In January of 2009, Whatcom County, Wash., had a rainstorm. Not just any rainstorm though—this rainstorm happened when there was already snow on the ground. The storm hit an area near the Middle Fork Nooksack Diversion Dam particularly hard, causing significant erosion, washout and landslide damage.
There were several stretches where the erosion cut into the roadway 2 to 3 ft deep, all the way up to 8 to 10 ft deep. Originally this site had two culverts passing under the roadway, but “the culverts had become plugged during the storm event. The debris load overtopped the road and culvert, and completely filled in the diversion structure located below,” explained Aaron Hartvigsen, P.E., of GeoEngineers Inc.
The slide made this road completely impassable, which was a major problem because it provides the only access to the diversion dam which is critical to maintaining a clean water source for the city of Bellingham, especially during dry summer months.
The area is subject to very high dynamic forces as a result of the debris flows that occur during major climatic events, so the engineers’ goal was to design a solution that would re-establish access and reduce the risk of future washouts and overtopping of the access road. Redirecting future water and debris flow away from diversion dam facilities was a major priority.
To accomplish these goals, GeoEngineers created a plan to realign the drainage channel and add a bridge using two flatbed railroad cars. The bridge required abutments and wing walls, for which Geoengineers recommended Redi-Rock.
The railroad cars sit directly on the abutments, and the wing walls help prevent erosion around the bridge structure. These walls utilized the Redi-Rock reinforced solution, which relies on a strong connection between the geogrid and the block itself. The upper walls were reinforced with Miragrid 5XT.
On the lower walls, “we arrived at a gravity system because of the shorter height requirement. Using gravity walls allowed us to reduce excavation on that portion of the project, because we didn’t have to cut the slope back as far,” Hartvigsen said.
In total, the project required 165 lineal feet of walls, up to 15 ft high (including embedment), with some tapers down to 4 ft high. This totaled 1,750 sq ft of wall facing. Once permitting and funding were finally secured more than two years after the event, the wall construction portion of this project was completed within two months.