Machine Assisted Planting on Slope Failure above Railways

18 MILE PIT PROJECT

Location: Moyie, British Columbia

Client: Ministry of Transportation and Highways

Objective: To restore stability of the slide site above the railroad track, and to establish vegetation in order to reduce surface erosion processes such as rilling and gullying to avoid further debris flow.

In the spring of 2003, a slide occurred below the 18-mile Pit, depositing debris onto the railroad track below. This debris obstructed the railroad track. The slide was caused by diverted water above the site.

Terra Erosion Control Ltd. was retained by MOTH in May 2003 to develop and implement a prescription for soil bioengineering treatment for the site. A combination of brush layers and straw wattles were used to stabilize the site.

This treatment implementation was partially machine-assisted using an excavator and a gradall hoe to build the various prescribed brush layers onto the slope. The excavator was also used to remove unstable materials lying between the head scarp and the tension crack above the slide headscarp. The remaining work was implemented manually. A special blend of growing medium was applied to the brush layer structures to provide nutrients and moisture retaining capacity.

The structures were composed of the following species: Willow (Salix scouleriana), Black cottonwood (Populus balsamifera ssp. trichocarpa), and Red osier dogwood (Cornus sericea). Native seedlings were planted and a mix of grasses and legumes were applied between the brush layer structures to address surface erosion and provide further slope stabilization.

The site was monitored the following summer and fall of 2003. Although the province of British Columbia underwent an intensive drought with several large forest fires during that summer, the 18 mile pit site thrived with vegetative growth without any irrigation, establishing native vegetation at the site. Subsequent monitoring was carried out during the fall of 2005. Average growth of 2.0 m in height was observed on the upper 2/3 of the site, with some mortality found on the lower portion. Overall survival was estimated at approximately 85%.