Urban centers and suburban neighborhoods of Washington State's Puget Sound region are facing increased needs for improved roadways to accomodate traffic volumes, improve safety, and to increase access. The cost of right-of-way acquisition, environmental considerations, and adjacent property owner concerns frequently require designers to limit the lateral spread of both new and improved roadways. The hilly terrain of the region frequently dictates the use of steep slopes adjacent to roadways to minimize impacting adjacent property. The proximity of roadways to environmentally sensitive areas such as wetlands, aquatic resources, greenbelts, or geologic hazards forces designers to minimize encroachment more so than was common in years past. The ever increasing sensitivity of adjacent property owners and the frequent success of organized opposition groups also lead designers to consider alternatives to expansive rights-of-way.

The common use of relatively shallow slopes, whether in excavations or fill embankments, is giving way to the use of steeper slopes adjacent to transportation corridors. Limitations inherent in the glacially derived soils of the Puget Sound basin require comprehensive geotechnical studies to successfully incorporate steep slopes into the transportation project. The efforts of the geotechnical engineer and the engineering geologist, working in concert with the other design team members, allow the selection of the solution appropriate for a given application. Fortunately, some of the relatively new techniques and materials available today provide an opportunity to create an aesthetically pleasing finished product. The use of geogrid and geotextile reinforced fill embankments, frequently in combination with pre-cast concrete wall facing materials, is becoming commonplace, yielding beneficial results.

The ability of a soil mass to resist shear is the primary factor determining a stable configuration of an excavated slope or a fill embankment. The use of geogrid materials, such as Tensar, or geotextiles specifically layered in a fill embankment during its construction, effectively increases the shear resistance of the soil mass, allowing the construction of slopes steeper than might otherwise be possible. This allows designers to limit lateral encroachment of rights-of-way. However, the steeper a soil embankment face becomes, the greater its susceptibility to erosion. This may be overcome by "wrapping" the geogrid or geotextile materials around the fill of the embankment face, limiting exposure of rainfall and surface water to the soils. By virtue of the open space incorporated into their design, the geogrid materials allow the establishment of vegetation on the embankment face. Combining the use of layered geogrid materials or geotextiles with pre-cast concrete wall units allows the development of a finished appearance, which still meets the technical goals of the design by limiting embankment width and effectively eliminating erosion.

Slopes or embankments constructed in this fashion frequently yield other benefits. These benefits include a greater tolerance of differential settlement than afforded by more conventional rigid retaining structures. These engineered features may also be constructed with limited use of heavy equipment; much of the work may be accomplished by hand labor and relatively small construction equipment. This characteristic alone is frequently a deciding factor for the owner or contractor needing to deal with traffic control in busy urbanized environments as well as in limited access rural areas.

Slopes composed primarily of sand may be able to stand at relatively steep inclinations, as much as 1.5H:1V (Horizontal:Vertical) without introducing slope instability concerns. The erosion resistance of such steep slopes, however, is a consideration which the designers need to address from the project initiation. The use of semi-rigid polyethylene grid materials, such as Presto Products Geoweb, offers designers a way to reduce the susceptibility of sand slopes to erosion, as well as to protect the slope from undue disturbance during the landscaping effort itself. Placement of the cellular confinement system products on the slopes provides regularly spaced barriers normal to the surface water flow direction, thereby dissipating the erosive energy of flowing water, and minimizing the downslope displacement of soil and landscaping materials. The polyethylene grid materials also limit loosening and degradation of the upper surface of the sand slope as landscapers execute planting tasks. The grid opening sizes allow a variety of planting to occur without the need of cutting the grid materials, thereby maintaining the integrity of the protective product.

The multitude of innovative slope stabilization and erosion mitigation products available to designers today presents an opportunity for the development of new and adaptive applications. Long faced with expansive slopes of hydroseeded grass, bland reinforced concrete retaining walls, or acres of industrial rip rap rock, travelers of our highways and byways are going to become accustomed to viewing the creatively designed and attractive landscaping attendant with new construction. LASN