Wind Farm Infrastructure

August 17th, 2012

Designing wind farm infrastructure can be a complicated process. The sites are generally located in remote upland areas to get the most from the wind. These upland areas tend to be covered by a peat layer (in Ireland) which provides engineers and planners with particular challenges. The sheer size of the turbines and the requirements for transportation and erection on site will also impact the design. This article takes a practical look at the tools used (and lessons learned) to design wind farm infrastructure from an AutoCAD® Civil 3D perspective.

Creating Geological Surfaces from Probed Depth Values

Probed depths or borehole logs provide us with information on the subsurface geological layers for the site. When importing and using this data in Civil 3D there are a couple of issues that need to be considered.

Issue 1: The surfaces created from the depth values in Civil 3D are of limited use. What we really need are the elevation values at each of the probe locations.  We need to convert the depth values into elevations.

Issue 2: Due to differing surface data resolutions (typically more points in the existing ground surface) the bottom of peat surface may not appear to accurately represent the geological layer – it may not ‘follow’ the lie of the land. See screen grab below. (You could, in some cases, see your subsurface extend above the existing ground in section). We need to create a surface that uses the probed depths and also ‘follows’ the existing ground in the areas where we do not have any probes.

There is workaround that has previously been posted on a number of blogs and forums that solves both of these issues. Here it is described as applied to a wind farm project - with a little bit of explanation of what is going on in the background from a Civil 3D point of view.

Solution:

1. Create a TIN surface from your probed peat depths and call it Probed Peat Depth.
2. Create a volume surface using the Existing Ground surface and the Probed Peat Depth surface and call it Peat Volume. The order in which you add the surfaces is important (base – Probed Peat Depths, Comparison – Existing Ground). You now have a volume surface that has depth values that are equal to the elevation for the bottom of peat.
3. Create a new TIN surface and call it Surface from Peat Volume. Paste in the Peat Volume surface. Pasting a volume surface into a TIN surface creates a surface with elevations equal to the depth values of the volume surface. We now have a TIN surface representing the bottom of peat.



The surface created in step 3 uses the probed peat depth values and follows the existing ground in between probes.  Note: This is not a true representation of the subsurface geology, but it is a good base with which to start.

Access Roads

With the access road locations (http://au.autodesk.com/?nd=event_class&session_id=6919&jid=610991) decided upon, we can then start to look at the design of the roads. We need to take into account the different subsurface geological layers when creating subassemblies to model the earthworks. In this example there are three layers—a peat layer overlying an intermediate layer of competent material under which there is a rock layer. The peat layer will be stripped from the site and the roads will be constructed on the competent layer beneath.

Typically the earthworks for the access roads will vary depending on whether the design is in cut or fill at a particular location. For example, in cut, the earthworks may need to slope up to meet the existing ground at a 1:1 slope. In fill, we may need to slope down to meet the underlying rock layer at a 1:1 slope first and then back up to existing ground at 1:1. The ConditionalCutFill subassembly can be used to control how the earthworks behave, depending on whether we are in cut or fill.

Step1: Create the Assembly

Cut Situation

• Add a conditional subassembly. Set the Type parameter to Cut and leave the min and max values as they are. Attach this to your assembly as shown below.

Note: The Layout Width and Layout Grade do not have any effect over the behaviour of the subassembly. Rather, they just control how the subassembly appears on screen. In fact the appearance of these subassemblies can be off-putting when you are first using them. My rule of thumb is to ignore how they look on screen and instead concentrate on ensuring that their properties are correct.

• In the cut situation we want to slope back up to the existing ground at 1:1. We need to add a subassembly to model this. Select the LinkSlopeToSurface subassembly (Generic tab of Tool Palettes) and set the Add Link In parameter to Cut Only. We will set this to target the Existing Ground surface in the corridor later. Attach this to the end of the conditional cut subassembly as shown below.

Fill Situation:

• Add another conditional subassembly—this time setting the Type parameter to Fill.

• In the Fill situation we want to slope down to rock first at 1:1. Use the LinkSlopeToSurface subassembly again. Set the Add Link In parameter to Fill Only. Attach this to the end of the conditional fill subassembly.

• From the rock layer we want to slope back up to existing ground at 1:1. Use LinkSlopeToSurface once more and set the Add Link In parameter to Cut only. Attach this to the end of the previous subassembly.

You can mirror all of the earthworks subassemblies to the other side of the assembly to speed things up. Just select the subassemblies you want to mirror and choose Mirror Subassemblies from the ribbon. The complete assembly will look something like this (I have added a ditch on the left also).

When you are setting targets later in the corridor it is important that you have named each subassembly so that the name identifies what surface you will be targetting. e.g.,Cut to Rock Left 1:1.

Step2: Create the Corridor and Set Targets

Create your corridor and set the targets in the corridor properties as required (see screen grab below for an example of how they might look).

Now we have our various earthworks conditions being controlled automatically by one assembly. Should changes be made to the vertical design of the roads later, the cut/fill conditions on the conditional subassemblies will look after any changes to the earthworks for us. See sample of what the resulting sections look like below. (Existing ground - green; top of rock - magenta; road surface – red.)

Cut Situation



Fill Situation (on right-hand side)

If you are looking to explore the conditional cut and fill subassemblies a bit further, I recommend working through the example at the following link: http://usa.autodesk.com/adsk/servlet/index?siteID=123112&id=7151908&linkID=9240695

3. Hardstand Areas

Hardstand areas are constructed to provide sufficient space for the cranes to operate during erection of the wind turbines. The hardstand areas must be large enough for the cranes to operate in and also provide storage space for materials.

The hardstand areas are effectively a widened region on the corridor. Typically the hardstand areas are flat and widen at right angles to the corridor (see image below). This can cause problems when targeting the hardstand widen alignments using an assembly on the centreline alignment. Civil 3D targets perpendicularly from the baseline alignment and will not model the hardstands correctly at the widen region.

There are a number of methods for modelling this type of widening.

a) You could add offsets   to your assembly and use the offset alignment to provide the offset value. This method gives mixed results when the widening is perpendicular to the main alignment.
b) You could also create a featureline defining the edge of the hardstand and then use the grading tools to model the earthworks. This has the advantage of correctly modelling the grading in tight corners where the corridor would otherwise overlap. The disadvantage is that you have a number of ‘parts’ to your model and increased margin for error.
c) The third method involves adding the hardstand alignments as new baselines to the corridor and applying earthworks assemblies along these baselines. This results in one object (the corridor) controlling the earthworks thus reducing the amount of ‘parts’ in your model. This is the method that has given the best results and the one we are going to look at here.

Step1: Create Hardstand Alignments and Profiles

Create alignments defining the left and right edge of the hardstand and then create profiles along these alignments. As mentioned above, the hardstand areas need to be flat—the profiles along the edge of the hardstands need to be at the same level as the centreline profile. To achieve this we will use a dummy corridor to provide levels along the hardstand alignments.

To create the dummy corridor, first create an assembly that has 0 percent grade and is wide enough to extend beyond the extents of the hardstand. The LinkOffsetandSlope generic subassembly works well, as shown below.

Next build a dummy corridor along the centreline using the assembly (see image below).

Create a surface from the corridor and finally a surface profile along each of the hardstand alignments sampling the dummy corridor surface. This gives our levels along the edge of the hardstands.

Step 2: Create Hardstand Assemblies

The hardstand assemblies will be applied along the left and right hardstand alignments. The left and right assemblies will consist of the left and right earthworks subassemblies used in the main access road assembly.

Create your new assembly. Select the earthworks subassemblies from the main access road assembly. In the case below I am selecting the ditch, conditional cut/fill, and generic subassemblies used to model the earthworks for the left side of the road.

Copy these to your hardstand assembly and repeat for the right-hand side. Your finished assemblies will look something like the following.

Step 3: Add Baselines and Set Corridor Properties

Next add the hardstand alignments to the main corridor as new base lines. In the corridor properties add a region to each of the new baselines for the chainages of the hardstand.

Anyone who has used corridors to model earthworks in tight corners will know that the downside is that the corridors do not resolve the overlap on the insides of bends similar to the grading tools– see screen grab below.



To resolve this issue we can use a workaround. In the corridor frequency for the hardstand regions set the sampling frequency to a value greater than the total length of the alignment and set the additional sampling frequencies to “No.”



This will result in no automatic corridor sampling frequencies being applied to the region. We will then add in sampling stations manually at points along the region ensuring there is no corridor overlap in the earthworks.



This will not result in a perfectly modelled corridor, but the differences in terms of volumes calculations are tiny in the grand scheme of things. The benefits achieved by having one corridor where you can easily make edits and create surfaces for volume calculations far outweighs those of having a 100 percent perfect model. 

4. Earthworks Volumes

When calculating the earthworks volumes for the site there will usually be a number of subsurface layers that we will need to take into account. For example, a typical site might consist of three layers: a peat layer overlying an intermediate layer of reusable material under which there is a rock layer. All of these will probably be intersected by your proposed access road surface at some point and volume calculations will involve comparisons between two or more of these surfaces.



In calculating the volumes we will look at two methods: Volume Surfaces and Materials from Cross Sections. While using volume surfaces is a great method for calculating volumes I have found that using materials is more flexible. You define a material as the volume you wish to calculate. For example, I might create a material called Volume of Rock

Cut and define it using the access road proposed surface and the top of rock surface. Materials are particularly useful where you have more than two surfaces bounding your desired volume. It also allows you to generate a cumulative volumes report for each material on a per cross section basis.

Volumes to Be Calculated

1. Volume of peat to be stripped from site
This is straightforward. Create a volume surface using the existing ground surface as the base and bottom of peat surface as the comparison. Extract the border from your final corridor top surface to give you a polyline representing the extents of the works. Add this polyline to the volume surface as a boundary. This can be added from the Toolspace.

2. Volume of rock cut
This is the volume bounded by the top of rock and the access road proposed surfaces. Create a material (Sections menu<Compute Materials) using these two surfaces. Set the Quantity Type and the Conditions as shown below.



The conditions in this case are telling AutoCAD Civil 3D that we want everything above the formation surface and below the rock surface.

3. Volume of rock fill
This is the volume bounded by the bottom of peat and access road proposed surfaces. Create a material using these materials. Conditions are everything Below the proposed surface and Above the peat surface. Quantity Type is Fill. See completed material below.

Volume Cut/Fill Rock Materials on Cross Sections:

4. Volume reusable material cut
Between the bottom of peat and top of rock layers on this project there was a layer of material that was deemed competent enough to be reused as fill elsewhere. The volume required in this case is bounded by three surfaces: Proposed surface, top of rock, and bottom of peat.

Create another material as before adding in the three surfaces. The Conditions are everything below bottom of peat and above the other two surfaces, Quantity Type is Cut. See below:

Volume Reusable Material on Cross Sections:



Once you have your materials calculated you can generate a volume report—Sections menu<Generate Volume Report—and select the report template you wish to use. This provides an easy-to-read, cumulative volume report for each of the cross sections.

Donal McMoreland is a civil applications engineer for Amicus Technology, Calway, Ireland. He holds a bachelor's degree in Civil Engineering and has worked in Ireland, Australia, and Canada in the water services and land development sectors. For the past four years, he has developed and delivered customized AutoCAD Civil 3D training courses to civil engineering consultants and local authorities throughout Ireland. Donal is one of the founders of the Irish Civl 3D user group and author of the Civil 3D blog www.civilintentions.blogspot.com. For comments or questions email Donal.mcmoreland@amicustec.ie.

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