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.