Using Base Course for Road Surface

[ChipperB]

I am reviewing a set of plans that constructs a new road over an existing road. The existing road is on a 5 percent grade and shows a lot of erosion and some bare rock outcropping. When it was first built it caused a lot of flooding to the area downstream at the bottom of the hill. The 22-foot wide new road road will cover the old road with 9-inches of compacted TxDOT A1 or A2 Base Course material. Also, the design calls for small ditches on each side of the road. Crown is drawn on plan, but cross-slope is not specified. The designer is regrading the upstream area so that the volume of water going to the road is reduced.
I have concerns that the Base Course material will be washed out at the first good rain. I am going to ask the designer to provide his design analysis. Can someone give me some tips on what to look for to make this type of road successful? At this point in time we cannot afford HMAC or concrete, so really need to work with cheaper materials.

Thanks In Advance For Your Support!!!

 

[Ron]

Graded aggregate base courses achieve their stability, in part, from their confinement. If you leave the top open, without protection, that confinement is lost and the surface will erode because of a lack of cohesion.

As a compromise, consider injecting asphalt emulsion into the base course as it is placed or add cement to stabilize. You can also combine the two for a durable surface/base. Keep in mind that you might not have a lot of skid resistance.

Another option is to place a single or double chip seal after placement of the base.

 

[ChipperB]

Thanks Ron. I too had thought about cement stabilizing. Also, Texas we have something called black base, which is just asphalt impregnated base.

The TxDOT A1 base is supposed to be good unconfined when its compacted, but the A2 base-not so much.

 

[BigH]

On an "open" base course, expect a lot of wash-boarding effects. You can check out some papers/manuals on what non-asphalt/concrete road surfaces should be as far as gradation and PI. I like the idea of cement stabilized base - usually I found that 3 percent gave good results on a project I did overseas. However, I would suggest that you consider putting on a prime coat and then a single or double course chip seal. You can do this over a non-stabilized base course.

 

[HuntPA]

I also would highly recommend the "tar and chip" sealing. Even if the base is concrete stabilized, there still may be pockets that do not get fully stabilized leading to potholes, erosion, and then a worse road than you have now. If the big worry is erosion, I would concentrate on sealing the surface to guard against it. I like a 2% crown.

 

[civilman72]

A properly graded gravel road can hold up to normal traffic if prepped correctly. But truck traffic and sharp curves can make the best gravel road washboard.

You generally want to use the same gravel material used for a basecourse, but with more binder (fine) material.

Require at least a 3% crown.

Chip-seal, as mentioned above would be preferred, but if costs are truly restrictive you can consider a soil stabilization material such as mag chloride. It will significantly decrease road maintenance.

 

[Phidelt218]

I would go with a chip seal or use a surface course aggregate as a wearing surface. I have not seen a base course used plainly as a surface course. I would think you would get faster then normal washboarding and form potholes faster. It would also not be as smooth of a ride due to larger gradation and fewer sands/fines. In Alaska I do not typically inject asphalt emulsion but it does work well.

As for cross slope. Ensure it is not 0%. That is poor design and will shorten life of road. 2% is typical for paved and 3% for gravel, normally crowned but can be sloped all the way across too depending on the road alignment, terrain and all.

 

[ACtrafficengr]

What kind of traffic will you have (AADT and %trucks)? Unless it's very low, your long term costs will be lower if you surface it. The exact break-even point probably depends on materials, vehicle types and speeds, and weather, but I've heard 150 vpd used as a rule of thumb.

I'm not familiar with Texas DOT specifications, but if A1 and A2 are a base course materials for a paved road, you'll need to surface it. Paved road base course gradation makes a lousy gravel road. Good material for a gravel road has more fines in it. This, of course, means it's a lousy base for a paved road.

The Cornell (Univeristy) Local Roads Program has some good info on gravel road materials. If this is a public road, and assuming you're in TX, the people at the equivalent Texas program would be a good resource, too.

 

[cescientist]

While deciding about these factors, please also keep in mind the ride quality.

Fahad
Editor at

http://cescientist.com

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