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Gravel Road - wearing course

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Folks,

We're building a 1/2 mile gravel road (client does not want asphaltic concrete) up a relatively steep incline (10~18%).  We have a local quarry that supplies a 3/4" downgraded crushed rock that I've used in the past and liked - it's got a good clay component and compacts well - it meets Caltrans Class 2 gradation requirements and this is the material I consider to be best suited for the application.

The contractor disagrees completely about using a Class 2 base and wants to use either a local pit run shale material which is considerably cheaper or, as a better option (in their opinion), a decomposed granite.  The contractor tells me the Class 2 would be the worst performing option in the long term.  They're an experienced engineering contractor and I trust their judgement, but in this case I cannot convince myself as to why DG would outperform a 3/4" minus crushed rock for a gravel road - but I also do not have first hand experience with using DG for a gravel road.  The area gets about 30" rainfall per year and the road is traveled by slow moving vehicles.

Do any of you have any experience and/or opinions regarding the best aggregates and gradations for a steep gravel road?

Regards,

Marc Mitchell, P.E.