Vegetating steep reinforced slopes
Establishing vegetation on a very steep reinforced slope—1 horizontal to 2 vertical—can be difficult. The vegetation's performance is affected by many factors, including the orientation of the slope, slope infill material, topsoil thickness and quality, method of seeding, type of seed, irrigation, and competency of the installer. Ron Farmer, P.E., of SEH, presented case studies of several reinforced slope installations in the Twin Cities metropolitan area.
Farmer's research was conducted on three installations with different variables. At two sites the slope exposure was to the east, while at the other site exposure was to the south but was shaded. In one instance the reinforced slope infill consisted of clayey sand till, and in the other two instances the infill was Mn/DOT select granular borrow. Select topsoil borrow was used in all three cases. Seed mixes varied from crown vetch to Mn/DOT 500 and Mn/DOT 5B. One installation was also seeded with annual rye for initial growth and planted with daylilly plugs. Seeds were applied by hydro-seeding in one instance and hand-mixing the topsoil and seed in the other two cases. In two cases irrigation consisted only of rainfall; in the other case, a homeowner watered the site.
Farmer found that slopes with partial sun performed the best—that is, an eastern exposure or an exposure that is shaded during a portion of the day. "When planning a vegetation program," he stressed, "you must remember the face orientation and pick the right seed for the orientation." Also, he noted that an infill with sufficient fines promotes moisture retention and vegetation growth. However, backdrains may be necessary to ensure slope stability.
Other findings include:
- An aggressive seed mix—one that really takes off and grows quickly—provides the best vegetation coverage and inhibits growth of invader species.
- Seed application by hydro-seeding does not promote adequate growth.
- Slopes where seed and topsoil were mixed together provided the best overall coverage.
- Seeding with an annual rye to promote an early cover did not significantly aid in long-term growth.
- Specifications should require watering by the contractor to establish growth.
While the process seems promising, Farmer warned that it is "pretty labor intensive" and isn't an approach most contractors like to do. Many contractors have not developed business relationships with expert subcontractors for this type of work. And because contractors haven't installed many of them, the vegetated slopes tend to be more expensive—about $25 per square foot of face.
For more about the project, contact Farmer at rfarmer@sehinc.com.
Source material adapted with permission from Ronald Farmer.