How much can I slope gravel?

jayhaitch · Jul 13, 2007

I want to extend the gravel parking pad in front of our attached garage around the side and back, to make a parking pad for the landscape trailer behind the garage. However, there is quite a slope from the side of the drive to a tree line about 20 ft away. Behind the garage is quite level. I could build a retaining wall, but it would far less work to just grade, spread, and compact gravel.

What is the maximum slope gravel can accomodate without constantly washing into the trees?

jeffinsgf · Jul 13, 2007

What kind of gravel are you using? Its composition will have a lot to do with how you deal with your issue.

Fundamentally, you want to start at the bottom of your fill area with large rock. Around here we use "4 by 6", which is crushed limestone no smaller than 4 inch and no larger than 6 inch. If your total elevation change is going to be measured in feet instead of inches, you might want to start even larger. You'll need your large rock spread farther and wider than the next layer, creating a taper to your grade. Again, depending on your total elevation change, you'll next want to move one grade smaller on your rock. 2 by 3, for example. Lay in a lift of that, and then use 1 inch crushed limestone with fines (crusher run) for the last 3 or 4 inches and the final surface.

Using large rock as the foundation minimized wash out, as does locking it altogether with crusher run.

dirtrod · Jul 13, 2007

Not that I would recommend it but part of my driveway is a 22% grade. Most of the 1/3 mile driveway is about 18% and we do get some serious down pours here on the Oregon coast. I started first with some larger random sized road bed rock and after a season of use to pack that in I followed with 4 inches of ¾ minus crushed rock with fines. The 18% portion has had no problems and 22% portion has only had minor washing and only during severe down pours.

I should add that after the first season and prior to adding the ¾ rock we went over it with a vibrating roller.

jayhaitch · Jul 14, 2007

Thanks!

18-22%? Yikes! I think I'd need a winch on each vehicle if I had that here in our winters

Actually, my rough calculation is about a 16% grade. I could make it somewhat less with some slope to the rear. Glad to hear it's do-able.

Big rock to small rock sandwich: got it. That's what the excavator guy did for our driveway, 2" down limestone base during construction, well compacted with his track crawler back and forth after almost a year of construction, then 3/4" down limestone, again compacted with his crawler. Only a few ripples now, almost a year and one cold winter later. The parking pad was added after. He didn't compact it nearly as well, and the edges are definitely loose.

I talked to a commercial rental yard, and they recommended a vibratory plate compactor, which they said compacts up to 26" depth, which is more than a ride-on roller. Yet I have a bad back, and can't twist with a heavy load, though I can do quite a bit as long as I maintain proper body mechanics. Have you guys used a plate compactor? Back killer or not? Should I just rent a ride on one?

Greyfields · Jul 14, 2007

For stability, don't exceed a 2:1 slope (that's 2 horizontal to 1 vertical foot). The word "gravel" covers an immense range of products, though. If it's washed gravel, meaning round and uniform in size, then it may even fail at a 2:1. If you can get 5/8 or 7/8th minus rock, it will be graded, meaning having proportions of clays, silts, sands and gravels (up to 5/8 or 7/8 inch) and it will be far far stronger. You always wanted to use well graded gravels and crushed rocks.

well graded = poorly sorted = strong material
poorly graded = well sorted = weak material (but good for things like infiltration and underground drainage)

Egon · Jul 14, 2007

Around here larger rocks [ say 2 ft. + diameter ] are stacked around the edge of area to be built up. The interior is then filled with material at hand. Larger equipment is required to handle the rocks.

For compaction go with a vibratory roller if you have any doubts about your back.