Crane hardstands for installation of wind turbines: a handbook

Crane pads engineering and construction handbook. Copyright STOWA

Due to the pandemic this year we have been forced to go on vacation by car – the majority of flights from Germany to southern Italy have been cancelled, so we are making a 2000+ Km car trip with our three kids.

While my wife was driving I searched something to read and I have found something unexpected and extremely interesting: a 121 pages handbook only on wind farm crane pads!

The owner of the document is a bit unusual – STOWA, the Dutch knowledge centre of the regional water managers. However it makes sense if you consider that many projects in the Netherlands are on reclaimed lands and water authorities and municipalities are involved in the permitting process.

Behind the document there is a supervisory committee composed by the very best wind companies in the country, such as Fugro, ABT, H4A, Tencate and many others.

They will give you a very detailed view on the topic: after an introductory section describing the standard wind turbines and cranes now in the market the handbook describe in detail the process, from the geotechnical investigations to the design to the execution, operation and maintenance.

I have worked personally with some of the authors and I can guarantee you that they are very experienced professionals, so this is an extremely valuable document.

Plate bearing test for wind farm roads & hardstands

Plate bearing test elements. Copyright image

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 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.

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.


Crane pads shape and dimension

Reading the wind farm BoP related info of several companies in the sector, such as Gamesa, Vestas and Nordex, I’ve found a variety of proposed shapes and dimension for the crane pads.

Basically a rectangular solution looks like the more reasonable option, but sometimes triangular, polygonal and even circular pads are found. I think that they waste space and that they can be complicated to construct.

The easiest solution is to align the pad to the access road: doing so it is possible to use the access road to assembly the boom of the main crane (if it was disassembled when moved from a position to the other).

The standard size for blades of 45 meters would be around 40x45 meters: this is a “Full Storage” solution, because it allows storing blades, tower, nacelle and all the materials in the platform.

Other solution are used in mountainous wind farms where earthwork is expensive: in this case a temporary storage area is made somewhere nearby on a flat zone, but this solution is more expensive and time consuming, because it needs more trucks movements and several loading/unloading with the auxiliary cranes.

The biggest problem is how to allow to the trucks to turn around: for instance, in the Gamesa manual for BoP no clear solution is given to this problem (attached picture).

I normally study this problem on a case by case basis simulating the truck movements with AutoTURN, as no general solution can be provided.

Another solution I use every now and then is to split the storage area: blades on one side of the road, and tower elements on the other. It can be useful when, to minimize earthworks, a 2 level crane pad is realized.

Below as an example you can see Gamesa and Nordex solution.