The Slate Creek Trail Gabions
Planning and installing rock filled gabions for stream crossings.
By Dave Brandt
While the number one rule of trail construction is never put a trail in a drainage, sometimes there is no way to avoid it. The Deerfield Trail on the Pactola/Harney Ranger District in the Black Hills National Forest of South Dakota travels partly through the Slate Creek Canyon crossing a large creek many times.
At one point, the tread drops off a seven-foot cliff face into the creek at high water or into a mud bog at low water. In another section, the trail dives into the creek for 24 feet along a cliff and then returns to higher ground. A relocation was explored, but it would have required a steep climb above high cliffs and would have to be built out of huge down-sloping slabs of slate rock with no soil in sight. It was also doubtful that the recreating public (hikers, bikers, and anglers) could be convinced to leave their favorite creek for a mile.
I had seen gabions used for a road bank stabilization and proposed we use them to create tread out of "thin air." I asked a Forest Service engineer to look at the situation and he recommended gabions also. I now felt more realistic in my proposal. As soon as we had about 10 eager rock gatherers we could recruit for two days, we hit the creek. We had to transport the five folded wire gabions by ATV, which was an adventure in itself. The last one quarter mile, the gabions had to be carried by hand.
The ground where the gabions were to be placed had to be leveled with a pick and bar, a splashy job. Then the gabion was opened, wired together and put in place. Not more than one or two gabions should be filled at a time because if a partly finished gabion had to be left at the end of the day, it would create a potentially dangerous obstruction in the trail, and a liability to the trail owner.
When filling the gabion with rock, place large rock parallel to the inside edges of the gabion cage to enhance the gabion stability and to keep sharp rock edges from poking out the sides of the gabion. When the gabion is half full, wire should be attached to the outside wire mesh, pulled over the rock fill, and twisted until tight to keep the gabion from squashing or bulging out the sides. When the gabion basket is full of rock, close the top and wire it securely.
The finished gabion should be checked for protruding stray wires that might poke a hiker. The greatest difficulty in the project was finding enough rock in the creek and in the cliff area to fill all five gabions, and then finding enough soil to place on top of the gabions to create a tread. At the ends of the gabions, we placed large rocks for steps.
Two nine-foot long gabions, joined together, were used at the cliff drop off area, and a single nine foot long gabion was used in a nearby low spot. At the 24 foot section of trail that was always in the creek, we used two 12 foot long gabions wired together. I proposed we try to curve the long gabions around a jutting side rock and it worked, to my surprise. The engineer suggested 12 inch high gabions be used, but I opted for 18 inch high gabions to provide more freeboard from the 100 year flood we seem to have every year. The only drawback I can think of is that I would not recommend gabions be used for stock unless a substantial tread is installed on top of the gabions to prevent horses hooves from tangling with the wire mesh. We constructed these gabions where horses do not travel.
Additional soil will need to be added periodically as the present soil settles and sifts between the rock fill. The gabion cages are made of heavy gauge galvanized wire (river type), so they should last a long time in all creek conditions. It is also one structure that few, except the most hardy, would consider taking home to install in their trophy den.
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Updated March 18, 2007