Geo-technical analysis of the switchback area of the 70-Mile Butte Trail, Grasslands National Park PREPARED BY BERNARD LEPLANT, MSc (geology), PhD (Please see the full letter to the Park administration under FILES: LEPLANT_GEOTECH_OPEN-LETTER) In an open letter to Parks Canada, Colleen Watson-Turner identified the switchback area as a deeply problematic area on the 70-Mile butte Trail, both aesthetically and structurally. Her proposed solution, complete rehabilitation of the trail “as it was before” and the relocation of this section of the trail, needs to be considered. That said, because the trail was cut into 65–70 million year old rock formations, putting things back together as they were before would be most challenging. It’s a bit like cutting down a tree and splitting the wood, and then deciding to put the tree back together. Nonetheless, I feel that there are quite possibly other ways to remediate the switchback area. To do so would require the administration of the Grasslands National Park to consult a geo-technical engineering firm with some expertise in slope stability. In the meantime, to help us to move forward, I would like to share with you my geo-technical understanding of this particular area. As a trained geologist I have a certain amount of knowledge and experience about slope stability in various materials and the following information might be useful to GNP people who are working on the trail. Over the switchback area, the trail was deeply cut on a steep NE-SW oriented slope. That slope was already at the limit of stability for loose material (25–30 degrees). Fortunately, it was covered by various grasses and juniper; all plants with deep root systems that added to the stability of the slope. Although the Frenchman Formation and the Bearpaw Formation through which the trail was cut are considered rock formations, they consist mostly of poorly consolidated to unconsolidated sand, silt and clay that break down quite easily. From a geological perspective, the layers of the Bearpaw Formation in this area are mostly flat lying. These layers consist essentially of mud deposited in a marine environment and contain marine fossils. The Frenchman Formation lies on top of the Bearpaw and is separated from it by an erosional unconformity. The layers of the Frenchman are often dipping in various directions. This is because they were deposited in a changing, non-marine basin environment on top of eroded layers of the Bearpaw. They contain fossils of ceratopsian dinosaurs and other fauna and flora from the end of the Cretaceous Period. The choice of slope to cut the trail and the actual route used by the trail designer to reach the top of the butte were far from the best possible choices. The slope is so steep that cutting a bench-cut trail required a deep vertical cut (over one meter). That, in turn, has created an area of instability up-slope and a large fill slope. As can be seen in this area, whenever such a deep vertical cut is made in a slope of unstable material it results in crumbling and slumping of large chunks of material. If the material is removed and pushed down slope, it does not reduce the instability of the slope. The slope remains unstable, while increasing both the width of the trail and size of the fill slope. In other words, because of the way the trail was cut on this steep slope, it created so much instability in both the cut slope and the fill slope that if nothing is done, the trail will, like the snake eating its own tail, erode itself away in 3 to 5 years, depending on the amount of rainfall. Right at the start of the climb, the trail stays in the lower part of the gully for too long, requiring a large vertical gain before reaching the top of 70-Mile Butte. That created the need for a double switchback which only compounded the problem of deep cuts, erosion and slumping. The cuts are so high and the fill slope so wide that they almost run into each other in some areas. Slumping, already occurring in this tight area of the switchback, is cutting into the fill slope and pretty soon, if nothing is done, it will cut into the trail bed itself. I do understand that the project is still under construction and that measures will be taken to mitigate the slumping, control the water flow and the erosion. I don’t know for sure what these measures will be: perhaps a retaining wall wherever the cuts are unstable, some compaction of the fill slope. etc. A geo-technical engineering firm should be consulted. In her letter, Colleen Watson-Turner outlined another possibility, that of the complete rehabilitation of the slope as it was before. That too should be considered. In Moose Jaw, on one of the large murals depicting a tipi, horses and First Nations people, there is something written: “We will be remembered by the trails we leave behind.” PHOTOGRAPH OF FOOTPATH LEADING FROM EAGLE BUTTE TOWARDS 70 MILE. AUGUST 28, 2014
Posted on: Wed, 03 Sep 2014 14:28:07 +0000
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