Rolling Crown Switchbacks: Routing Trails on Steeper Grades
Joey Klein, IMBA Trail Care Crew
The worst erosion problems on contour trails are usually found on the turns. Most of the turns we see could be defined as climbing turns or descending switchbacks; that is any turn that drops straight into the fall line. Trail users that are forced into the fall line have to resist gravity, causing them to brake. If riders have to break hard, soils may become loosened. When water is channeled down a trail and through a turn it carries away the loosened soil. The combination of these two forces are what causes erosion.
When a climbing turn exceeds a 7%-12% grade (depending on the area), erosion is sure to follow. Erosion can be minimized, by using a grade dip above the turn to sheet water off the fall line section. Climbing turns can also be placed strategically around ridges, south facing slopes or toes where water accumulation is minimal. All these techniques can help, but on steeper side slopes a "Rolling Crown Switchback" is preferred for sustainability.
A "rolling crown switchback" is a structure that ensures the turn is happening on a near level deck. This deck is slightly crowned so that water sheets off, away from where users are turning. The upper leg of the trail, descending into the turn, is excavated down and that soil is used to build the lower leg up. This brings the two legs together, where they meet at a rounded point on a near level deck. The upper leg is insloped at 5% and a drain is created out the back of the switchback to keep water out of the turn. Insloping begins about 25 ft above the switchback and can be initiated with a grade dip. The trail slope of this section should be no greater than 5% to 10%.
The lower leg will need a crib wall to contain the excavated soil taken from the upper leg. Typically, for every 8%to10% of side slope a foot of elevation is needed on the crib wall (depending on how much excavation can be done on the upper leg). Example: 40% sideslope divided by 8% to 10% = 4 ft to 5 ft high cribwall. (A switchback built on a 40% side slope, divided by 8% to 10% will need a crib wall 4 ft to 5 ft high). Backfill for the cribwall can be taken from trail being excavated on the upper leg. The deck will be crowned at a 5% outslope, draining back into the upper leg's inslope and out the front over the cribwall. The lower leg is outsloped at 5% and the trail slope should not be greater than 10%.
The turning radius of the rolling crown switchback can vary, and it should be consistent with the flavor of the trail, i.e. a very narrow technical backcountry trail may not need a switchback with a huge turning radius. However a wide, smooth, heavily used trail near a city definitely requires a wide turning radius switchback. An 8ft turning radius, which is the same as a16 ft diameter turn is usually plenty in most cases. For cyclists and equestrians a minimum turning radius of about 6ft (12ft diameter turn), could be appropriate on certain backcountry trails. When possible, build trail structures bigger than they need to be and sustainable for unforeseen user numbers.
Most contour trails require a direction change eventually to gain or lose elevation. These turns will always need maintenance and if there is a way to minimize reversals, choose it. More often than not, this will give users more trail mileage and less time spent on repairs. When utilizing switchbacks put some thought into how they fit in with the flow or type of trail. If there is a long smooth straight upper leg, users will carry speed into the turn impacting the switchback. If the trail can twist or roll gradually before the turn it helps to slow users down. A rough surface, including rocks and other technical features, is also a good way to keep speeds down through switchbacks. Always try to use gentle grades and a wide turning radius.
A properly built RCS will outlast any other type of turn. They require lots of initial work but they truly are one of the most rewarding trail projects imaginable. Also, remember that when working with nature, she calls all the shots and you can only do the best you can with what she gives you.
| #1 LAYOUT: The most crucial stage. Find an open area with a relatively gentle sideslope and an open area for the drain. Decide what turning radius is appropriate for the switchback. Bigger is usually better. Shoot for 8-ft radius (length from rounded point where two legs meet, to outside of turn) or 16-ft diameter (distance between where turn is initiated and ends). |
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| #2 PREP: The area of the rolling crown switchback (RCS) must be cleared of organic material. Get down to good mineral soil, to minimize settling and rotting, especially in the zone where the cribwall will be placed. |
| #3 ROCKS: Rocks are the first choice for wall builders if they are nearby. Large angular rocks are easy to place and stack well. Gather rocks of all sizes; boulders that are TV to microwave size for the wall and lots of tennis ball to melon size for fill. Stockpile rocks above the wall, out of the way. When ready to start on the wall, start with the largest boulders where their girth is needed most, usually the middle or low point. Work your way up or out so that gravity holds every rock next to its lower neighbor. The first boulders have to be placed in their own holes and should be unmovable when set. |
| #4 BOTTOM TIER: All tiers will be canted back towards the hill and each rock should be butted up next to its neighbor. Boulders should be freestanding and not able to rock back and forth. Dig individual holes for particular boulder adjusting hole depth to allow bottom tier to have a common height so next tier sits easily on top of bottom tier. Backfill with rocks and then dirt to just below top of boulders. Tamping of backfill dirt is crucial. |
| #5 NEXT TIER: Stack boulders from lower end of wall working towards high end so gravity pulls all boulders together. This tier will rest halfway on bottom tier and backfill. Stagger vertical cracks like a bricklayer (breaking the joints) and keep rocks canted back towards hill. |
| #6 BACKFILL & TAMP: Start with baseball size rocks to save fill dirt for top of layer. Cover backfill rocks with dirt and tamp; compaction in layers is key! Backfill material may be taken from above RCS on the trail and sometimes a borrow pit may be needed. |
| #7 TOP TIER: As top tier forms, the drain for the upper leg is excavated. |
| #8 CHECKING GRADE: The upper leg trail grade and inslope are shaped and these excavated soils can be used for the crown. Trail grade descending into RCS should be close to 7% and inslope should be 5%. |
| #9 TAMPING: Each layer of fill must be compacted by tamping; McCleods work great. The soil should be moist to help bonding and compaction, bring water in if necessary. |
| #10 DEBRIS: Close off zone between legs to keep users from shortcutting. Use natural forest debris if available. Routing a RCS intentionally around a natural barrier, like a blackberry thicket, cacti or a rock outcropping is also a great choice. |
| #11 FINAL TAMPING: More tamping and fill to ensure crown is highpoint of deck. |
| #12 FINISHED WALL: Wall is tilted back towards tread, with tread outsloped over the top of wall to allow drainage for lower leg. |
| #13 SWITCHBACK: In-sloped upper leg descending into turn with forest debris between legs to keep users from shortcutting. |
REMEMBER: It is up to the land manager if rock can be pulled from the nearby area with out impacting habitat. Holes and borrow pits should be caved in and smoothed out afterwards. Rocks are heavy and seem to enjoy pinching fingers, breaking toes and tweaking backs. Carry rocks as a last resort, flip them, walk them or wheel barrow them when possible. Rock bars should be used for prying, rock hammers are good for making small adjustments to rock shapes and come-a-longs may be useful for moving. Working with rock takes patience, skill and strength. Nothing is as rewarding or as lasting as rock, just ask the Egyptians. When constructing a switchback the same day as putting in new trail above and below that switchback consider this: Flag the upper and lower legs coming into the RCS but halt construction 75ft before actually reaching the RCS. This will allow for any unforeseen adjustments that may take place during construction of the RCS.
A special thanks to all that helped with the RCS pictured here on the Tahoe Rim Trail. Families and friends of the Boy Scouts from Tahoe City, members of the Folsum-Auburn Trail Riders and crew leaders from the Tahoe Rim Trail Association and TCC2 teamed up to complete this colossal 12 hour work project. It was the biggest and best RCS we've seen yet. Build it once, ride it forever!
Thanks,
Joey Klein, TCC2
