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:


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.

Wind farm internal roads bends additional widening

One of the problems frequently found in wind farm project and construction is that road bends with a reduced radius often needs an additional widening to allow the passage of the trucks with the WTGs components.

Normally the biggest problems come with the blades. I suggest using a commercial software such as AutoTURN to estimate the actual additional road widening needed, because the tables provided by wind turbine manufacturers are almost invariably inaccurate and often based on wrong hypothesis.

The point is that there are too many variables to simplify the problem and give a single "one size fits all" value:

  • Angle between entry and exit tangent of the bend: the lesser the angle, the bigger the widening.
  • Bend radius:  smaller radii means greater widening.
  • Type of vehicle (number of wheels, center of turning circle, dimensions).
  • Different possible trajectories chosen by the driver.
  • Use of real wheels independent control.

Simulating software are great because they use real word data: the trucks are equipped with GPS equipment and the movements of the vehicle is registered and transformed in an algorithm that allow to replicate it in your AutoCAD project, with realistic results and cost effective solutions.

In the next image the trajectory of the different components of the truck in a bend with reduced radius are detailed. If the tractor unit follows a path in the center of the road, the rear wheels (orange lines in the drawings) will need an additional road widening both before the beginning of the bend and inside the bend.

It will also be necessary to clear an area appropriate for the transit of the blade tip (outside the bend, cyan line in the drawing) and for the truck body (inside the bend, green line in the drawing).


Figure 1: Standard wind farm internal road bend

It is also noteworthy that most of the trailers for WTGs components transportation allow orientating rear wheels independently from guiding front wheels.

For this reason, the road widening can be completely internal to the bend (using the steering control of the rear wheels) or external, sweeping the area outside the bend.

These solutions are normally more demanding in term of additional required area. They are used in situation where, due to existing constraints (buildings, structures, property boundaries, etc.) the standard widening cannot be used and a solution only inside or outside the bend must be found.

Here you have an example of a widening only in the interior side of the bend:

Figure 2: Wind farm internal road bend. Widening only in the interior.

This is a non standard solution, and as you can see it needs more space.

The third possibility is to use only the area outside the bend:

Figure 3: Wind farm internal road bend. Widening only in the exterior side.


This solution needs an enormous amount of space, and we use it only in exceptional situations.

Wind farm civil works projects: typical errors

Here you have my collection of errors I frequently see when I check wind farm project developed by external companies.

Being a quite peculiar sector is no surprise that I normally found several mistakes: here you have the most commons.


Error #1: two levels crane pad / foundation area designed without considering the slopes of the foundation pit.

Here the problem, as you can see in the transversal section, is that the slope of the foundation pit “enters” inside the crane pad and the road nearby, reducing the available space.

The only way to build something similar is with a "2 steps" constructive approach, i.e. to build the foundation, to close the hole and after to build the crane pad.

An example of this 2 steps approach can be seen in the following drawings, taken from a real wind farm. The ground was very steep, so first of all we calculated the elevation of the bottom of the foundation pit to ensure the necessary soil covering that help balancing the overturning moment. In several cases the result was that the center of the foundation was too far away from the border of the crane pad (around 16 meters), making difficult the work of the main crane (a standard distance is around 10 meters).

So we had to approximate the border of the platform to the foundation filling the area in between until the required distance is reached:


Error #2: crane pad and foundation on an embankment.

The problem here is that the foundation must be realized below the natural ground for stability reasons. Normally the depth of the bottom of the foundation pit is around 3 meters, calculated from the lowest point of the terrain around the circumference of the foundation.

But if in the project the WTG is shown on an embankment, probably the stair or even the tower door will be below ground. In the transversal section you will see clearly the problem.



Error #3: road and crane pad at different levels

Here you have a top view of a platform and the access road. Everything looks fine:

But then, when we check the longitudinal profile, we discover that the road is going down (while obviously the platform stays at the same level, 646 meters). The only way to build something like that is to use a wall to retain the earth inside the crane pad, but this is obviously not the case.

At the end of the platform, the height difference is almost 5 meters:

In the longitudinal profile above you can see the platform in yellow and the road in dark blue.


Error #4: insufficient vertical transition curve

Sometimes the Kv parameter for the vertical transition curve, defined as  is not adequate.

This happens when it’s lower than 400 – 500: in these cases, the truck can remain “stuck” because it touches below.


Floating roads on peat

Floating roads are a solution developed in Scotland during the construction of several Km of internal wind farms roads on very poor, organic materials such as peat.

It is an interesting constructive technology, developed on a very compressible, decayed material normally considered as the worst possible subgrade for road construction. In a normal highway project, peat would be removed

Usually floating roads are stabilised haul roads with one or two layers of geogrid. This approach leads to several advantages:


  • Reduced road thickness (and subsequently less weight)
  • Better distribution of pressure
  • Untouched surface layer, lesser impact on vegetation
  • Less construction material needed
  • Usually cheaper


The suggested approach to floating roads construction is based on six steps:


  1. Carrying out a detailed survey to define the hydrology of the area and the paet type (according to the “Von Post” system, there are 10 possible classification for this material depending on the decomposition level)
  2. Identifying the value for in situ peat strength
  3. Estimate the expected traffic loading
  4. Design the road
  5. Monitoring the construction
  6. Recording action and outcomes for future projects


Among the design assumptions, it’s relevant to remark that a rut with a depth up to 10 cm is to be expected.

The first step is an in depth site investigation, where several parameters are recorded: peat depth and classification, side slope angle, hydrology and permeability. Many in situ test are available, from probing and sampling to more sophisticated techniques such as ball penetrometer or Mexecone penetrometer and ground penetrating radar. After, in laboratory, water and organic content can be defined, together with vane testing and direct simple shear tests.

When all the data is collected, normally is the geogrid manufacturer who will design the cross section of the road, deciding the type of grid (there are several dimensions possible – they must match with the aggregate size), the number of layers and the height of the compacted stones.

The designer will use semi empirical rules, using as inputs shear strength of the peat, weight of the road and expected number of equivalent axes.

It is standard practice to use at least 2 layers of geogrid: a lower one directly on the existing material, and an upper layer on the top of the embankment, used as a support for the controlled granulometry crushed gravel.

Good practice during windfarm construction

“Good practice during windfarm construction” is a document produced by several Scottish agencies (Scottish Renewables, Scottish Natural Heritage, and the local Environmental Protection Agency and Forestry Commission).

It provides several useful advices on wind farm construction, based on the experience they gained with the development of several pretty big projects (for instance Whitelee W.F., with 140 WTGs, 86 Km of roads and 940 Km of cables).

The document is based in a northern Europe environment, where many roads are on peat and heavy precipitations are expected. Here the main points of the documents:


  • They suggest designing the drain for a 1:200 year event and the use of pre-earthworks drainage (a solution seldom used in dry countries).
  • Regarding the shape of the ditches, they explain the differences between “V” shaped ditches (they maintain more the vegetation, but they have more erosion) and “U”shaped ditches (they allow easier access and egress to wildlife).
  • Considering that many times internal road are built without camber to allow the use a narrow truck crane, they propose the use of a series of cross-drain to divert the flow to the side ditches.
  • Several techniques are suggested as protection measures: silt traps, silt fencing, straw bales, settlement lagoons or even the use of flocculant.
  • They explain that cable trenches can act as a water drainage route, so in case of strong gradients clay plugs and impermeable barriers should be used to limit water flow.
  • They recommend not to install MV cables in soft peat, because they sink adding tension and potentially breaking.


Last but not least, there is a detailed explanation of floating tracks (road construction on peat using geotextile or geogrid). The suggestions are:


  • To build them only in flat areas, to avoid the risk of a slip or circular failure.
  • To leave vegetation and tree roots in place, and lay the geogrid directly on the existing terrain.
  • To avoid leaving open trenches or drainage ditches in the proximity, because the cyclic loads due to truck passage can lead to a soft materials migration and collapse of the road.


If you are interested you can download the entire document here: Good practice during windfarm construction

Tensar Triax geogrid use in wind farms

Here you have a real world example of geogrid use.

We are in a wind farm in southern Spain, and thanks to previous experience with this technology the client decided to use a Tensar Triax geogrid TX160.

This is a triangular geogrid (the “old school” version was square). It seems that the triangular geometry guarantee a better distribution of the loads

The same geotextile  has been used in many wind farms (more than 100) around the world, with several big project in Germany and UK where in some cases more than half a million square meters have been used. Often the soil was peat (turf), with a very low CBR (even less than 1).

The saving in crushed stone using a geogrid can be around 30% to 40%, so with a price around 2-3 €/sqm it is normally a cost effective solution.

When we opt for this solution we’ve had helpful feedback and hints from Tensar, as they can study the existing info and provide a design based on the soil conditions and the available materials. There are several models available with a different triangular size, so it is not easy to choose the right model.

We haven't had any problem during the first half of the civil works circulating with machinery and trucks, but when we started moving the narrow track crane on the internal roads between the pads significant damages appeared: