Another Topsoil Question

John_Mc · Jul 28, 2002

I had a barn/garage built a few months ago. Some of the area was filled with sand to build up and level it off. On two sides, the ground slopes away pretty significantly. Since I don't have any topsoil over the sand, it started to wash away. I was concerned about the erosion undermining the cement slab, so I've temporarily covered the slopes with plastic sheets to divert the runoff.

I've been slowly covering the area with topsoil as I get time, and will eventually seed it. I'm presently at about 1 or 2" deep with topsoil. I'm shooting for 4" or more (hopefully 6"). My question is this: would I be better off tilling what I've got down now to mix it a bit with the sand before I put the rest of the topsoil down? I'm thinking this may help the topsoil "stick" in place a bit better. The sand is hard-packed in most places, and I'm wondering if I don't do something to mix the layers a bit, I may get the topsoil sliding off the slope of the sand underneath when I get just the wrong combination of weather and/or tractor traffic over the area.

Any thoughts?

John Mc

patrickg · Jul 28, 2002

Yea verily, if there is much slope I would till the soil into the sand about as deep as I could. I fear you are probably right in your concern about driving a tractor around the slope under worse case conditions of moisture in the layers.

I spent a day digging in the botom of a partially dried up wet spot (was a creek bed prior to some pond building) to get moist dirt to pack around a washed out culvert in a driveway. I too have concerns of bonding the new dirt to the old dirt and that is without the sand problem that you have. Get some professional advice, you might be safer to remove some or all of the sand.

Patrick

Turfman · Jul 29, 2002

Soil layers on slopes contributes to movement when the various layers get saturated. Perhaps one of our civil engineers can help here. What I remember from my basic soils classes is that the sand will saturate and the layers will shift. This is a particularly bad problem with 2:1 clays. What is the top soil like?

Tilling the two together to "blend" would probably help some, but I'd be more interested in getting some grasses growing on it to have a root structure which will bind everything together.

Egon · Jul 29, 2002

Tilling in the dirt will not hurt. You could also till some cement into the sand. This will harden it and make it more erosion resistant. Putting a layer of crushed gravel over the sand may also help. This may be what should have been used as fill in the first place as it can be compacted. Sand doesnt compact as well as we think as it is usually gap graded. Also make sure the roof runoff is so directed that it avoids the sand. Straw bales or anything that will decrease the velocity of runoff will help.
Another possibility is to dig out a strip of sand and replaced it with crushed gravel. This may act as a retaining wall but it should not be done to close to the foundation.
Egon

cowboydoc · Jul 29, 2002

I agree with Egon. Sand is a terrible material for using on a slope and expecting it to stay. Get some crushed rock and put down around there to keep everything from washing away. It's a real pain to mow around buildings anyway, esp. on a slope. Even if you do get grass growing there you aren't going to have a very stable base with all that sand even if you rototill it in and it will probably continue to shift. If it was mine I'd take the sand off down to the original base and then put road mesh over it and put rock over the top of that. You'll never again have to worry about it.

John_Mc · Jul 29, 2002

Thanks for all of the responses, guys. I wondered about the sand as it was being put down, but figured the guy knew what he was doing. I'm not sure I can dig down to the old base at this point. I'd be too worried about undermining the slab that was poured.

I did dump a bunch of mixed rocks and soil around to try to make the slope more gradual, but I still have a good bit to go. I've been spreading some good topsoil over the top and the areas where the runoff was the worst. I'll be seeding it as soon as I resolve some of the remaining issues.

I'll try to include some pictures if I get the chance. At least my tractor has a home in out of the weather now.

Kodiak · Jul 29, 2002

If your going to till it. Then till in a layer of straw with the top soil this will help bind it all together. As well as retain moisture for the new grass. The new grass will just grow through it, and the straw will eventually decompose.

John_Mc · Jul 29, 2002

Thanks for the tip on tilling in straw. I'm actually thinking of tilling what I've got now (~2" of topsoil over sand) to mix the layers a bit, then putting down more topsoil over that. I assume I'd want to till the straw in AFTER putting down the last layer of topsoil?

John Mc

Kodiak · Jul 30, 2002

John:

I would just till it all together. This will help keep all of the layers bound together to avoid erosion. The new grass will just grow up threw it. and any sticking up will be cut off the first time you mow it. Also a loose layer of straw on top after you seed it will help keep it moist and protect the seed from UV rays and birds.

Soundguy · Jul 31, 2002

Couple things to consider with erosion, and migration of sediments and soils.
Unconsolidated material ( fill sand ) can do some funny things when wet, and also in earthquake situations.

As someone pointed out, in a wet area, a 'slump' may occour.. to picture this, think of the side of a hill 'melting' away from a road... that is slump.

Wet fill material also becomes plastic, and will/can flow during earthquake situations. I've seen firsthand what has happened when houses with large stemwalls ( 8+feet ) sitting on fill material. The material flows, and one side of the foundation gives out, and the whole house gives way to the side with the failure.

Lots things to controll it..
Vegitation helps prevent soil migration. Temporary silt fencing allows water to pass but traps sediments. ( this would be expensive and time consuming for that individual.. especially on a small basis. )
Staked terraces are another answer.. again.. this is more suited for big projects.
Gutters with downspouts and a splash pad ( or as the new DOT specs call them.. energy dissipators..... )
Vegitation will be easiest...
Soils with large subteranian clay pockets can do some interesting things too.. in most cases, clay is an aquaclude ( think barrier ) and can in some places, create an artificially high water table.. this is a perched water table.. and isn't much more than a resevior trapped on top of an aquaclude... Clay can swell and contract a great deal as moisture content changes.. makes for some neat topology. Also hydrology in areas with aquifers and seasonal rainfall / drought conditions create those lovely sinkholes. ... Just occoured to me that I'm rambling a bit and have really gone way off topic.... sorry guys.

Soundguy

<font color=blue>" Perhaps one of our civil engineers can help here.