Don't get bent out of shape my Algerian friend. I've been involved with a number of 'third world' roads. Geotechnical investigations would vary. As you say there is no cuts - it is a straight road and you said it is "unimportant". I would think the following. A few test pits to identify the soils to a reasonable depth, say 2 to 3 m. Depending on your road length, you would probably take a test pit at 100 m intervals; perhaps 200 if there is no apparent change of stratigraphy.
Take the samples for sure. I would presume that the natural soils would be your subgrade. How much unsuitable material is there? (i.e., topsoil, uncontrolled fill). For laboratory tests, if clays, do the Atterberg, natural moisture content and sieve analysis. I don't think it is so important to obtain the % clay. If the soils are granular, obtain the sieve analysis for classification purposes. You can then go to any older text book and, based on the classification, it will tell you the "general" suitability for the material to be used for road subgrade, fill, etc. This gives you an idea of the overall suitability of your subgrade.
In the upper soils that by judgement you would expect to be acceptable as subgrade, do a Proctor test (yes, even in granular soils as many labs will not have the means of doing relative density (and most everyone does field roadwork using Proctors anyway). At 95% of the MDD, do a CBR (soaked) as you will want something like a CBR of 6 or 8 for subgrade under the subbase.
Field tests, you could try the TRRL miniature dynamic cone test - this could be done at a number of spots and would give you, through rough correlations what the CBR value would be - best if you could calibrate to a couple of locations.
Once you have this data, you would determine if you need an improved subgrade - i.e., select subrade that would take your subgrade soaked CBR value to 12 or more. Knowing this, it is a matter of having data on the type of traffic loading, average daily vehicles and such - you can then go to AASHTO charts to determine your pavement design. (You'll need to know some other environmental things as well - i.e., freeze-thaw; rain-dry.
Sometimes we have put on the select subgrade and then carry out Benkelman beam tests (I'm old) or using other more current deflection methods to confirm your pavement design. - you could do this after laying subbase too; and base course as well before HMA.
I don't see the need for more fancy tests - you could determine the resilient modulus, and all sorts of other parameters and the go to elastic solutions, etc. - but for normal roads the above is the general means that the roads are developed.
Now if you are into mountains, swamps/marshes, soft soils (lacustine lake beds), etc. you will need to use more judgement but the overall steps are basically the same - what I think I am saying, knowing your others posts, is that you don't need to overthink.
Now, gentlemen, hit me with your best shot! (and OG, Ron, fattdad always have great comments that you can learn from - oldest guy from an originator's perspective!, Ron from a forensic perspective and fattdad from just plain great experience)
