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Curt Oldenburg, Mark Stahl, and Brian Spring
The above-normal rainfall of the 2023 water year (October 2022 to September 2023) and the lack of strong off-shore winds this last fall (e.g., no public safety power shutdowns (PSPS’s) this year in Homestead) were very much appreciated around the valley from a fire-hazard point of view. However, not so welcome were the effects of the high rainfall and runoff which contributed to two ongoing landslides downslope along the unpaved parts of Pixie Trail.
The HVLT and County Open Space / Marin County Parks are working on mitigating these two downslope landslides. The unpaved Pixie trail extends from Edgewood down to the eastern gate at Janes Street. The two Pixie Trail landslides on County Open Space land are referred to as follows: Major Landslide (#1) which starts at the western trailhead of the Dharma Way trail and extends ~80 ft. east towards the Cape Court access; and Minor Landslide (#2) located in the small valley between the Cape Court access and the gate at Janes.
As reported in an earlier Homestead Headlines article, local geotechnical firms were overloaded with work from last year, and the HVLT had difficulty until recently engaging experts to address these slides. With concerted effort by HVLT and the County, the geotechnical firm, Kleinfelder, has now provided a Technical Memorandum for Pixie Trail Landslide #2 (Minor) and a Draft Technical Memorandum for Pixie Trail Landslide #1 (Major). These memoranda provide plans for how to address the slides. Kleinfelder determined that both slides were failures of old dirt fill used to construct the Pixie Trail roadbed, which was originally graded for horse-drawn wagons to access places such as the Hillside Dairy (currently site of Walsh Estates).
As per the Kleinfelder plan, the smaller eastern slide (i.e., “Landslide #2 (Minor)”) requires a relatively simple repair. The geotech firm assesses the slide to be an active soil & rock debris flow which is less than five feet deep. The mitigation approach in their plan is to excavate the debris from the slide zone and dig into the exposed rock and soil to create a few horizontal surfaces (benches). Next, large boulders (“rip-rap”, rocks roughly 15-24 inches in diameter) will be placed on top of the benches. The boulders will be placed such that the top of the surface of the boulder-filled zone will roughly match the pre-slide slope (grade). The voids of the boulders will be filled with native soil and allowed to vegetate with local grasses and shrubs.
The design of the repair of the larger western slide (i.e., ““Landslide #1 (Major”) is more complex and will likely involve the installation of Gabion boxes. Gabions are wire boxes, approximately 3 ft. x 3 ft. x 6 ft., filled with rocks. Similar to the approach for Landslide #2, soil will be removed at Landslide #1 down to a stable layer, “benched”, and two-three rows of Gabions will be placed on top of the benches to provide support to restore the original trail width and level.
Both landslides are winterized for the current rainy season. Work on the landslides will be in the spring or summer months. The cost to mitigate Landslide #2 (Minor) is in the $50K range, while the repair of Landslide #1 (Major) is in the $200-300K range.
The next step is for the HVLT representatives and County to review the plan, propose suggestions, and approve a final plan. Then the plan will go out for bid by contractors capable of doing the work. Full financing of the repairs has yet to be resolved in collaboration with CSA 14-HVLT-HVCA, and Marin County Parks). Once work begins, a Kleinfelder geotechnical person will be onsite to direct the contractor’s work.
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