Case histories from Poland, Canada, and U.S.
By Stephan M. Gale, P.E., D.GE., Mark Kurtz, Fred Chuck, P.E., and Dick Janse
The location of a renewable energy project is based on many technical and political issues. Sites are selected because of available wind forces, power distribution, and land leasing issues, but not normally on the ground support condition for access roads. Soft subgrades have to be dealt with.
This article will review the the types of geotextiles and geogrids used on various subgrades and water conditions for high-demand, short-term gravel access roads on renewable wind energy projects throughout Europe and North America through a series of case histories.
Geosynthetics are being used on a regular basis to stabilize renewable energy access roads throughout the world.
For each wind turbine, approximately 50 concrete trucks, steel reinforcement, the tower, the blades, and the turbine must access the turbine site over the course of a one- to two-month time period. The traffic loading considered for design is an axle load of 89 kN, a tire pressure of 690 kPa, and 100–500 axle passes. The developer usually sets a guidance for maximum rut formation of 50mm.
After this short-term heavy loading, the road must then perform as a low-volume maintenance vehicle access for the next 25–50 year design life. The use of geosynthetic materials successfully reduces maintenance concerns during construction and enhances longevity of the roads throughout the life of the project.
1.2 Wind capacity
The European Union (EU) installed 9.6 gigawatts (GW) of wind energy construction in 2011, bringing the total installed capacity to nearly 94 GW, enough to supply 6% of the EU’s electricity demand according to the European Wind Energy Association (EWEA). Germany has the most installed EU capacity followed by Spain, France, Italy, and the U.K. (North American Wind Power, 2012).
The United States installed 6.8 GW of wind energy capacity in 2011, bringing the total installed capacity to nearly 47 GW. According to the American Wind Energy Association (AWEA), the U.S. wind industry added more than 35% of new wind capacity during the past four years. There are 38 states in the U.S. with utility-scale wind installations—the leading states are Texas, Iowa, California, and Minnesota (Anderson and Gibbson, 2012).
Canada installed 1.2 GW of wind energy capacity in 2011, bringing the total installed capacity to 5.4 GW. Ontario has the largest installed Canadian capacity (North American Wind Power, 2012).
For estimating purposes, each wind turbine is approximately 1.5 MW; thus 9,600 MW or 9.6 GW of construction relates to approximately 6,400 turbines installed in the EU in 2011, 4,533 turbines installed in the U.S. in 2011, and 800 turbines installed in Canada in 2011.
2. Historical perspective
2.1 Initial use
Starting in the mid-1960s, the use of polymeric materials such as woven and nonwoven geotextiles and extruded geogrids were seeing initial use in road construction.
Early publications identify that a two-directional polypropylene geogrid with strengths under 35 kN/m were developed in the U.K. in the late 1970s and early 1980s. The late 1970s also saw the first commercial application of a reinforcing fabric in an embankment in the Netherlands.
It was concluded from some of the early test sections within highway projects in the Netherlands that elongation at break for reinforcing geotextiles should be below 10%, fabric modulus at the working load should be in the range of 5% elongation, and the strength should be at least 150 kN/m.
In the early 1980s, the use of reinforcing fabrics spread into the U.S. through the efforts of the Army Corps of Engineers. These products are the workhorses of the wind access road stabilization system over soft ground in the 21st century.
3. Design practice
3.1 Unpaved road design with geosynthetics
The design of geosynthetic inclusions in short-term high-demand haul roads relies on the mechanisms of base reinforcement and subgrade restraint.
The performance of geosynthetics in base reinforcement applications typically is determined by product specific testing. Laboratory or fields tests with specific products, specific cross sections, and various subgrade conditions are required to quantify the contribution of geosynthetic reinforcement to the pavement performance.
Design procedures for subgrade restraint and base reinforcement are documented in the literature and include Bender and Barenberg (1980), Giroud and Noiray (1981), Giroud and Han (2004), and Holtz et al. (1995). The design protocol we use includes a geosynthetic reinforcement modulus and a filtration and survivability component input. Additional input includes traffic loading (axle load and number of passes), subgrade soaked California Bearing Ratio (CBR) test value, and a final rut deformation requirement.
This article details three case histories, not the design calculation for the roadway section. The design calculation for high-demand short-term haul roads is the topic of a future paper.
The development of a road section involves the following steps: 1) assess applicability, 2) perform an unreinforced design, 3) select the target goal in terms of short-term structural section performance and long-term service life, and 4) evaluate the benefit offered by various geosynthetic materials.
4. Case histories
4.1 Golice, Poland
A wind farm project in Golice, Poland, in 2011 required 8.2km of access roads to reach 19 2.0 MW turbines.
Soil borings by the geotechnical engineer showed a sandy humus topsoil layer of 1m or less over plastic clay soils. The original plan was to remove a portion of the topsoil and then construct a 40cm aggregate surface. Because of the nature of the silty clay soils, the project team was concerned about the high moisture content and instability of the plastic clay soils and the loss of strength during rainy weather.
Road building activities occurred March–July 2011. The design of the aggregate-surfaced roadway section was based on the Bender and Barenberg approach considering the modulus of the geosynthetic and assuming a CBR of 2% for compacted silty clay subgrade at or near an appropriate water content for stability.
A 4.5m-wide roadway section was designed, with 40cm aggregate surfacing, with a 2% slope from a center of road crown. Because of the wet conditions during road building activities, most of the topsoil was removed to get to drier clay soils. Some areas of clay were aerated to dry out prior to placement of a geotextile stabilization layer.
The geosynthetic stabilization chosen for this project was a multifilament polypropylene woven geotextile with a tensile strength (EN ISO 10319) (MD and CD) of 30 kN/m at 3% strain and 56 kN/m at 5% strain and a water permeability (EN ISO 12956) of 20 l/m2s.
The aggregate surfacing was placed in two layers. The contractor originally tried to achieve stability by using the high-strength geotextile and an imported sand soil. When the 20cm-thick layer of sand could not effect a stable surface, rock (0-31,5mm) was added to the sand to achieve stability as shown in Figure 2.
This layer was then compacted and leveled and then a second 30cm layer of crushed gravel (0-31,5mm) was placed and compacted as shown in Figures 3 and 4. By the time the second gravel layer was placed, the geotextile had tensioned and no further rutting or cracking was evident in the roadway surface during compaction with a smooth drum roller.
The project was successfully completed by November 2011.
4.2 Lameque, New Brunswick, Canada
A wind farm project in Lameque, N.B., Canada, constructed in 2011, required access roads to reach 30 1.5MW turbines.
The island of Lameque is primarily covered by swamps or forests. The geotechnical engineering report identified organic site soils with a CBR in the range of 0.5-1.0%, as shown in Figure 5.
Typical access road construction in Canada is to remove 15-30cm of topsoil, place a 20cm-thick aggregate layer, and then cap the section off with 10cm of a final driving surface. But this typical section was not appropriate for the Lameque site because the subgrade consisted of peat-type soils.
For this project, two cross sections were developed based on research of rural roads in Scotland and North America. The roadway sections were based on the use of geosynthetics and the only available aggregate source on the island, a soft friable sandstone. The plan was to build over the vegetation-cleared surface.
Road building activities started in August 2010, with major completion by the end of November.
A 5.8m-wide roadway section was designed with a geosynthetic stabilization layer at the subgrade interface. The geotextile was placed directly on grade without removal of tree stumps or any of the peat, as shown in Figure 6. The existing root mat was not disturbed; this was critical for support of initial equipment.
The geosynthetic stabilization chosen for this project was a multifilament polypropylene woven geotextile with a tensile strength (ASTM D4595) (MD and CD) of 35 kN/m at 5% strain and a water permeability (ASTM D4491) of 20 l/m2s. The product used was manufactured in a width of 5.2m.
The aggregate stabilization layer was then placed. The contractor placed 70cm of the sandstone aggregate. The surface was rolled and ruts filled in with additional aggregate during the construction work.
In exceptionally soft areas with standing water, the stabilizing aggregate section was increased to 90cm and a biaxial geogrid with an ultimate strength of 35 kN/m was used and was placed immediately overlying the multifilament woven stabilization geotextile for additional reinforcement.
The purpose of the geogrid was to provide aggregate interlock. The high-strength geotextile provided separation and a much higher tensile modulus. The project was successfully completed by March 2011.
4.3 American Falls, Idaho, USA
The geotechnical engineer estimated a CBR of the subgrade of 1.1%. The concern was the moisture susceptibility of the silts and clays, especially during a rainy construction season.
The project was to be constructed in the spring of 2011. It turned out that that spring was very wet and the site’s soils lost their support capacity after a few truck passes.
The original access road design included the stripping of the topsoil and then the placement of a needle-punched nonwoven geotextile on the subgrade surface, an extruded polypropylene geogrid on top of the geotextile and then 23cm of the locally available, crushed basalt aggregate as shown in Figure 8.
Initially, a test section with the above cross section was used, except the initial aggregate layer was increased to 30cm. Failure of the 35 kN/m ultimate strength extruded polystyrene geogrid occurred after only a few passes.
Because of the wet conditions during the construction, the developer/contractor replaced the geogrid with a multifilament woven polypropylene geotextile, a product with a tensile strength of 35 kN/m at 5% strain as shown in Figure 10. The geotextile was placed on the subgrade, topped by 30cm of 75–100mm-minus crushed basalt aggregate.
As the section was installed, rutting developed as the geotextile tensioned, which was then filled in with more aggregate as shown in Figure 11. The section survived installation. A final surface of 3cm of a 25mm-minus aggregate was used as the surfacing for the access roads for the project. Once this section was decided, the contractor was able to install and cover 900m of roadway in a day.
There is an ever-increasing demand for wind energy projects through Europe, the Americas, and Asia. These projects require tandem axle/heavy vehicles to access remote locations over soft soil. Geosynthetic materials play a critical role in both short-term and long-term haul road performance.
Steve Gale, Gale-Tec Engineering Inc., Minneapolis, Minn.
Mark Kurtz and Fred Chuck, TenCate Geosynthetics, Pendergrass, Ga.
Dick Janse, TenCate Geosynthetics, Almelo, Netherlands
Anderson, A.C. and Gibbson, J.B. 2012. North American Wind Power magazine, Vol. 9, No.3, April 2012.
Bender, D.A. and Barenberg, E.I. 1980. “Design and Behavior of Soil-Fabric-Aggregate Systems,” Transportation Research Record 671, TRB, National Research Council, Washington D.C., USA: pp. 64–75.
Giroud, J.P and Han, J. 2004. “Design Method for Geogrid Reinforced Unpaved Roads,” ASCE Journal of Geotechncial and Geoenvironmental Engineering, Vol. 130, No. 8 pp. 776–786.
Giroud, J.P. and Noiray, L. 1981. “Geotextile Reinforced Unpaved Road Design,” Journal of the Geotechnical Engineering Division, ASCE, Vol. 107, No. GT9: pp. 1233–1254.
Holtz, R.D., Christopher, B.R. and Berg, R.R., 1995. Geosynthetic Design and Construction Guidelines, U.S.
Department of Transportation, Federal Highway Administration, FHWA-A-Hl-95, National Highway Institute Course No. 13213, Washington D.C.: p. 396.
North American Wind Power magazine, Vol. 19,
No. 4., p. 11, March 2012