Big Oak Flat Road Emergency Repairs
Repair actions will include the following:
1.) An 8" diameter underdrain pipe will be installed along the uphill shoulder (cutbank side) of the road. This will require a trench (two feet wide by four to eight feet deep) along the cutbank shoulder. This trench will be lined with a permeable geotextile fabric. The pipe will be surrounded by eight inches of drain rock (3/4"-1" fractured rock) This underdrain will flow to a 24-inch diameter culvert pipe that will replace the existing 18 inch culvert at the low end of the failure area. Note: This trench will be installed within the new stabilization area.
2.) The existing paved road surface and the base/subgrade material supporting the road will be removed to the depth of the failure area (approximately 3,000 cubic yards of material along the uphill lane and another 8,000 cubic yards along the downhill lane).
3.) The historic stone headwall will be salvaged and repaired in-kind. Photo documentation will occur prior to work (and will include documentation of the stone retaining wall although no work is planned for the wall).
4.) After all the failed material is removed, a fabric and/or pipe drain system will be installed to allow the slope to drain by multiple ways if water were to enter the slope in the future. This will include sheet drains as well as multiple perforated pipe drains flowing out to the edge of the failure area. This series of drainage systems would be placed in combination with the new backfill material and the geogrid material noted below.
5.) Weather permitting, the intent is to salvage as much existing material as possible and blend it with good subgrade/base material to create an acceptable backfill material. The backfill would then be placed and compacted in 2 to 4 foot lifts until the new fill material is level with the existing road grade.
At the beginning of each lift of fill material a uniaxial/biaxial/triaxial geogrid material would be placed as a tie-back to help support the road and prevent this failure from happening in the future.
6.) As part of the backfill operation, the outside/exterior fill slope will be armored with large rocks (2 to 6 foot in diameter) to support the exterior of the new fill slope. The lower 2/3 of the fill slope is armored with existing large rock and the intent would be to salvage as much of this rock as possible, but more rock will need to be added because the existing rock was not adequate, more support is needed to prevent future failure.
7.) The road will then be repaved with 4 inches of new asphalt pavement to match the existing width, along with a replacement curb and pavement markings.
8.) Approximately 50 trees, mostly fir (24 inches diameter or smaller), will need to be removed to provide for repair access to the embankment.
9.) Staging will occur on the road itself and on road turnouts/parking areas.
10.) A temporary access haul road will need to be provided to allow for safe access for equipment south of the existing slide area. Impacts will be minimized by avoiding large trees and other vegetation.
On February 14, 2017, longitudinal cracking was discovered on the Big Oak Flat Road approximately 6 miles north of the El Portal Road intersection. Due to safety concerns, the road was closed on the evening of February 21, 2017, because the crack width had widened to just over three inches and one lane of road had sunk 4-6 inches in five days. The existing fill slope supporting the Big Oak Flat Road is approximately 50 feet high. On February 28, the crack had widened to two feet and had sunk two additional feet.