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Stabilization and reinforcement of wind farm access roads with geosynthetics

Final Inspection | October 1, 2019 | By:

This project is the construction of the first commercial-scale wind farm in North Carolina. It was for a renewable power generation facility providing clean energy. Constructing the wind farm required subgrade stabilization and reinforcement for more than 60 miles (97 km) of access roads that needed to be built in order to erect 104 turbines in the first phase of the Desert Wind Project.

The engineered geotextile in place. Photograph courtesy TenCate Geosynthetics Americas

As the only source of entry, the access roads had to support construction traffic over soft, wet North Carolina farmland soil. The access roads would be used by aggregate and concrete trucks, by delivery tractor trailers (with heavy turbine components), and for moving cranes between the wind turbines. The original design specified geogrid with dense-graded aggregate (DGA). The subsurface investigation revealed that the in situ soils consisted of peat and organic clays with a California Bearing Ratio (CBR) value as low as 0.7% and silty/clayey sands with a CBR value of less than 4.0%. From the start of the project, the grading contractor had trouble with the roadways being too saturated. Multiple attempts at stabilizing the subgrade with geogrid were unsuccessful. The only way to stabilize the roads was to add additional stone. However, the aggregate imported by railcars was very expensive. The contractor tried cement stabilization as an alternative; however, this method required too much cement and an excessively long curing time. This option was immediately ruled out for both time and cost reasons.

Designs with an engineered geotextile were recommended after evaluating the subgrade soils along with the anticipated truck and crane loadings. An engineered geotextile having a high flow rate, good filtration and confinement was deemed necessary due to the extremely saturated conditions of the low CBR soils. A geotextile having a high strength at a low strain was recommended for the worst conditions to help minimize the aggregate necessary for stabilizing the roads. After a successful roadway test section of 900 linear feet (274 m), the engineering team chose to minimize construction delays and optimize overall costs through the integration of the separation, reinforcement, filtration and confinement functions into a single product along with the aggregate. 

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