sloping gravel driveway 4

[petervb]

I have a problem driveway that is constantly eroding. Every year or so, part of a 2500 ft long driveway erodes, especially at a high slope area. Most of the drive is manageable with diversion swales sending water off the road. in one spot though, two conditions are a problem: high slope and hard granite rock. the slope for 188 ft is 15 ft for 8%. The road at that point goes through a fault, or old geologic drain forming a solid rhyolite ridge with a channel cut in it. the road is laid on top of that channel, but the ground rises on both sides, leaving no room for drainage channels etc.
I've had rotted granite laid down, but it washes out in the hard rains of texas. There is NO limestone available, just granite and river rocks of different sizes. The local materials supplier

http://www.colliermaterials.com

has a variety. I've thought of filling the erosion with large cobbles and over toping with a larger sized granite. Any ideas?

 

[miningman]

I suppose the use of explosives to reduce the grade of the steep section is out of the question but perhaps using expansion chemicals aka smokeless powder might be cost effective. Even in hard granite this stuff is remarkably effective.

https://www.dexpan.com/

 

[cvg]

decomposed granite is not very erosion resistant. any sort of crushed rock would be better. if you cant afford to pave the steep section, maybe use a cement treated base material.

https://www.azdot.gov/docs/businesslibraries/CMChapter_3_Section_304.pdf?sfvrsn=5

 

[oldestguy]

A tough situation you have. I struggled with that sort of thing for years. I finally bit the bullet and paved it with a two course layer of asphalt-concrete blacktop).
the thickness I had placed was at least 4 inches. You still need to do some shaping of the subgrade and compacting down a well graded sand and gravel base course. That would not be required on solid rock, but will be needed over frost susceptible soil. For a driveway under 4 inches of blacktop, at least 8 inches of base would be my choice. For weak sub grade, more would be what a long term life design would be needed. To avoid eroding the base course out from under the blacktop at the edges, you may want to widen and sloped down the blacktop and divert the water away, or pave the ditch also with blacktop. In summary, that paving was done about 25 years ago and it still holds up well. It was well worth the cost.

 

[GeoEnvGuy]

High ground on both sides on an 8% slope, so your road is the ditch. Check dam spacing in ditches for 6% slope is 25 ft. You can consider making the road into a V and have the water run down the middle and place gravel and rock in the middle, with larger rock at the bottom of the slope.

 

[petervb]

Thanks to all of you for the excellent advice. Mostly. The explosives idea is too far out; I don't have a government budget!

to oldestguy-- can you run a 25T gravel truck up that paved drive? I still have 2000 ft to maintain. I had thought of pavement, my neighbor did that, but he is scared to have heavy vehicles on it.
to cvg and GeoEnvGuy -- I can get crushed rock, 1" river pebbles, larger sized granite, but no limestone. I'm considering making a larger sized rip-rap drainage feature on each side with a couple of check dams. They are irritating to drive over, but a chevron shape would make things better.

 

[petervb]

For GeoEnvGuy-- is there standard practice for sizing of granite gravel for a particular slope? 8% in this case.

 

[GeoEnvGuy]

FEMA Technical Manual Overtopping protection for dams

Water going through rockfill in chapter 8
The Abt and Johnson (1991) relations apply to flat slopes of 50:1 to 10:1 (2 to 10 percent).