Hello
Grasshopper is not easy but it is a sort of automation of hand modelling with Rhinoceros. So if you can model this tunnel in Rhino you must have a certain idea of a work flow.
We mostly are not tunnel specialist so it is not easy to know what is a real parameter and what is an implied parameter (obtained via others parameters).
Nice beginning project.
Welcome to the parametric world.
The main issue with anybody helping you, who is not an expert in your field, is that you have to explain the underlining logic behind what you want to achieve in greater detail. From there on, there are plenthy of people on this forum who will be happy to guide you trough the what is what and how it works.
For sure, I began to give a try. First wonder they use Gradians for the angle not degrees nor radians! There is not converter to Gradiant.
A first try, follow some tutorial to understand the basics of Grasshopper, hidding, searcing components …
The logic in there is simple, draw a first point bottom of the tunnel
Rotate this point
Mirror it
Make an arc
Make the step
tunnel LD.gh (12.9 KB)
It’s a remnant of old surveying principles. Often the angles in infrastructure design are still displayed in gons. I have never understood what is the benefit of using it these days…
Thanks for the information. There are many strange units everywhere, I was in Artillery and we used Millième (French one (6400 = 1 rev) not Russian (6000 = 1 rev) ). I now more work on astronautics and USA uses for inertia slug.ft², strangest unit I ever saw. For my job I took a XML format of data where you put the unit. So you have not to bother to convert data.
Nice
Just wanted to save you two minutes 
Btw, if you remember you have helped me a lot in the past with a certain little tool you’ve made for me. Still using it daily. Wanted to thank you again
Whole section of tunnel. The cut (shatter) of main circle is not a robust method that I used. There are always multiple solutions in Grasshopper to replace the human action/Mouse click used in Rhinoceros. *
@davidsmavrov, I am happy you still use my little tool. I have too much little tools. I may one day publish a big dll with all the tools or a plugin. For your converter, put “deg” on the label
At first, I would thank you all for your answers, I didn’t expect so much help. I appreciate that.
I’m looking at your model @laurent_delrieu to understand and learn as you worked it out.
However, I’d like to share my “first trying” so you can tell me if it’s a good way to model it.
I started to import the Autocad file into rhino and from there I tried to model the external lines of section doing some circles with the related radius and then from there taking out some points from which I should draw the arc. But, I’m stuck here because I don’t know how to get the third point for the command “arc3pt”.
As you can see, my model is very simple because I just started to learn this software and I don’t know all the functions.
Btw, thank you a lot againModel 1.gh (11.0 KB) Model 1.3dm (12.1 MB)
Arc are more difficult from CAD as you have not the mouse. On Rhino/Autocad you decide which side you want the arc. It is far better to make arc than split circle.
But don’t forget that arcs are infinite rotation of a point. So Use the rotation component to rotate one of the point of the arc. It will give you the third point. In Grasshopper you will have more Maths to do, also knowing the axis, cross vector …
You have posted a Rhino 7 (R7) .3dm file. Some of us are still using R6 and can’t open that file.
You can either ‘File | Save as’ R6 format or internalize your relevant geometry in the Grasshopper file.
Or not and get help only from R7 users. McNeel now wants annual $400 to $600 upgrades. 
Do any adjoining arc need to have tangential continuity? If so that puts a constraint on where the centers of the arcs can be located. The center of each arc needs to be located on a radial lines of the adjoining arcs.
Yes, you’re right the adjoining arcs need to have tangential continuity otherwise when I change the radius they will not make the section properly.
“French expression” “Et bien on n’est pas au bout du tunnel” 
Meaning that we are still in a difficult situation … I mean the discussion was about using Grasshopper to make a parametric tunnel.
Looks like rest of the conversation has gone off the path of the original topic… Anyways I just wanted to mention that I just happened to serve as an artillery officer also and we also used 6400 per revolution for angle measurement, but it was called mils as in NATO standards. Seems it’s a rounded up value of a milligradian (1/6283).
