Wednesday, 30 May 2012

Wind Farm Access Roads - Hardstand Area Design

The following post forms part of an article written for this month's edition of AUGI World magazine on using Civil 3D for designing wind farm infrastructure, pg 54.
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% grade and wide enough to extend beyond the extents of the hardstand. The LinkOffsetandSlope generic subassembly works well, see below.
Next build a dummy corridor along the centreline using the assembly. See 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% perfect model.

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