Maximum wind farm internal road gradient

This is another standard problem I found in the wind farm I’m working with: mountainous areas, with difficult access and very strong inclination.

The standard maximum slope imposed by several manufacturers (for instance Repower and Gamesa) for safe transport on gravel roads is about 6% to 7%. Above 7% other technical solution may be necessary, depending on the trailer used to pull the T1 and the nacelle.

With an average quality track surface a 6x6 tractor unit can pull the nacelle approximately up to a 9%-10% slopes.

Than it is necessary to pull the truck with a bulldozer, using a steel bar or a steel cable (see pictures below). In this case the inclination was around 15%, and we used a D8R Caterpillar and a steel cable), without particular problems.

It must be noticed that in steep sections long horizontal alignment are preferable respect to closed bends.

What we normally do in extreme cases (above 15%) is to build a ramp using concrete slabs or an asphalted road: this solution is not only more expensive, but can also introduce additional problems, such as the need for environmental authorizations or from other authorities.

Here you have a 17% asphalted curve:

Concrete slabs are normally cast on site on a layer of aggregate stone, and they have dowel bars for load transfer and sealed contraction joints. An initial texturing is made with a burlap drag or a broom device, while final texturing is made with a spring steel tine.

You can find more information on the subject in the JPCP Design and Construction Guide

Jointed Plain Concrete Pavements Design Construction Guide

 

Geotextile vs Geogrid: which is the best solution?

This is a debate we are living each time we have to build internal roads on soils with a low (<5) to very low (<2) CBR.

Basically when the others alternatives (mainly soil substitution and soil improvement) are not feasible we are adopting two different solution, either a strong geotextile with reinforcement and separation properties or a geogrid (coupled with a thin geotextile used as a filter if necessary).

Presently both of them are working well, but only after many years we will know which one works best. I'm hearing very different opinion on the subject, so there is not an universal consensus.

As geotextile, both woven and nonwoven alternatives seem reasonable. Both of them provides separation of the aggregate from the subgrade and have high tensile strength and modulus, adding reinforcement to the foundation soil. Right now the woven solution is widely preferred.

As woven geotextile we have used the US250 from US Fabric, with the following properties:

PROPERTYTEST METHODENGLISHMETRIC
Tensile StrengthASTM D-4632250 lbs1,112 N
Elongation @ BreakASTM D-463215%15%
Mullen BurstASTM D-3786450 psi3,102 kPa
Puncture StrengthASTM D-4833100 lbs445 N
CBR PunctureASTM D-6241900 lbs4,005 N

And as nonwoven, something like the US Fabric US 160NW  looks like the best option:

PROPERTYTEST METHODENGLISHMETRIC
Weight - TypicalASTM D-52616.0 oz/sy203 g/sm
Tensile StrengthASTM D-4632160 lbs711 N
Elongation @ BreakASTM D-463250%50%
Mullen BurstASTM D-3786305 psi2,103 kPa
Puncture StrengthASTM D-483390 lbs400 N
CBR PunctureASTM D-6241410 lbs1,824 N

Regarding geogrid, it has been used in several wind farms all around Europe. I had a meeting with the representative from Tensar, and their product looks interesting.

It is a triangular net, providing support to the stone aggregate. It works equally well in every direction.

We have used it in Spain, and it have been used in several other projects in UK and Romania.