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Trail Construction and Maintenance Notebook

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Natural Forces at Work

Dirt, Water, and Gravity

Dirt, water, and gravity are what trail work is all about. Dirt is your trail's support. Terra firma makes getting from point A to point B possible. The whole point of trail work is to get dirt where you want it, and to keep it there. Water is the most powerful stuff in your world. Its mission is to take your precious dirt to the ocean. The whole point of trail work is to keep your trail out of water's grip. Gravity just is...

It is much more important to understand how the forces of water and gravity combine to move dirt than it is to actually dig dirt, install waterbars, or build puncheon. If you work trails long enough, you will see hundreds of examples of trail structures built with little understanding of the forces at hand. Such structures don't work and the dirt goes downhill. You will save time, money, and your sanity if you get grounded in the basic physics first.

Water erodes soil surfaces by picking up soil particles and carrying them off. It builds soil surfaces by getting tired and dropping soil particles. And it alters soil structure by hanging out with soil particles.

Water in the 'erode mode' strips tread surface, undercuts support structures, and blasts apart fill on its way downhill. How much damage is done depends on the amount of water involved and how fast it is moving.

Water has "carrying capacity." More water can carry more dirt. Faster water can carry more dirt. You need to slow water down and get it off the trail. When and where you can do that deter-mines what sort of water control or drainage structure you use.

Water has "deposit" ability. If you slow water down, it loses its ability to carry soil. If you abruptly turn or block water, it slows down. This has some advantages if you are restoring eroded tread and use check dams to capture waterborne soil. It works to your disadvantage if your waterbar happens to be the abrupt turn and the soil drops, clogging the waterbar (Figure 4).

Photo of a washed out waterbar.
Figure 4--Too much water and sediment washed this
waterbar out. Keep the water moving until you get
the suspended soil where you want it. This sounds
simple, but most failed water diversion structures
are ones clogged with deposited soil.

Water can also affect soil strength. The general rule of thumb is that drier soils are stronger (more cohesive) than saturated soils, but it is also true that fine, dry soils blow away. The best trail workers can identify basic soils in their areas and know their wet, dry, and wear properties. They will also know about plant indicators that will tell them about the underlying soil and drainage.

You will have mastered dirt, water, and gravity when you can:

  • Move surface water off of the trail.
  • Keep surface water moving, without taking tread material with it, until it is off of the tread.
  • Keep trail tread material well drained.

Critter Effects

Gravity has a partner--the Critter. Critters include packstock, pocket gophers, humans, bears, elk, deer, cows, and sheep. Critters will burrow through your tread, walk around the designated (but inconvenient) tread, tightrope walk the downhill edge of the tread, shortcut the tread, roll rocks on the tread, chew up the tread or uproot the tread.

Gravity waits in glee for critters to loosen up more soil. If you recognize potential critter effects (especially from humans, deer, elk, domestic livestock, and packstock), you can beat the system for a while and hang onto that dirt. How?

  • Don't build switchbacks across a ridge or other major "game route."
  • Don't let tread obstacles like bogs or deeply trenched tread develop.
  • Make it inconvenient for packstock to walk the outer edge of your tread.

Your trail strategies are only as good as your understanding of the critter's mind.

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Updated: 4/14/2014
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