best bed for cobblestone driveway?

Depending on your climate, locale, and drainage issues I think I would go with the engineer. Your contractor is basically building a sidewalk to place your driveway on top of. If you have that much heavy traffic running on the drive anyway the concrete will break up just as well and there is no natural drainage with concrete. At least with stone dust water and moisture will drain/wick away and leave you with a relatively dry compacted base. I personally have a 30 year old 20' wide x 80' long drive with a bed of 1" stone 12" deep covered with 4" of grade 9 w/ fines (much coarser than stone dust) with 2" of asphalt over that and have never had any rutting problems and I have good drainage from all the stone.

score the concrete?

thanks for the reply. one other engineer had suggested to score the concrete to help 'guide' the eventual cracking of concrete issues. also, the contractor was going to put 6" of 3/4 gravel below it for drainage. so, you feel the stone dust and item 4 will be good for both support and drainage?

I bet the engineer called for 95% compaction or better in so many layers for the subsoil. I also bet he specified that the Item 4 be placed in layers of a certain depth and compacted to a certain level. Then called for the stone dust and cobbles.

He's calling for compaction of the subsoil in order to control its behavior and minimize any effects on the driveway. Same thing for the Item 4, all while taking into account adequate drainage. The stone dust will not shrink or expand greatly w/ water absorption, nor will it bond together and harden. No reason to think this will deteriorate or settle in a way that would cause tire ruts.

This is a much better approach than the gravel and cement.

95% compacted

Hi, thanks for the reply. You are correct. The engineer is calling for compacted subgrade and the 95% compacted item 4. Not being an excavator, but being responsible for work done on our property, how does an excavator achieve that level of compact? Do they use a roller after each couple of inches applied? Or do they just keep driving over it with the excavating treads of the machine?

and, is it worth having any thickness of concrete layer? or is the 6" of item 4 compacted to 95% and then the 1.5 inches of stone dust cool to hold 5x5x10 bluestone cobbles without depressions?

we appreciate your knowledge and opinion! Everyone seems to do these things slightly differently.

dl

translation?

appreciate your input and help. sounds like folks like item 4 better than concrete.
but what does ... mtl i'd also use IF i want'd your d/w
mean? do I need to have my kids translate this for me, lol?

' mtl i'd also use IF i want'd your d/w ' = material i would also use if i wanted your driveway ( smiley face ) - often write phonetically as i see/hear it

My vote - No concrete. Properly compacted stone will provide a very stable base that will flex when needed. I am no expert but from experience I can tell you water will run off, not drain threw a properly installed paving so be sure to incorporate proper drainage in your design.