Road Design & Earthwork Optimization Software for Wind Farm Networks

I have been contacted by Erin Wasney, Business Development Manager at Softree Technical Systems.

She proposed to write a guest post on RoadEng, a software developed to design long roads and large networks of low volume roads faster and easier than other civil design software.

I am more than happy to share his post with you.

(Beginning of guest post)

Onshore wind farms often require a large network of roads to install and access turbines, and as previously shared by the team at www.windfarmbop.com there is often a need for quick planning and analysis. As followers of the blog, we were inspired to share some information on our software, RoadEng.

The video below shows an example of a wind farm project created in RoadEng Civil Engineer. As a software tool, RoadEng is a detailed geometric design software, but its focused functionality allows for it to be used for quick planning, analysis and visualization.

Created more than 30 years ago to support the design of forest road networks in complex terrain, RoadEng is now being widely used in renewable energy projects. There are many similarities between the requirements of forest road networks and those required to develop and maintain windfarm projects, mainly:

  • Projects are often time and cost sensitive, both from a planning and implementation standpoint,
  • Road networks are usually low volume resource roads that may include greenfield construction or upgrades to existing access infrastructure,
  • Pads and/or landings are usually required and built as part of the road portion of the project,
  • Location of the road is often influenced by other project factors and being able to quickly update a road design to account for other project parameters is important to help facilitate timely decisions and avoid expensive redesigns often encountered when using slower, less dynamic/interactive software, and
  • Often the personnel planning the access infrastructure have additional roles in the project.  Having a fast, easy-to-learn software that doesn’t require a drafting background to quickly become proficient makes adopting RoadEng as a design platform easy but more importantly reduces the need for more project personnel.  This often allows decision makers to have a more wholistic understanding of the project, do more design themselves, increasing productivity, and avoid some of the challenges associated with design review and communication.

In RoadEng, the horizontal and vertical alignments are connected, and a cross section geometry is attached to the 3D centerline automatically.  Pre-built customizable components make building smart cross section geometries easy, and in situations where more complicated cross sections are required (as in the video below), components can be combined and/or linked together:

As the user creates or adjusts their alignment, all aspects of the design update in real time; no need to manually prompt the software to recalculate and since the software is light, users can avoid having to deal with the frustration of click-wait, click-wait, click-wait as their computer struggles to keep up with the computation requirements. In other software, it is common for users to truncate their projects into short, workable segments to reduce computing requirements, in RoadEng it is not uncommon for users to do detailed designs for multiple alignments in a single file for roads over 50km long and based on large LiDAR data sets.

Aside from just considering site geometry, road costs are often significant and are worthy of careful consideration during the design process. RoadEng offers several tools that help designers quickly evaluate how much effort is required to build a project.  These tools include:

  • Traditional mass haul diagram, with cut and fill quantities
  • Opti Haul diagram, tracking excavation and fill volumes by material type; solving for optimal material movement by individual material types, definable quality requirements and movement direction constraints
  • Alignment Costing, including an ability to easily compare alignment options and get the associated sub-grade construction costs for each option (we call this “design time costing”).

Finally, construction costs and time spent designing can be further reduced by using Softree Optimal. It is a patented earthwork optimization add-on tool for RoadEng that can help reduce costs by generating a vertical alignment that minimizes earthwork costs (embankment, excavation, and movement costs for sub-grade materials).  According to a study completed by FP Innovations (2017) on low volume resource roads, vertical optimization reduced the estimated construction cost by 13% to 22%, on average, depending on road design standard.

Other notable functions included in RoadEng for wind farm design:

  • 3D symbols for turbines – allows for visualization of the turbines in the context of the roads and pads
  • Drainage tools – hydrology tools and watershed calculations, as well as a culvert editor tool for quick additions of culverts and cross-drains to projects
  • Graded pad object optimization – balances cut & fill for graded pad objects
  • Multi-Plot report builder – semi-automated creation of construction documentation
  • Field-focused tools – creation of Avenza georeferenced maps, GPS integration during design

Although not ideal for every civil engineering project, RoadEng performs well in rural infrastructure applications, particularly for quick planning and analysis, as is often the case for wind farm road networks.

Infraworks for due diligence and preliminary designs

If you either work for a WTG manufacturer or any electric utility the situation in which you are asked to do a complete analyse of a project very quickly (like in 2 or 3 days) is very common.

You are usually asked to do this task for several reasons:

  • Preparation of a first rough layout with preliminary quantities.
  • To compare different alternative WTG layouts.
  • For a technical due diligences.

Until not so many years this type of assessments was done using the specialized design software: MDT, Civil 3D, Istram-Ispol, Clip, etc.

These programs are more focused on a detailed design and the processes are not easy to adapt to a different scenario, where quick results are required.

In the last years Infraworks has emerged as a powerful alternative.

It is a planning and design platform from Autodesk that has experienced an impressive development. It offers a bunch of very useful tools that can be easily adapted to the BoP design of a wind farm.

Some of the principal qualities are:

  • It allows engineers to quickly create a preliminary design in a realistic environment, making possible to position the point of view anywere in the wind fare and immediately be able to visualize the existing conditions.
  • The program supports data from multiple sources: GIS (shapefiles, geodatabases, etc), CAD, raster, and all kind of BIM-data. These data is integrated into an interactive 3D model.
  • Cloud technology is integrated through BIM 360. Different users from the same team can work dynamically on the same projects in remote - i.e. from any point in the planet, a feature especially useful in these "work from home" days.
  • Multiple alternatives called “Proposals” can be generated for the same project.
  • The program allows to extract quantities, create shadow analysis, analize conditions with style maps using “style rules”, etc.
  • It is that it is very user-friendly: it takes only a few days to learn the basics and start doing your own projects.

One of its major advantages included in the last releases is that it is possible to import detailed designs from other software such as 3Ds Max, Civil 3D or Revit.

What I like the most is the dynamic flow of work implemented in the most recent versions between Infraworks and Civil 3D. In this sense, I like to think about Infraworks as a good complement to Civil 3D. We can either:

  • Start the design in Civil 3D and export it to Infraworks.
  • Create the predesign directly in Infraworks feeding the model with data available in all kind of source formats: shapefiles, raster, etc.

The synchronization between both programs is not bidirectional: any change done in the Civil 3D model is automatically included in the Infraworks model but not the other way around.

Going more deeply in what the program can offer, here are some comments specific for the wind farm BoP items:

Crane pads

The software offers a quick way to obtain quantities for big construction areas such as crane hardstands.

The option “Land Areas & Grading Behaviours” allows the user to easily calculate quantities for gradings and landfills.

One point to improve is that currently there is no way to export this data to any other format: the information must be extracted "manually" to an external BoQ.

Foundations:

It works in a similar way as explained before with the hardstands.

For example, we can quickly model the foundation bottom pit and calculate the excavation volumes.

We can do it to analyse a certain position or, taking advantage from GIS format files and other formats, do an overall analysis of all the positions.

Roads

Infraworks has two typologies of roads, which will be used depending on the needs of both the designer and the project:

  • Planning roads: These are lightweight roads that use spline geometry. You can add planning roads to a model or import roadway data as planning roads. This format does not allow to extract quantities from the model.
  • Component roads: these are configurable roads in cross section, vertical and horizontal geometry. They provide a precise control of geometry and grades. This is the type of road we are interested in when creating a BoP design in a wind farm because they have available features such as the grading tool and the mass balance quantification.

The interesting thing about this tool is that we can have a full control of all the parameters from a 3D environment and we can export to quantities to a .csv file format.

Additionally, the intersections between roads are automatically created and very easy to handle and edit.

Drainage

Hydraulics analysis and drainage calculations are hard to deliver in few hours - at least if you want to do a good job.

However, Infraworks offers a useful and easy module to deal with this matter. Some of the main available services are:

  • Watershed analysis: it includes point watershed creation, watershed analysis along roads and calculation of flows for a calculated watershed according to different methods (rational, regression, etc).
  • Culvert design analysis with automated culvert placement, culvert analysis and culvert reporting
  • Roadway inlet and pipe analysis, including automated inlet and pipe placement and inlet and pipe design analysis.

As a drawback, it is worth point out that drainage tools are only available in combination with a BIM 360 account.

Furthermore, depending on the kind of service required, we will need to have "cloud credits" available for certain processes such as culvert analysis.

In any case, it is difficult to find in the market a tool that gives such a powerful and interactive tool for preliminary drainage calculations.

Creation of realistic videos

Useful realistic videos or tours around the model can be created in a very easy way. They could not only look really fancy in a presentation but also are useful to assess whether there is a major mistake in the predesign. You ave an example at the beguinning of this article: the creation of this video took no more than 15 minutes from when the model was ready.

Other options:

There is also the option to create advanced customized cross section profiles.

This could be of great use to model other elements linear elements such medium voltage electrical cable trench or high voltage lines.

Final comments

Beyond all the great functionalities described so far, we must not lose the perspective: as it is now, Infraworks is not intended for a full detailed design. As a user you frequently feel that the program lacks "advanced capabilities" that would be highly appreciated.

Autodesk is aware that the software has a great potential and the company is working constantly to improve the product. To have a good idea of the expected developments in future releases I recommend to visit the webpage “Infraworks Public Roadmap”:

These new developments are based on the feedback given by the users. Suggestions and proposed new features are posted in the “Infraworks Idea Forum”. I have personally posted some of them myself.

https://forums.autodesk.com/t5/infraworks-ideas/idb-p/129/tab/most-recent

I also include the link to the Youtube official channel which I consider a good audio-visual reference for those who want to crack on with the software:

In short, Infraworks is already a tool to take into consideration, not only in preliminary designs but also as a support for final designs.

It will be interesting to see whether future developments will transform in a main reference for designers - not only for wind farm projection, but also for all kind of civil works modelling.

Modifications to wind farm access roads: a step-by-step guide

One of the problems that occurs frequently to engineers working on wind farms is how to modify existing access roads to allow the transit of special vehicles.

I was contacted by several readers of the blog who had this issue - the last one was Egil from Norway who led me to write this article.

In general, the problem is usually a curve that is too tight, a change of slope that is too fast (i.e., a road crest in the vertical alignment where the change of slope is too sudden and the truck “hits” underneath) or a combination of the two.

I am describing in this post the procedure I have followed in the past. If you have followed different steps please drop me a line.

The example is taken from a project where the blade truck was hitting below (there is usually very little space under the trailer – as little as 20 centimeters).

The first step is to send a topographer to create a cartography as detailed as possible of the area.

Both GPS and “traditional” topography will be fine while I would avoid LIDAR because you would have too many points to work with. It is sufficient to work with a simplified model.

You will usually receive the results in AutoCAD DXF or a similar format, ready to use.

The next step is to create a tridimensional model of the existing road. I usually work with AutoCAD Civil 3D – however there are several similar software in the market.

Then I use AutoTurn to calculate the path of the truck. I really like this software as it gives me a very reliable simulation of the wheel path and the swept path area.

AutoTurn simulation of the the path followed by the truck. The problem area is on the bottom.

The following step is to use Civil 3D (or your favourite software) to calculate longitudinal & cross sections (representing the ground "below the truck" and "as seen from the wheels")

This help me deciding if and were changes to the existing roads are needed. You can see in the image below where the trailer is "touching" the road. In this example I was already aware of the problem as I was contacted by the colleagues on site.

A section showing the blade trailer (hatched rectangle) and the elevation of the existing road below. As you can see the truck is hitting the ground below.

The next step is to modify the geometry of the road "manually".

This is done changing the elevation of the points in the topography one by one in the area of the problem recalculating the longitudinal profile and the transversal sections until I reached a shape that looked OK in terms of torsion, slope and free space below the truck.

The elevation of the problem area before and after (points marked in red)

You can see in the image above marked in red the points that I have changed. At two points the ground has been lowered, at one point it has been raised.

When you are satisfied with the solution you can export all relevant information (digital elevation model, cross section, longitudinal profiles, etc.) and send it to the topographer on site so that the changes can be implemented.

In the case of bends, if only minor modifications are needed, the same procedure can be followed changing points as needed.

Plate bearing test for wind farm roads & hardstands

Plate bearing test elements. Copyright image www.southerntesting.co.uk

Plate bearing test (also known as “plate load”) is one of the in situ investigations most frequently used during the construction of wind farms.

Its objective is to confirm that roads and hardstands meet the minimum requirements for compaction and bearing capacity.

Basically, it is performed pushing a steel plate against the material to be tested (usually the subgrade or the surface layer of roads and hardstands).

Three different standard plate sizes are available – the larger the diameter, the greatest the depth reach by the test. The biggest plate (762mm) is the one usually used in wind farms.

The plate is loaded by hydraulic jacks and it is connected to a counterweight (usually a truck or a construction equipment with a weight of several tonnes).

The jacks push the plate and the settlements are then measured at set increments of the loads.

Two different load cycles are performed (that is, the plate is loaded, unloaded and subsequently loaded again).

The results of the test are two “load / settlement curves”, one for each load cycle. With the curves two value called “Strain modulus” can be calculated – one for the first loading cycle (Ev1) and one for the second (Ev2).

I will not enter in the detail of how Ev1 and Ev2 are calculated (nothing too complex by the way – the formula is relatively simple).

Ev1 and Ev2 gives you a value in MN/m2 linked to the bearing capacity, while their ratio (Ev2/Ev1) will tell you if the compaction is sufficient.

Usually this test is done in wind farms road (every 500m or every Km are two standard values) and in the cranes hardstands (at least one test for hardstand, but frequently I have seen 4 tests, one below each of the crane legs).

The test is relatively slow: usually only a few positions can be tested every day. If needed the results can also be correlated with CBR values.

Technical requirements and tests for wind farm roads and hardstands

Two readers asked me how to assess the quality of the civil works in a wind farm.

This is a very broad topic and it requires some previous knowledge in geotechnics and road construction.

However, I think that someone might found useful an introductory article with some of the requirements that I would recommend to include in the technical specifications and test during construction.

The objective of these requirements and tests is to confirm that the materials used are appropriate and that the works have been properly executed.

As the readers of this website are from different geographic areas, I will not suggest a specific national or international norm such as ASTM, AASHTO, UNE, etc.

Finally, I also intentionally did not specify the value that I consider appropriate for each parameter.

However, if you need support to find an adequate standard or to define a specific value, please feel free to contact me.

For the embankments, I would recommend to define:

  • Maximum grain size. This requirement will avoid the presence of rock and boulders in the wearing surface of the road.
  • Appropriate gradation by sieve analysis (that is, the “granulometry” or particle size distribution). This requirement will ensure an appropriate interlocking between the particles and it is linked to almost all other property (e.g. stiffness, fatigue resistance, permeability, etc.).
  • Atterberg limits (Liquid limit and Plasticity index). You do not want to use plastic material behaving like clay.
  • Low organic matter and soluble salts content. This requirement will insure that only appropriate materials are used in the construction of the embankment.
  • Very low swelling or collapse potential.

The material should be properly compacted. Compaction grade is defined by the percentage achieved compared to the “optimum Proctor value”.

You also want a low deformability. This is usually defined the “Strain modulus” (or “Ev1” and “Ev2”) – basically the deformation of the material under two load cycles as resulting from a Plate Load Test.

I would also recommend defining a minimum CBR (“California bearing ratio”) value for the top and the core of the embankment.

If possible it is always a good idea to build a test section of the road, for a full-scale trial of the construction procedure. It can also help to determine the most appropriate compaction moisture – the actual value could be different from the value given by the laboratory test.

Graded aggregate is the material used in the upper layers of the internal roads.

It is made of well-graded crushed stones. The typology of arid to use and its granulometry depends on the local availability of materials.

In addition to the parameters defined for the embankments I would recommend to specify:

  • Sand equivalent (because you want as little clay as possible).
  • Loss by abrasion (because you want to use for the wearing course a material that can last for several years, ideally more than 20).

It is also advisable to define the maximum difference between the actual roads finished surface compared to the theoretical surface defined in the project.

Finally, compaction must be verified. Again, the standard option is the plate bearing test.

Wind farms Road Survey: 3 things you should ask

One of the first point that I try to spot when I receive an offer for a wind farm is an unusual dispersion in prices among subcontractors.

Usually, it means that there has been a misunderstanding regarding the actual scope of works.

Several items have usually very stable, predictable prices. This is the case for instance with the main transformer, which usually will have a similar price between the various offers (unless someone is proposing a Chinese transformer assembled in North Korea, or something similar).

However, other prices may vary wildly. In my opinion, the most unpredictable item is “the works included in the Road Survey”.

If you are reading this post you probably know what a Road Survey (or Route Survey) is.

Basically, it’s a document explaining how the various wind turbine components will reach the wind farm area. Depending on several factors the logistic can be very easy or extremely complicated.

For instance, it’s not strange for a project to use 2 different harbours, maybe one for the blades and another for towers and nacelles.

Also, the road from the harbour to the project area can be flat and easy or full of bends and critical points where works are necessary.

All this point are normally discussed in the road survey. If you are working at a wind farm development, you should have a similar study. Otherwise, you could discover at a later stage that there is a critical point somewhere in the access roads.

The most critical points are usually houses and land plots with owners reluctant to concede right of way, but you can find a lot of obstacles on your ways (e.g. overhead lines, rivers, huge earthworks, etc.).

So, what is the problem with Road Survey?

Basically, it’s a non-standard document. Each company is doing it in its own way. It can be redacted by a transport company or by some external consultancy more or less experienced in wind farm.

The result is usually similar – something that it’s very difficult to understand and to quote for the company that is going to do the works in the real word.

Often your only alternative will be to ask for a lump sum quote, making comparisons and considerations about price fairness of different bids impossible.

Also, it could be difficult even for someone knowledgeable about the business to understand what is really necessary. Is the necessary “bend widening” a quick, 10m3 work or is it an extensive (and expensive) intervention?

Sometimes you have some kind of control on this document, for instance if you are requesting (and paying) it.

If this is the case, you should be sure that some key points are included in the report:

  1. UTM coordinates of the points where works are necessary
  2. Pictures of the area (including orientation of the photo - looking North, East, etc)
  3. Exhaustive work description, with at least a tentative bill of quantities (m3 of cut/fill, number of trees to cut, meters of New Jersey barrier to remove,  etc.)

A good job would also include some kind of topography, but this is probably asking too much.

Slopes stabilization using Vetiver

I’ve recently worked on a slope stabilization preliminary project.

Among other action, the use of Vetiver grass for stabilization purposes has been suggested by the customer, who had used it successfully in another wind farm in the same area.

Although this technique is used in Italy and other European country I was not an expert, so I’ve done a search to see the peculiarities of this solution and I want to share the key points I’ve discovered.

Basically, Vetiver is the colloquial name of a plant originating from India and South East Asia.

There are 3 species:

  • Chrysopogon zizanioides (formerly known as Vetiveria zizanoides), from southern India: this is the one that you’re going to use – deep roots, sterile and not invasive.
  • Chrysopogon nemoralis, from Laos and Vietnam: shorter roots and not sterile.
  • Chrysopogon nigritana, from South and West Africa: not sterile.

 

Vetiver can be used both on cut and fill slopes, up to a 1:1 gradient (45 degrees).

The number of plant that you will need will vary – the horizontal distance between plants should be from 10 to 15 cm, while the distance between rows (measured on the slope) will vary from 1 m. (highly erodible soil) to 2 m. (more stable soil).

First and last rows should be planted at the top and toe of the slope.

As cuts and embankments are not fertile (or at least, they shouldn’t be) fertilizer and watering at regular intervals will be needed. Later on regular cutting will be needed.

It growths very fast (up to 3,5 meters in 12 months).

This solution has several advantages:

  • Above all, it’s cheap. This is particularly true in countries where labor wages are low. Also, there are no heavy long term maintenance costs.
  • It’s a natural erosion control measure (at least, more natural than a plastic geotextile and less ugly than a rip rap).
  • It’s very effective.

 

There are also some potential problems:

  • Intolerance to shading (but this can be an advantage if you use it only as a pioneer plant for initial stabilization).
  • Roots don’t penetrate well below water table.
  • Need an initial establishment period of 3 to 6 month depending on the climate. During this phase it must be protected from livestock.

 

You can read more on the subject on this interesting book from the Vetiver System network.

Wind farms roads and crane pads made of laterite

I’m currently having the pleasure of working at a wind farm project in Senegal.

One of the challenges I’m facing is the use of unconventional materials, at least based on my European background.

For instance, the internal roads and crane pads will be probably made of laterite, a solution very common in tropical climates successfully used in several countries for the subbase and base layers.

These days I’ve been investigating on the peculiarity and the technical requirements of this material.

As a starting point, I’ve selected the useful “Guide pratique de dimensionnement de chaussée pour les pais tropicaux”.

On page 36 you will find a catalogue of low traffic roads suggested cross sections. Considering the subgrade CBR of the wind farm site, I’ve selected 2 layers of 35 cm (foundation) + 15 cm (base), both made of laterite.

The granulometry can be found at page 60 (foundation) and page 73 (base).

There is also a requirement about the maximum increment of fine particle percentage (less than 8%), while the PI is greater than normal (<15).

Finally, you’ll also find that maximum CBR swelling is 1%.

The document “Characterisation of laterite for road construction”, from where I’ve stolen the graphs above, explains that compaction can greatly change the granulometric distribution of this material. Therefore, it is wise (although unusual) to ask for a granulometry check after compaction.

Another useful research is this “Review of Specifications for the Use of Laterite in Road Pavements”. The authors suggest using the Brazilian standards. You’ll also find an extended bibliography, a detailed analysis  of the criteria followed in several countries and “real world” results on existing roads in numerous countries.

CBR value - road aggregate thickness correlation

After a long search I’ve finally found a direct correlation between CBR and gravel thickness for unpaved gravel roads.

I’ve discovered that often wind farms are built in areas with a low (<5%) to very low CBR.

Somehow empirically, we’ve started with a standard 20 cm layer of gravel. We’ve learned the hard way that often this is not enough.

I’ve also seen that several competitors in their EPC projects opt for a double layer subbase+base with values such as 20 +20 cm, or even 30+20. Even if it may look expensive, this solution is probably cheaper on the long run, above all in wind farms in rainy areas and poor drainage where the road can be easily washed away.

I’ve also commented in another post why I think that national norm methods such as AASHTO are not applicable for wind farms (basically, because traffic is very low).

Therefore, I’ve been searching for a direct relationship between CBR, axle load and gravel thickness and I’ve found this:

According to the nomogram for instance with an axle load of 10 Tonnes and a CBR of 2%, you would need about 35 cm.

If you are based in Europe, you will probably want to use a more common value of 12 Tonnes axle loads.

The picture has been taken from a document made by Terram (a geotextile producer).

Please note that I don't know the source, but the numbers that it generates appears reasonable.

You can download it here.

AASHTO green book equation in wind farms road design

I’ve been recently asked to justify the roadbed thickness for a wind farm I’ve designed.

For several reasons (mainly because the majority of documents are redacted by non-civil engineers) the engineering companies supporting our customer ask for a written demonstration that the road design comply with the requirement of the famous AASTHO 1993 green book.

Unfortunately, it is not possible to use it for wind farms, and I’ll explain you why in this post.

As you will probably know, AASHTO defined an empirical equation  after a series of full scale test done about 50 years ago in the USA, the famous “Road test”.

This equation, very large and complicated indeed, gives as a result the “structural number” (SN) – a number that can be used to define the required roadbed thickness.

The formula looks very complicated, but the idea behind it it’s pretty easy: given the expected number of vehicle using the roads (defined as standard “equivalent single axis loads”) and other physical and project related variables you can define the correct thickness of the various materials selected for the road bed.

This is how the equation looks like:

Where

W18 = Predicted number of 80 kN (18,000 lb.) ESALs (equivalent single axis loads). Basically different type of vehicles (car, trucks, bikes, etc.) will use the road. To simplify the calculation, all this different axes are concerted to “standard axes”.

ZR = Standard normal deviate.

So = Combined standard error of the traffic prediction and performance prediction. Both ZR and So choice depend on the type of the road (for a major highway you will need more confidence in the result, while for a local road you can assume some risk).

SN =Structural Number (an index that is indicative of the total pavement thickness required).

Basically, each layer has a thickness (D) and a “layer coefficient” (a) representing the quality of the material.

In wind farm construction normally only one or two gravel layers are used.

Therefore the equation SN=a1D1 + a2D2m2 + a3D3m3+… will simplify becoming SN=a1D1

a1 = Layer coefficient. Gravel would be around 0.14

D1 = Layer thickness (inches).

ΔPSI = Difference between the initial design serviceability index, p0, and the design terminal serviceability index, pt. This concept is needed to incorporate in the equation the quality of the road at the beginning of the considered timeframe, p0 and the quality of the road at the end of the life span (pt).

MR = sub-grade resilient modulus (in psi). This number indicates the quality of the sub-grade.

Said that, let’s see why this beautiful and highly effective equation is of little (if any) utility for wind farm design.

Basically, a highway or an urban road is damaged by the recurring transit of heavy loads – that is, bus, trucks, etc. This trucks use the road for several years, causing accumulated damage.

What happens in a wind farm is that, when the WTGs are installed and producing, no one will use the internal roads – only a few service cars every now a then. The ESAL number will be almost zero.

What normally damage wind farms internal roads without heavy traffic is poor drainage, incorrect roadbed material selection or poor construction (e.g. incorrect compaction), not cyclical mechanical loads above the elastic limits.

Therefore we normally design the roadbed based on the CBR value: we know that with a very good CBR in dry climates 20 cm are normally enough, while for low to very low CBR (>5) we use 40 to 50 cm.

Below CBR=3% special solutions are normally needed.