Design - Roadside and Site Development - Soil Bioengineering

Soil bioengineering (SB) uses live plant materials to provide erosion control, slope and stream bank stabilization, landscape restoration, and wildlife habitat. These techniques are used alone or in conjunction with conventional engineering techniques.

Soil bioengineering employs a partnership of the professions of soil science, landscape architecture, civil, hydrological, and geotechnical engineering, and horticulture.

Soil bioengineering is an excellent tool for stabilizing soil surface erosion. These methods should not, however, be viewed as the only solution to erosion problems.

Please see the topics below for more information.

  Soil Bioengineering Before technique    Soil Bioengineering After technique
Before and after pictures of Soil Bioengineering techniques in Raymond, WA

Slope Treatment
Slope treatment using a combination of brush layers, bare root plants, dead stakes and compost socks (SR 18 Maple Valley to Issaquah/Hobart Rd)


  • Projects usually require less heavy equipment excavation. As a result, there is less cost and less impact. In addition, limiting crews to one entrance and exit route will cause less soil disturbance to the site and adjoining areas.
  • Erosion areas often begin small and eventually expand to a size requiring costly traditional engineering solutions. Installing soil bioengineered systems while the site problem is small will provide economic savings and minimize potential impacts to the road and adjoining resources.
  • Use of native plant materials and seed may provide additional savings. Costs are limited to labor for harvesting, handling, and transport to the project site. Indigenous plant species are usually readily available and well adapted to local climate and soil conditions.
  • Soil bioengineering projects may be installed during the dormant season of late fall, winter, and early spring. This is often when other construction work is slow.
  • Soil bioengineering is often useful on sensitive or steep sites where use of heavy machinery is not feasible.
  • Soil bioengineering systems are strong initially and grow stronger as vegetation becomes established. Even if plants die, roots and surface organic litter continues to play an important role during reestablishment of other plants.
  • Once plants are established, root systems reinforce the soil mantel and remove excess moisture from the soil profile. This is often the key to long-term soil stability.
  • Soil bioengineering provides improved landscape and habitat values. 


  • Soil bioengineering has unique requirements and is not appropriate for all sites and situations. On certain surface erosion areas, for example, distribution of grass and forb seed mixes, hydromulching, or spreading of a protective layer of weed-free straw may be satisfactory and less costly than more extensive bioengineering treatments.
  • On areas of potential or existing mass wasting, it may be best to use a geotechnically-engineered system alone or in combination with soil bioengineering.
  • Project areas require periodic monitoring. On highly erosive sites, maintenance will be needed until plants are established.
  • Established vegetation can be vulnerable to drought, soil nutrient and sunlight deficiencies, road maintenance sidecast debris, grazing, or trampling, and may require special management measures to ensure long-term project success. 

Tools and References

Native Plant Suppliers
Supplies, materials and equipment for erosion control and soil bioengineering use can be very specialized, and therefore, difficult to find. The following are some sources of: native plant supplies, erosion control products and services, and WSDOT approved products (WSDOT Qualified Products List).

Many of these products and services have not been tested or approved in any way by WSDOT. WSDOT does not endorse any particular products or service providers over others, whether they are included or not. This is not an exhaustive list, please contact the organizations below to add your product.

Native Plant Nurseries
Native Plant Resources for the Pacific Northwest from Metro King County's Department of Natural Resources' Water and Land Resources Division

Products and Services
WSDOT's Qualified Products List

Soil Amendments
Soil conditions are the key to a successful revegetation or restoration project. Many times, soil properties must be changed by adding soil amendments. Information on some of these is listed below.


WSDOT's compost specification falls under Section 9-14.4 of the Standard Specifications.

Juli Hartwig, PLA
Roadside and Site Development Manager
Phone: (360) 705-7242