the turn, and beginning of the inside radius. It also serves as a guide to the diggers as to how wide the tread should be as it approaches the turn.

From BTI measure inward the distance of the radius. Again, strictly speaking this should be perpendicular to the ramp, but straight into the slope should be okay. Set a stake and mark it "CT" – Center of Turn. Just under that write the radius (in inches). In some cases it might be useful to mark offsets to the grade at the bottom of the turn.

Next, determine the point where you want the tread to return to the upper flag line. (Refer to the Offset Turns diagram.) Most likely this will be anywhere from eight to twenty-five feet up the grade from BT, but the best placement depends on the context and is somewhat subjective. (Fortunately it is also not critical.) Set a stake, mark it "RT", and also mark the grade.

Now comes the trickiest part, setting "ET" – End of Turn. From CT measure into the slope the radius. Now look at the line from there to RT. You want to swing ET – maintaining the radius from CT – a little further until the line from RT is tangent to the turn. E.g., it makes a right angle with the radius.

If the stake ET is likely be disturbed by excavation, have a fleet-footed assistant run up the slope and place a pin flag (a large orange one would be good here) just above the top of the cut-slope and in line with the line of CT-ET. (To reset ET horizontally, just measure out from CT towards the flag.)

Before determining the elevation at ET you need to check the overall grade. (Recall how swinging a ramp into the slope requires additional run, but the additional run from having a radius turn may suffice.) You will first want to measure the rise between RT and BT, then measure or calculate (see trail math) the length of the turn and ramp. If this is sufficient run for the grade required, fine. If not, extend the turn out from the vertex, and possibly increase the radius, until you have adequate run. (Then reset BT, CT, and ET.) Don't forget that extending the turn past the vertex (the initial point BT) lengthens the lower ramp, which may raise the elevation of the turn/platform.

So the turn is looking good. Measure the distance (run) between RT and ET, multiply by the grade (and divide by 100) to get the rise. Which is to say, how far the grade drops from RT to ET. As the stake at ET will most likely be disturbed, mark an offset on stake RT (which may be a little further away than is handy) or stake CT (where the offset might be minus if the offset mark is below the grade).

The grade mark at RT is to ensure that the grade coming into the turn is not so far off as to disturb the turn. You may want to put a similar stake and grade mark below the turn for the same purpose.

Finally, you should put some stakes on the inside circumference at 25" intervals between BTI and ET to protect the inside radius from being shortened by zealous diggers. Add additional stakes up to RT and beyond as may be useful; all of these can be marked for grade.

To check that all the elevations are consistent, stretch a string down the inside of the turn. Or hang some flags horizontally at the offset mark, as was described above.

The outside edge of the turn (or platform) can be marked if desired; pin flags are sufficient for this as all elevations should be taken from the stakes now in place.

In some cases I would also run a pink or orange ribbon from CT back towards RT to remind everyone not to be shortcutting the yet to be built switchback.

If you have curb logs or walls you should set stakes at each end showing their location and elevation as was described above.

And there you are – one potentially perfect turn ready for excavation. Once you understand these basics it should be straightforward to adapt them to other types of turns.

Summary.

Pin flags and stakes are used to ensure adequate control of horizontal and vertical location of construction. Vertical control (elevation) is more important than horizontal control; for traverses generally the recommendation is to mark the vertical location of the outside edge of the tread or bench, and adjust the horizontal location to fit. Stakes are generally placed slightly beyond the outside edge, with the elevation indicated with grade and offset marks. The length (run) of turns is always measured on the inside circumference; staking may be necessary to defend the inside radius from being shortened, which would thereby reduce the length of the turn and increase the grade. The location of a turn should be carefully checked for hidden problems prior to staking, and any adjustments made.

The staking of a reversing turn begins with a check of the flag line, and then careful determination of the horizontal and vertical location of the vertex. After any adjustments (and possibly offsets pertaining to certain types of turns) this is used to determine point BT, the Beginning of the Turn. From this are determined the points BTI (BT Inside) and CT (Center of Turn). From CT measure up the upper flag line a suitable distance to get point RT, then set ET (End Turn) at the specified radius from CT and suitably aligned with RT. Measure the rise and run and check that the grade is within specification; mark the proper elevation for all of these points. The location and height of curb logs and walls should also be marked on suitably placed stakes. Place additional stakes and flags as may be needed.

Back to Switchback Theory and Principles.

Notes.

Construction flagging/staking is more than just a communication of where to construct. It is also a definite statement of what is proposed, which can be re-measured and examined and verified, corrected if necessary, and refined if feasible. This leads to not only a better result, but also to greater confidence that the result will be as expected.
Plastic flags of various sizes and colors attached to a wire staff. I have found 2"x2" yellow flags to be suitable for this use.
I know of various instances in parks and trailheads where a walkway has been precisely located horizontally per a plan as specified by an architect, with dubious and even unfortunate results vertically.
The one argument I have heard for flagging the top of the cut-slope is that inexperienced diggers want to know how high up the slope to clear. There is a simple enough technique for this – i.e., from the flag line clear as high as you can reach with a standard length McCleod. In any event it is not that critical. If your diggers need the guidance and assurance of flagging at the top of the cut-slope, fine, provide it for them. But this is separate from the flagging of grade. It is not reasonable to control grade (elevation) from the top of the cut-slope.
I find that whether flagging or staking on the outside edge is disturbed or not is largely an issue of technique. If your workers tend to flail about widely, well, that is likely a problem in itself. At any rate, the key principles here, of respecting grade, and especially of measuring it, do require some care, and even discipline.
Heretofore I always scraped, then dug and graded, then – almost as an afterthought – placed the curb log. I had to do it this way, because I did not know how deep to bury the curb log until the grade was nearly done. But now that I have learned to measure I can place the curb logs first, with minimal disturbance of the slope, then scrape, dig, backfill, and grade, with full confidence that everything will come out even. There is some saving of effort, less disturbance of the slope, and a great deal of satisfaction in having the grade come in right where you planned to have it come in.
My expectation is that you will read this several times until you have a good grasp on what has to be done, and how, and even why. Then imagine a scenario and go through the steps in your mind. (A good way to occupy your self while waiting in the dentist's office, or riding the bus.) This should give you a good sense of how to proceed in the field, even if you don't do it exactly the way described here. Indeed, you will undoubtably have to "show some flexibility" and adapt to your specific conditions. Which you won't be able to do if you blindly follow this as a check-list.