Wednesday, 7 December 2011

Wind Farm Access Roads - Drainage Ditch Locations

Wind farm access road design often involves the placing of a drainage ditch on the uphill side of the road only. This appears to be a straight forward task but is complicated by the fact that this side can change as the access roads wind their way around the hilly terrain and there is no subassembly that can automatically achieve this.

You can display contours and slope arrows on a surface style to identify the areas where the uphill locations are in relation to your roads. This can be cumbersome, however, particularly if you are dealing with a large surface where performance is affected by the need to regenerate all those slope arrows.

The uphill side of the road can be identified more efficiently by using a combination of a temporary corridor and a volume surface.

First create an assembly using the generic subassembly 'linkoffsetandslope', setting a slope of 0% and offset from baseline to say 20m (wide enough from baseline to give a good indication of the lie of existing ground).

The assembly layout appears as below:

Next create a corridor using this assembly, the access road alignment and vertical design for the access roads. Create a surface on this corridor using top links. Now create a volume surface (with 2D elevation banding style) using the existing ground as the base surface and the corridor surface as the comparison. In the surface properties on the analysis tab run an elevation analysis with one range and set the maximum elevation for this range to 0m.

In plan we can easily identify the uphill side of the road along our access roads.

In this case green areas identifying the sides of the alignment where we need ditches need to be applied. We can now split our design corridor at these locations and apply subassemblies with drainage on the correct side to each region.


  1. Good tips - I do a lot of windfarm work, do you have any more tips you can blog about??

  2. Hi Neil, thanks. I have a few more posts waiting to put up over the coming weeks. There is a workflow I presented at AU 2010 for slope stability that you can download. I think you may have seen this judging by the comment on that page:

  3. Yes - I've used the slope stability technique a few times now.

    I've also got an assembly (built with the SAC and a little help from Peter Funk at Autodesk) for floating roads that I use regularly. It checks for the highest side of existing ground and creates the road but it needs to be modified to also check that if the existing ground slope > 1:12 (say) the assembly doesn't apply.

  4. The SAC isn't something I have gotten my head around yet, its on the list somewhere though! Sounds like an interesting subassembly. I saw some discussion about floating roads on the C3DUKIE forum and the feedback seemed to be that you would need to use a custom subassembly. I have found the conditional cut/fill great for the earthworks when targetting the bottom of peat layer depending on whether you are in cut or fill. i will get a post up about it shortly...