The construction of a 60 (120MW) turbine wind farm within upland forestry and moorland included the widening of 47km of existing forest tracks and the construction of some 18km of new access roads. The access roads needed to be able to withstand the trafficking from construction vehicles as well as the trafficking from the turbine construction activities.
The use of multiple layers of Tensar TriAxTM geogrids with site-won stone combined to form new access roads and to permit the widening of existing forest tracks. A saving of 15% in carbon emissions was achieved over an unreinforced solution.
Arecleoch Wind Farm lies 3 to 4km to the south west of Barrhill in South Ayrshire and involved the construction of 60 wind turbines together with associated infrastructure. The site consisted of moorland and extensive coniferous forestry which was felled to permit the construction of new access routes. Existing forest tracks which were less than 5m wide were widened to meet the new width and alignment requirement. Site investigation indicated that the ground strength was low with the CBR of the site lying between 0.5% and 1%. This would normally necessitate the use of very thick stone layers in order to accommodate the trafficking with the associated environment-related problems of large numbers of vehicle movements and excessive settlement of the road.
The stone on the site which was to be extracted from borrow pits on the site was a Greywacke, consisting of poorly metamorphosed mudstones, siltstones and sandstones. This material responds poorly to processing, breaking down to produce excessive fine material which is unsuitable for use in the road and turbine construction works. The roads were therefore constructed using largely unprocessed stone which, although well graded, varied from 300mm down in grain size. The excellent trafficking performance demonstrated that mechanically stabilised layers can be created effectively from less-than-optimum fill, on occasions.
During April 2009, Tensar designed floating roads to suit the different parts of the wind farm site using single layers of TriAx geogrid for lightly trafficked spur roads and up to three layers of TriAx geogrid and 800mm of stone on the most heavily trafficked main access roads. In addition, a generic design for the widening of existing forest tracks was produced which could be used with the main design to ensure that the widened sections would perform satisfactorily under the expected trafficking. A site trial was undertaken to confirm geogrid selection and to ensure that the geogrids worked with the proposed type and grading of stone1. Construction has proceeded using TriAx geogrids during 2009 and will continue into 2010.
Paul Sheppard, Construction Manager for Farrans commented, ’’Construction of 40km has taken only 4 months using TriAx. Floating road construction is more economical than a more traditional cut and fill operation.‘’
Neil Sturdy, Contracts Manager for Farrans went on to comment, ’’We chose Tensar International on Arecleoch wind farm because we were confident on the performance of their TriAx geogrid. Over 63km of road, the use of Tensar TriAx geogrids enabled significant savings on our stone quantities.‘’
1Ground Engineering, October 2009, pg 32-34