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Old 04-08-2009, 02:08 PM   #1
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best bed for cobblestone driveway?


we're doing a driveway out of bluestone cobbles 5x5x10. The engineer has called for the cobbles to be set in stone dust, and underneath that 1.5 inches of stone dust, with a 6 inch layer of item 4. this over a well compacted sub grade.

Our contractor though suggests that we use 3/4 gravel with 2-4 inches of cement over that instead because he said that over time the item 4 will deteriorate and we will end up with tire depressions where we drive over the cobbles.

any thoughts on item 4 vs 3/4 gravel with cement? and with these cobbles which will provide the best base?

cement is expensive! so, we are wondering if it is really necessary and why the engineer did not call for it.

dl

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Old 04-08-2009, 02:37 PM   #2
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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.

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Old 04-08-2009, 03:02 PM   #3
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best bed for cobblestone driveway?


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?
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Old 04-08-2009, 10:31 PM   #4
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best bed for cobblestone driveway?


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.
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Old 04-09-2009, 05:45 AM   #5
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best bed for cobblestone driveway?


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
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Old 04-09-2009, 06:38 AM   #6
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best bed for cobblestone driveway?


you're building a flexible pavement,,, while conc does flex, they're call'd ' rigid pvmts ',,, if it were mine, i wouldn't have conc in it, either,,, ' item 4 ' is a nysdot term which mtl i'd also use IF i want'd your d/w

yes, lifts & 95% compaction - either vibratory roller or compaction plate,,, i'd use roller 'cause its faster however they may not have 1 onsite,,, who's responsibility or work ? - generally your site contractor,,, who cks compaction ? usually testing lab or your eng.

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Old 04-09-2009, 07:40 AM   #7
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best bed for cobblestone driveway?


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?
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Old 04-09-2009, 08:09 AM   #8
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best bed for cobblestone driveway?


' 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
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Old 04-09-2009, 09:33 AM   #9
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best bed for cobblestone driveway?


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.
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Old 04-09-2009, 09:37 AM   #10
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best bed for cobblestone driveway?


this may come as a surprise but, in 30 yrs of roadbuilding, we never built 1 on anything but item # 4 in nys,,, down here, we use sand clay - nevertheless, none're blt on conc

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