San Francisco’s Favorite Outdoor Volunteer Program

Upper Historic Hydrology Lab: Armoring

Roots are a major concern for any trail designer. They cut through trails, breaking apart rock walls and pushing paving stones out of place. They are forever uneven and slippery when they get wet, making them a legitimate safety concern. When a root crosses their trail, most designers simply rip them out. But, as with any trail treatment, that wont work everywhere.

Thick, matted patches of roots dot the Upper Historic trail. When faced with the question of how to deal with them, Sutro Stewards opts not to rip them out. Mt Sutro has rocky, erosion prone slopes, and roots - both living and dead - play a big part in holding things together. Also keep in mind that Eucalyptus trees have shallow roots systems, meaning ripping out roots that cross trails could cause them to topple over.

So we leave them in, but then what? There’s no simple way to take care of roots once you’ve decided to leave them in the ground. On Mt Sutro, volunteer crews have had success using a method called armoring. Armoring is a style of dry stone masonry most often used to build portions of trail form bedrock up. There are different styles, suitable for different trail usages. Some root armoring has already been done around the rock rose area of the Historic Trial that’s still strong and firm over a year later, which is significant in the life of a trail.

When Craig Dawson suggested we test the method, I jumped at the chance. After some research, I decided that raised-tread armoring would be the best method for our roots and on the April 7^th Volunteer day, I, along with my two volunteers Mike and Joe Sullivan, struck out to armor some roots.

As with all trail projects, you start by scraping down the organic matter to the bedrock, or in the case of Mt Sutro, the densest dirt you can find. You then lay down a thin layer of sand as a buffer between the bedrock/dense dirt and the first rocks of your structure. In armoring, you use gravity to help hold your rocks in place. We started on the downhill side of our root patch and buried some large stones as anchors. We then began placing the rest of our stone, fitting them together tightly like puzzle pieces and making sure that they didn’t shift. We got down a bottom layer of rock beautifully, but still had two layers to go when our rock, and our time, began to run out. Instead of armoring the roots to the level of the rail, we back-filled the remaining dips with a gravel, sand, and soil mix and tamped everything down tightly. We were disappointed that we couldn’t finish the project the way we had hoped but what we had done looked great and was much better than the knobby root patch we had started with.

In the almost month since we put it in, the almost armored section has performed well and will give me an excellent point of comparison when I complete a fully armored root patch. In fact, I will lead a crew this Saturday, May 5^th, on another armoring project. Come out and volunteer or pay us a visit at the intersection of the West Ridge and the Upper Historic trails!