Wednesday, April 6, 2011

Trail Construction Techniques on the Appalachian Trail

This was originally written as a lab for my Physical Geography class. While it doesn't relate directly to a mountain bike trail per-se, I think that any essay analyzing trail building techniques should be interesting to mountain bikers. Also, you will find that I reference mountain biking and mountain bike trails throughout the essay. 

PS I got 100% on it and a comment of "Very thorough and perceptive!" 


As an avid mountain biker, hiker, and rock climber, I spend a ton of my time on singletrack trails. I have ridden almost 300 different trails across the nation, and have hiked and backpacked on many more. I enjoy and use the trails, and I also help maintain the trails in whatever way I can. As a result, I am constantly observing how different trails are designed and built, and what special features they contain. During our hike along the Gooch Gap section of the Appalachian Trail, I observed a number of interesting features. Some of these observations were of good, sustainable trail building techniques, some were of bad or outdated techniques, and some were just very interesting.

In general, the construction of this section of trail was superb. It was high-quality narrow singletrack that was mostly bench cut into the side of the hill to allow it to drain well. These bench cut sections also featured frequent grade reversals to prevent water from running along the bench for a significant distance. I observed that these sections of sustainably-built trail lay in areas where the terrain was relatively flat. These sections of trail traversed along gently sloping hillsides and did not gain or lose much elevation. As for the tread of the trail itself, it was very smooth and flat and very easy to walk on. (See Figure 1.) There were roots and rocks sprinkled intermittently along its length, but no more or no less than I would have expected to see.

The most interesting single feature that I observed was the rock-armored spring crossing. At one point along the trail, there was a small spring/tiny creek flowing out of the side of the hill and across the trail. In order to prevent trail erosion and a big mud hole, trail workers had built a small channel through the trail to allow the water to flow, and then armored the sides of it with slabs of rock. This is an ingenious natural solution to the problem that foregoes the use of an artificial culvert.
The trail construction appeared very sustainable along the sections of trail that paralleled the hillside. However, there appeared to be some issues with the sustainability of the design on the single hill where the trail changed elevation quickly. On that hill, the trail turned and headed straight up for several hundred yards. I could tell that it was not very sustainable by the trench-like appearance that the hill section had in comparison to the bench cut sections. (See Figure 2.) The trench-like appearance probably resulted from water flowing straight down the trail. Since this section was not bench cut with grade reversals incorporated in like the rest of the trail, the water flowed straight down the tread. Many people refer to this as a “fall line” trail, meaning that the trail goes straight up the hill. As I could see from the eroded trench, this is a problem.

In order to slow the erosion, the trail builders incorporated several techniques to divert the water off of the trail at intervals. The major technique utilized was what some people refer to as “water bars.” These water bars are basically trenches dug into the side of the trail that are designed to divert the water off of the tread so that it disperses in the undergrowth instead of gaining momentum and flowing like a river down the trench. The water bars that I observed looked very thoughtfully constructed, and were placed at key intervals such as turns in the trail. The other major erosion control technique that I observed is referred to by some people as “dragon teeth.” For this technique, the trail builders embedded rocks in the trail, standing on end, in an angled line. This line of “teeth” also helps slow down and divert the water, but it blends into the surface of the trail much better than the large water bars. However, they do not appear to be as effective.

Based on my observations of the different sections of the trail, the bench cut singletrack with grade reversals has held up well over the years and proved to be a very sustainable method of trail construction. The fall line trail, on the other hand, has fared rather poorly. See the attached diagrams for an illustration of my findings. In my opinion, these findings are a result of poor, or rather, outdated trail construction practices. The Appalachian Trail was constructed back in the 1930s and trail building technology and theory has grown in leaps and bounds since then. In recent years, organizations such as IMBA (the International Mountain Bike Association) have pioneered new standards for singletrack construction. They propose that the angle of the trail should never surpass a certain grade, which is definitely less than that of the hill that we had to hike up. By keeping the grade low and incorporating regular grade reversals, trails will last longer with less maintenance. These trails will take more work to build initially, but over the long run they will require less work to maintain. IMBA is not alone in this endeavor: the U.S. Forest Service is slowly adopting these trail building strategies as well. For a diagram of a proposed reroute of the hill that we hiked, see Figure 3.

Figure 3

The Appalachian Trail is an amazing way to access the beauty of nature, and the people that maintain it do so very carefully and conscientiously. All of the trail features that they have added in order to prevent erosion have been painstakingly designed and installed. However, these findings suggest that their efforts are hindered by the outdated design of these trails wherever significant elevation change occurs. Perhaps in the future there will be enough funds and man power available to reroute the least sustainable sections of trail. But until that time, the Appalachian Trail Conservancy appears to be doing a splendid job preserving what they already have!


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Greg Heil is the Editor in Chief for He's been writing and publishing online since before blogging existed.

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