How much gravel do I need?

[DsGrouse]

At the cabin, we used what they call road bond, up there. Probably the same stuff Al mentioned. That stuff packs down over time and becomes almost as hard as asphalt.

I have several inches of road bond down in the existing driveway. It’s the widening where I’ll need some. Or the crushed rock will do as the road bond is just gravel and the local clay.

The bottom or lower spot perked out in about an hour as the top six inches is top soil.

My only concern is if four inches of gravel will be enough. From what the neighbors tell me it’s marginal. This rain that I had was brief and I doubt I would have problems. It’s the spring thaw that concerns me. This spring I had a river in my driveway and while it was mushy my van didn’t get stuck. The motor home was parked on the lawn and sunk in almost to the axle.

Which is why i would recommend a french drain. dig it deep enough to put 2" of gravel below the drain and 4" on top of it. The water will drain from the drive way much easier. You will have less wash out of your driveway It is well worth the effort

[John E Davies]

I second the drains! I had a similar situation in rainy Monroe WA where the gravel driveway sloped down to a concrete RV pad. The builder just put down 6” of crushed gravel “mix” for the driveway. The first time I tried to tow my big boat off the pad after the first storm, the rear drive wheels of the truck got to the transition and sank down until the axle hit. It had a Detroit locker but all that did was make them both sink evenly. It was liquid mud underneath. I trenched crossways about every ten feet and at the bottom of the slope and laid perforated pipe leading away. Bingo, no more problems.

You definitely don’t want your motorhome sinking down to the frame. Put in drainage and move all the ground water far away.

I am not sure 4” of gravel on top of the pipe is enough to prevent crushing from your RV, I would go a lot deeper and also use heavy solid drilled plastic drain pipe, not the flimsy flexible kind. To be safe I would wrap the pipe under the gravel with landscaping cloth so the holes won’t plug. Use a long straight 2x4 and a carpenters level and put a good constant slope on the pipes (no low spots). I can’t recall exactly but I think 1/8” per foot should be a minimum slope for any underground drain.

Start each run at the far side at an access drain with grating or a sewer cap so the system can be flushed. Dead end pipes that are unground are not reliable! With a constant grade and a flush every few years with a garden hose, they should work great.

If you don’t have a clear down slope to move the water to, you need to dead end the system in buried 55 gallon plastic barrels, drilled on the bottom. You can buy surplus ones for about $5 to $10 each here. My current house in Spokane has four buried French drains leading to barrels and they work very well for rare roof runoff (no gutters).

Do you have a backhoe attachment? If not a ditch witch trencher will be needed…

Thanks for posting lots of pictures, I really like what you have done so far. And I also like to spend other people’s money

John Davies
Spokane WA

[GunnyGene]

One thing I'd recommend is to put in a border around your gravel to keep it from migrating into the grass. Railroad ties or whatever. You don't want to be mowing around it if it has migrated. Just asking for high velocity rocks to hit something you don't want to hit, plus rocks will chip your mower blades.

[DsGrouse]

I am not sure 4” of gravel on top of the pipe is enough to prevent crushing from your RV, I would go a lot deeper and also use heavy solid drilled plastic drain pipe, not the flimsy flexible kind.

I should clarify, that it's 2" underneath the drainage pipe, 4 inches over the pipe, THEN the 4 inches of roadway gravel over that. On mine i had 8 inch pipe, in a 10 inch trench, the sides were supported by gravel also. I was told not to have voids, other wise the pipe would collapse.

By removing the topsoil, you've removed the way for the soil to deal with the water. It isn't so much the single rain, but the rain over a period of several showers keeping that clay damp enough to eat gravel or sink a car. The gravel will not help remove the moisture from the clay. The drains will provide a path for that water coming down off the hill, to the top of your photo, to bypass your parking pad. It will prevent the clay from staying too wet. You don't really have to get as complicated as I showed. I would put one french drain at the edge of the hill and run spurs from it (at John E Davies recommended 1/8" drop) across the pad to the opposite side of the parking pad.

A ditch witch will help, you can do it with a backhoe, but a ditch witch is much faster.

When it comes down to it, a dry parking pad will save you thousands in new gravel, hours of time in not having to re-grade washouts and the hassle of not having to tow out a truck that buries itself up to the axle. (my Tahoe and Audi sank over a period of a few days. it wasn't an especially rainy season. It had just rained minimally several times over a period of a few weeks. The fall prior to that, we had Hurri-not-cane Irene hit us. Days of torrential rain and the pad did not fail. As the clay underneath was dry from the summer. That level of dry clay sheeted the water off the pad. Yet a year later we have 10 days of misty and drizzly weather, the clay slowly soaks up the water, and Poof. I have to have the cars towed out.)

After Irene, and the following sinking, I put in the drains, Later that year, hurricane Sandy hit, and the pipes were jettisoning water from the pad.

[John E Davies]

I should clarify, that it's 2" underneath the drainage pipe, 4 inches over the pipe, THEN the 4 inches of roadway gravel over that. On mine i had 8 inch pipe, in a 10 inch trench, the sides were supported by gravel also. I was told not to have voids, other wise the pipe would collapse.

Thanks, that makes sense, but an 8” pipe? How much ground water do you have to move? I would use regular 4” for each cross pipe and go up to 6” where they join. If the goal is to move the water far away, those bigger pipes should be solid so the driveway water doesn’t just go down into that area.

John Davies
Spokane WA

[DsGrouse]

I should clarify, that it's 2" underneath the drainage pipe, 4 inches over the pipe, THEN the 4 inches of roadway gravel over that. On mine i had 8 inch pipe, in a 10 inch trench, the sides were supported by gravel also. I was told not to have voids, other wise the pipe would collapse.

Thanks, that makes sense, but an 8” pipe? How much ground water do you have to move? I would use regular 4” for each cross pipe and go up to 6” where they join. If the goal is to move the water far away, those bigger pipes should be solid so the driveway water doesn’t just go down into that area.

John Davies
Spokane WA

I didn't have a grassy slope as he does. I had a series of stepped rock walls on the back side of my property. Thus certain parts of that area of my property had a lot of moisture at certain times of the year or when there was constant drizzling. When I went to get the piping, I had the choice of soft piping in a styrofoam sock, or hard piping in the same type of sock. The hard piping was 8inch and the soft was four. The 8 was overkill no doubt about it, but like you, i chose the hard over the soft.

[Sir Henry]

I'm hoping to sell my motorhome so that won't be an issue. Most of the turn-around has a good base. I could almost not put down any gravel and it would be okay. The exception would be up against the hill where there wasn't a driveway before. I won't have time to line the edge with timber this year. The main driveway doesn't have a liner and I don't have a problem catching rocks there.

I'm only putting down four inches and I'm told I need six to eight inches to make it so I don't punch through. I still have some base down in the driveway so I should be okay.

The reason I'm using one inch gravel instead of the regular paving gravel is because when it gets wet it is very dirty. My concrete driveway gets covered in sand/dirt and I want to eliminate that and also eliminate the standing water. I'm trusting what the neighbors are telling me and what the previous owner told me about what is under the driveway.

[JEBar]

looking forward to hearing how your "figuring" comes out .... many years ago I when through the process of trying to determine the same thing for our driveway .... I learned that having someone who does that for a living can quickly come up with solutions than I could .... you are lucky to have had the rain .... its much easier to put in blind drains and such before the rock/gravel is put down

[Sir Henry]

looking forward to hearing how your "figuring" comes out .... many years ago I when through the process of trying to determine the same thing for our driveway .... I learned that having someone who does that for a living can quickly come up with solutions than I could .... you are lucky to have had the rain .... its much easier to put in blind drains and such before the rock/gravel is put down

I’m not going to put down drains. Gravity will take care of the water and I just eliminated the few low spots. I will need to put in a French drain behind the garage. That’s a project for next summer.

[GFK]

Looks like you are doing a good job.