Monday, 3 August 2009

Trail Tech 1 - Bench Cutting (Trail building 2nd August)

Trail building today involved more building on the Monkeys Hat - which meant more bench cutting!

Bench Cutting is not about making things to sit on (though it is nice to sit down once in a while) but about building a trail on the side of a hill. Done correctly, they can make a fab trail that makes the most of a hills features while remaining strong, lasting longer than flat trails, look good and are fun to ride. The steeper the hill, the more dramatic the bench cut is! You'll see bench cutting everywhere you ride mountain bike trails, especially at trail centres. The reson why bench cutting is so popular is all about 'sustainability'. Water (and Skidding!) is the main reason trails degrade over time. Water runs downwards and if a trail is not built with this in mind, it won't last long. The greater the volume and faster the water runs, the quicker it washes the trails away.

Doing it right is easy but it is also easy to get it wrong! We've all ridden bench cutting that just doesn't work - the trails are too narrow, you catch your pedals on the sides, sucking you into the hill, or the outslope camber is so severe that your constantly fighting to stay on the trail ... or you find your self riding in a trench that is full of puddles or streams and debris from the hill.

There are two types of bench Cutting, Full and Partial cut. Full cutting is where the entire width of the trail is cut into the hillside and Partial (or cut and fill) is where part of the trail is cut into the hillside and what is dug out is used to form the outer edge of the trail surface. On Cannock Chase, we generally build only Full Cut because the ground material is rubbish at holding itself up! One other thing we are lucky with is that the trail surface is right underneath where we are digging and as it has been well bedded in over the years, it doesn't need much compacting. Many places have to import a lot of material and surface all of their bench cut trails, but some ancient Ocean decided that Cannock Chase would be made from Sand and Gravel.

A Bench Cut has a slightly outsloping surface and a steeper (but not vertical) back slope. This is to allow water to run down the backslope and over the trail slowly and in a small volume so small that the trail isn't washed away. If the trail surface or back slope are too steep, the water runs faster and washes the trail away!


Bench Cut Diagram. Arrows show water direction

Step 1 - Planning
Initial trail planning will have accounted for the bench cutting route. This will mean that the trail will have been steered towards or away from natural features and obstacles - toward a nice gulley or around a big tree - while bearing in mind the width of the cutting needed for the trail.


Step 2 - Clearing and Cutting
The width of ground that needs clearing away is far greater than the actual trail width. This is to allow room for the back slope that is there to hold the hill in place. There are a number of methods of cutting the trail; either scrape the top soil away to clear the full area of cut and then start hacking into the ground, or just hack away until some sort of bench cut appears! I prefer the first method, but both are equally effective. A third way is to start cutting away at the outer edge of the trail, and work into the hill. This is very time consuming, but gives the best results.


Step 3 - Disposing of the waste
As we build Full Bench Cuts at Chase Trails, the material that is dug out is not rquired. It is often mixed with the top soil which is no good for trail surfacing anyway. The best place for it ... is spread out on the hill below the trail. We need to be careful not to pile it higher than the outside edge of the trail (trapping water), nor have the material in mounds on the hillside which look ugly and can cause landslides! It takes a few months for the waste to blend in but because the soil is loose and full of nutrients, it doesn't take long to start growing and become part of the hillside.

Step 4 - Making good
Once the bulk of the cut is done, the trail needs to be tweaked so that the trail works lengthways as well as cross ways. The trail surface is trimmed to form a good riding surface that is slightly outsloped (about 5 degrees), the edges are cleared to allow water run off and the back slope is trimmed and shaped (to about 45 degrees) to make sure it doesn't collapse or wash away.


And thats it! Sounds easy doesn't it. The hardest part of bench cutting is getting the angles right and still having enough trail surface to ride on.

Next time you're out riding on the side of a hill, have a look at how well the bench cut was done!

E:)

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