Another Topsoil Question

[John_Mc]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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.

 

[Carl_NH]

John,

Straw will decompose slower than hay so will hold back a slope better long term however mulch hay (previously had gotten wet before baling) will decompose faster add more organic material , so I would get some mulch hay - cheaper and as effective until the grass grows in. Also you can use hay for the surface after tilling and cover coat of topsoil if you don't mind the various hay seeds also contributing to your "lawn".

As you state you have put rocks in some places, so when you till this mix you will bring these to the surface, so I would mix hay and till in, then grade smooth and add 2-3" of fresh loam on top, seed and cover.

Carl

Carl

 

[mrcaptainbob]

Thanks for the 'rambling', soundguy. I learned a lot from that. You answered some questions about the 'suspicious' activity of the slope in the front yard. 15 or 20 years ago, I had about a 150 yards of top added to the west side of the yard. The base is clay. Suitable for the kiln clay. Now that part has 'eased' it's way down so that it's probably close to a couple of feet lower than when I had it graded. And the side has 'eased' it's way further west and more ramped. Another 100 years or so and should have the field next door just about filled!

 

[Soundguy]

Thanks. I've got to watch myself better. Sometimes I come home from work and start telling the wife about such and such a project I was working on today, and so forth...then I'll glance up to see her eyes glazed over....

As for the gradual migration you are seeing.. might even be closer to 'creep', as a 'slump' generally occours on / around a steeper slope... but then again.. 20 degrees is steep enough.

Soundguy

<font color=blue>Thanks for the 'rambling', soundguy. I learned a lot from that. You answered some questions about the 'suspicious' activity of the slope in the front yard. 15 or 20 years ago, I had about a 150 yards of top added to the west side of the yard. The base is clay. Suitable for the kiln clay. Now that part has 'eased' it's way down so that it's probably close to a couple of feet lower than when I had it graded. And the side has 'eased' it's way further west and more ramped. Another 100 years or so and should have the field next door just about filled!"

 

[bnixon]

While we are on the subject of soil....I have a few questions.

I need to bring in some dirt to level and grade my place.

Here in Houston we have what we call Gumbo Clay. I was thinking about bringing in a sand/top soil mix. This will be used to fill holes and build up the pad where my house will be built (in about 3 years). I want to start now so I can give it time to compact and settle.

Is this the type of dirt you would use?

Would crushed stone be any good for a house pad base?

Most people around here use sand. But it seems to wash real easy.

What would you guys recommend? By the way, my place is flat.

Thanks,

 

[Egon]

Crushed gravel would be the best for the house pad and other concreted areas. For the rest clay,topsoil and a little sand would work well as long as the the top 6/8 inches can be well mixed to form a sod bed for the grass to grow in.

Egon

 

[cowboydoc]

I agree with Egon on the crushed gravel. If you can for the topsoil I would try and find some black dirt to bring in. I have no idea what's available in your area but around here they usually will bring in the black dirt for projects like yours.

 

[dmccarty]

bnixon,

Before you spend money and time doing this house pad you
might want to talk to a soil engineer and see what they say
to do.

I'm finishing up the first phase of a driveway. I think I put
down or will put down about 336 tons of ABC. It was about
16 tons per dump truck load. ABC is a mix of fines up to
angular rock maybe 2 inches long. I got lucky given we are
in a drought and got some real heavy rain on various stages
of building the drive. This stuff locks up like concrete. I'm
very impressed. I have only had one rut from the 20 some
dump truck loads backing up the drive. And the rut was
caused when I spun my rear tractor tires spreading the
ABC. I have seen very little, maybe a 1/16 or 1/32 of an
inche compaction due to 54,000 pound trucks. Where I have
a flat stretch of drive I'm only spreading 4 inches or so deep
but I am using geotextile fabric which I'm sure is helping.

Part of the driveway has to go over a culvert. The culvert
is in a 100 foot section of drive that dips about two feet
down. So I have a good two feet from grade to cover the
culvert. The ABC is pretty much verticle in some spots, up
to that two foot height and it has not washed down at all. I
sat there one afternoon in a heavy downpour and watched
the culvert and the road to see how well things where going
to work. Everything was fine and nothing washed away.

But I would check with an engineer to see what they say. I
have seen building pads built in south east Florida that where
made of sand. Some where compressed and some where not.

Good Luck,
Dan McCarty