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.

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I’ve recently had the pleasure to be exposed recently to the FIDIC contracts structure. Therefore I want to share with you my impression regarding the possibility of using this type of contract in wind farm EPC tenders.

The first FIDIC contract was released in the fifties. After half a century, the FIDIC contract family is expanding over the years to match the necessity of the market.

As you probably are aware, the different types of contracts are commonly referred to with different colors (red, green, yellow, etc.) from their cover. For instance “red” is for construction (of projects designed by the Employer), and “green” is for small work (maybe less than 500.000 USD).

The three types more appropriate to my particular sector would be the red, yellow or silver book.

Although there is obviously much more, I will resume the main characteristics of each of these 3 contracts type in the following table. I’ve not included the other books because they are not applicable to the wind farm sector.

Design Employer Contractor Contractor carries the risks
Design Approval Engineer may approve changes or ask for variations Engineer approves or rejects before executions N.A.
Proposal Unit Prices Lump-sum Price Lump-sum Price
Payment Schedules Measured quantities Payment percentages Payment Calendar / Payment percentages
Engineer Yes Yes "Employer's representative"
Risk Distribution Employer carries design risks More Distributed Contractor carries the risks

So, which contract type makes sense in a wind farm EPC?

Let’s start to the easiest contract to discard: the silver book.

This typology of contract would be a full EPC where almost the full risk is suffered by the contractor. There is no engineering available, therefore quantities are estimated and risk such as subsoil quality must be included in the contractor price.

Obviously, this made the contract not operative for projects such as tunneling, wind farms or other “high geotechnical risk” activity.

Theoretically, it is possible to include in foundation price this risk. We even have done it in the past, for clients not willing (or not able) to pay even for a preliminary geotechnical survey. But the price of a piled foundation can be 2 times the price of a standard, shallow foundation. Therefore numbers become huge quickly, and this approach kills the majority of the projects.

Also, the client is not really willing to pay for something that it might never get (that is, major civil works). This basically eliminates the possibility to use the silver book, a contract that makes sense in situation where both party knows that little deviations are to be expected (maybe some type of plant).

These leave us with the yellow and red books.

In the red, engineering is made by Employer and payments are made on the basis of the real quantities executed. Employer carries the risk for contract amount increases: as you might guess, this point is not particularly welcome by banks or by companies such as Vestas.

In the yellow, Employer prepares only the «Employer’s Requirements», including Draft layout, Operational Parameters, Technical Specifications and Financial Proposal.

Tenderers submit their technical proposals together with their financial proposals, including at least methodology, basic design and drawings, bill of and similar supporting documents.

So maybe you think that finally we’ve found of dream contract, the FIDIC yellow book.

Well, no. The problem is that contractors are unwilling to give a real closed price: basically, construction companies don’t have an engineer department big enough to properly follow each tender and give a good price. Being a really specialized business, often you ask price to good local companies with no wind experience.

This lead to misunderstandings, unreasonably low or high prices and (even worst) discussion during execution, when you already have a closed price with your client and accept a reclamation of the subcontractor would go against contract margin.

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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.

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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.

Here you have some interesting pictures that we’ve received trying to define the meteorological tower (also known as “met mast”) foundation for one of our wind farms.

Strangely, our customer is going to use a wired met mast for both towers (the permanent mast of the wind farm and the temporary mast used to calibrate the power curve).

As you can see from the pictures the mast has an interesting hinged base joined to the foundation of the WTG with 4 screws - looks like an effective technique.

The mast is tower model is KT470 from Kintech engineering.

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This is the first post after a long silence (more than 6 months), fortunately due to good news (the birth of my first son, who reduced dramatically my free time).

I want to thank José Ramon, one of our experienced Project Manager, who pushed me to start again ;-)

This post is about geotechnical parameters relevant for wind foundation. I’m in debt with Ana from Esteyco, a very good geologist who explained me how this work.

She explained me that the most important parameters that we need define in a geotechnical survey for wind turbines foundation design are 2: bearing capacity and deformability.


Bearing capacity is defined by c (cohesion), ϕ (friction angle) and qu (unconfined compressive strength) and it is used in Ultimate Limit State (ULS) design.

In this type of verifications, loads such as extreme wind, abnormal loads or earthquake are used to check the foundation for overturn, slide and collapse (soil failure).

If we are working with soils, we normally define these parameters using in situ tests such as SPT (in sands), CPT, Pressuremeter test and Vane test, or with laboratory tests  such as triaxial (CD, CU, UU) , direct shear test or CBR.

In rock, we could use a compressive strength test, rock quality designation (RQD) or triaxial, although the bearing capacity of rock (even heavily fractured and weathered) is normally so high that these tests are normally not necessary.

Deformability is defined by E (Young's modulus) and G (Shear modulus).

These parameters are used to check if the foundation is compliant with vertical settlement, differential settlement and, above all, rotational stiffness.

These verifications are done with a Serviceability Limit State (SLS) design.

In case of soils, these parameters are defined with in situ test like SPT, CPT, down hole, cross hole (normally not used because we don’t have boreholes nearby normally) and pressuremeter. If we define them with laboratory test, we would use triaxial or oedometer test.

In both case, we prefer field tests to laboratory tests, mainly because it can be difficult to have undisturbed samples with their in situ characteristics.

It is important to highlight that in some cases, we could meet the bearing capacity requirements but not the deformability minimum conditions and vice versa. For instance, soft sandy soils with some combination of WTG and tower can offer a reasonable bearing capacity but an excessive deformability.

In this post I want to show some figures about the evolution of the wind sector worldwide, both on the demand and supply side, and what happened in 2012.

From my point of view as Vestas employees the biggest new has been that GE is now the biggest wind turbine manufacturer. They now have a market share around 15% percent, with Vestas slightly below (14%) still winning the battle of the installed capacity.

The market is fragmented, with 10 more or less big companies and several smaller ones. It’s peculiar that the “small” manufacturers together have sold more than 20% of the installed turbines: apparently, they are not so small.

The Chinese manufacturers (Goldwind, Sinovel, Mingyang, etc.) felt the pain of a slowing internal market, falling down in the market share fight. Basically, they install only “at home”, in a closed, protectionist Brazilian style environment.

About 45 GW have been installed in our planet last year (2012), with an increase of 19% of the installed capacity. Right now there are about 286 GW of installed turbines.

Installed GW Cumulative
2007 20 94
2008 28 122
2009 38 160
2010 39 199
2011 42 241
2012 45 286

Apparently in 2013 the installation of new turbines is slowing down, although figures are only preliminary.

The country with the biggest number of wind turbines is China, with around 54.000 WTGs, followed by the USA with 51.000. Of course they are the biggest markets in the planet.

If you wonder how many turbines are installed in the whole planet, the cumulative figure is around 222.000 machines.

In Europe, Germany is leading the sector with around 23 thousand machines, followed closely by Spain with over 20.000. Looking the distribution of installed capacity by continent Europe is still ahead:

Installed capacity (2012, GW)
Americas 72
Europe 110
Asia 95
Pacific 7
Africa 1
Others 1
Total 286

Offshore is developing quickly in Europe, thanks to the Mega-projects in Germany and in the UK. Global installed offshore capacity is above 5.000 MW, usually concentrated in very big wind farms. Preferred foundation technology has been monopile (if you are interested in the subject, you can read more here about offshore WTG foundations).

As you will probably know the market is moving towards biggest turbines, being the average size of the newly installed generators around 2 MW.

Regarding owners, Iberdrola is the company with the most installed MW (13K), followed by 2 companies that I don’t know, Chinese Longyuan (10K) and NextEra Energy Resources, from the States (about 10 GW as well).

To close the post, the answer to a typical question regarding wind energy:

What is the contribution of wind power to the global electricity generation?

About 3%.

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I’ve recently discovered the existence of precast foundation for wind turbines. Strangely enough, this solution isn’t having a big success, at least as far as I know.

By the way, there are several clear advantages: first of all, an important time saving.

According to the developer brochure, only 2 days are needed to complete the foundation: the first to install the pieces and the second to connect the tower tensioning the bolts.

Then, as with all serial products, there is the advantage of tighter controls on the quality of the materials and the production

The manufacturer also suggest that the excavation volume is reduced, although I don’t understand why (in the end, it is still a gravity foundation, so the dimensions should be similar).

I also don’t see what happen with the conjunction element (embedded ring or anchor cage in the newer models): I suppose that it will be substituted by bolts enclosed in the precast modules, but I can’t visualize how it will work without the lower flange.

I don’t know how many companies are actives in this business: the picture below are taken from a presentation of Artepref, a Spanish company specialized in precast components.


Although I don’t normally post stories about wind farms I’ve worked at I want to do an exception for Talinay, a project with whom I have a relationship almost emotional.

Located in Chile, in the Coquimbo province, near the Limarí river, it has been a project entirely developed by Vestas with internal founds and my first “hands on” EPC experience.

We’ve had the pleasure of optimizing the layout both in the preliminary phase, working together with the wind & site team, and in the constructive project. It was tough, because it is located in a mountainous area where impressive earthworks are needed.

It is a mix of V90 and V100 turbines of the 2MW platform, with an installed capacity of 90 MW.

Connected to the grid in March 2013, it has been constructed at an amazing, “china style” speed: with almost 400 peoples working together on site during the busiest period, it was a record for the foundation (5 per week, with two concrete plants on site working full time day and night) and the turbine installation (4 WTG per week).

The wind farm was completed in 6 months.

Almost all the big players of the sector have been involved: from the engineering side support was provided by IDOM, SISENER and ESTEYCO, while the main subcontractor was GES who worked with local and international subcontractors (among them, Hormigones Melón, Burger Gruas and CJR).

On the electrical side, it was one of the first (or maybe the first) PASS installed in Chile. The transformer was developed at lightning speed (155 days ex works), while the substation was made by ABB and Siemens.

Now is property of the Italian utility ENEL Green Power, who also signed a service agreement. Part of the money of the deal comes from a loan from Denmark’s Export Credit Agency (EKF). ENEL has an aggressive expansive approach in the Chilean market, where is developing several other wind farms (some of them with Vestas).

Below you will find several interesting pictures: two trucks pulling a tower section due to the high slope of the road, PASS switchgear and line trap, concrete plant and other interesting views of the wind farm.

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