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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One of the problems of installing a wind farm in an area with a dense bird population is the possibility of impacts between the rotating blades and the animals.

A standard solution used in the industry is the use of a (very expensive) bird radar, a quasi-military technology that can spot very small flying objects and stop the turbines (hopefully) in time.

Working at the development of a wind farm in Jordan we have discovered the existence of an alternative: a Spanish company (Liquen) is developing an “artificial vision” system to be installed on the WTGs.

Basically the system is composed of 2 sets set of high definitions cameras installed on the WTGs or on the MET mast that are filming the space around the turbines, plus speakers for dissuasion sounds.

The software can analyze the images in real time and recognize if a bird is flying toward the machine. If this is the case, several countermeasures are possible:

  • Warning and dissuasion, using annoying signals. Clearly this signals are optimized to the type of birds in the area
  • Stop control, whit short duration (<2 minutes) stop of the affected turbine and automatic restart to minimize production loss

According to the company, real time detection is really fast (less than 1 second) and it is possible including in adverse weather conditions (fog, snow, rain) whit a very low power consumption.

The main problem that I see is that the system is effective only when there are at least 200 Lux (that is, from sunrise to sunset). So there is a clear problem with nocturnal birds – I wonder if in the future it will be possible to integrate the system with some kind of night vision, like the military infrared technology.

Another weak point is the low detectability of very small bird (12 centimeters or less): they can be easily spotted only if they came in a large group.

This technology looks promising and it’s not so “embryonic”: it has already been installed in a Vestas wind farm in Greece and several other wind farms around Europe (Spain, Norway and Greece).

If you are interested you can find more information in the DTBird Brochure or in the results of a study in a real wind farm in Norway.

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After the connection of a Wind Farm to the grid several parameters are used to analyze the smooth operation of the installation.

The more relevant are:

Capacity Factor

CF=\left( \frac{E(kWh)}{P(kW) T(h)}\right )

Is a parameter used frequently in power producing  plant. A high capacity factor means that the plant is working almost continuously (for instance a nuclear plant), while a low capacity factor may characterize a power plant working only in peak hours (like some hydro plants).

In the case of wind farms, capacity factor depends more on the wind that on the needs of the grid.

To be economically reasonable, a wind farm needs to have a capacity factor of more than 25%. Translated in hours, it would be around 2190 equivalent hours.

This parameter is probably too “global”, as it doesn’t add information about why the wind farm was not producing: was it for a low wind, for a technical problem of the WTG, for a disconnection from the grid?

Or maybe it is due to a scheduled maintenance or to the wind sector management (the automatic planned disconnection of some WTGs in particular wind conditions)?

Technical Availability

TA=\left( \frac{T(Available)}{T(Total)}\right )

This is easily defined: basically is the ratio between the hours the WTG was available for production an the total number of hours in the considered period. If there is a fault in the grid, or if wind condition is above or below the maximum, it doesn’t count as “unavailability”.

Production Availability

PA=\left( \frac{T(Producing)}{T(Total)}\right )

This is parameters start to be interesting from an economical point of view: is the ratio between the total number of hours producing and the hours in the considered period. It will be less than 1 due to grid disconnections, WTG problems and wind outside the operational limits.

Effective Availability

EA=\left( \frac{T(producing)}{T(wind)-T(disconnected)-T(stop)}\right )

This parameter give a very solid information about the quality of the turbine, and the “real” availability: is the ratio between the hours of production and the hours of wind speed between the operational limits, minus the hours disconnected by the grid (for a grid problem or order) minus the justified stops (for visits, preventive maintenance, etc.)

Some weeks ago a document describing a new type of wind turbine foundation, the “iCK foundation”, landed on my desk.

Also known as “Gestamp Hybrid Towers” (GHT) it has been developed and patented by iConkrete and Gestamp. Essentially it is a shallow foundation made of a slab to achieve a uniform pressure distribution, a central reinforced ring with his pedestal and several reinforced beams below the slab.

You can see how it looks like picture here:

The basic idea is to obtain a T section, for a better use of materials: compression is distributed on the top of the T head, with a reduced depth of the neutral axis.

This geometry, according to the developers, gives and improved fatigue behavior for concrete and a higher resistance reserve.

Among the other potential advantages, a (partial) prefabrication of the foundation’s elements is possible. This lead to a “cleaner” work and to a saving in time.

Moreover, the smaller excavation promises other savings due to reduced earthworks: this foundation is more superficial than the standard one, with an average depth of almost 3 meters. The excavation volume avoided can be around 50%.

Last but not least, according to a test design made by Gestamp, a 15% of steel can be saved thanks to a better use of materials.

If the soil below the foundation has a low bearing capacity, a geotextile can be used.

This solution can be adapted to any type of tower (steel, concrete or mixed).


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