backfill

[merlebo02]

I am building a house on a floating slab.. I now have the blocks placed and need to backfill "with something" before I pour my slab.. I have 2 options that I am contemplating on using: Red sandy dirt or straight sand.. I have seen both used in our area but not really sure which I should go with.. Could someone help give me some direction on my best choice.

 

[Billrog]

Under the slab straight sand is your best option for compaction dampen it good but don't soak it or the ground under will swell then settle as it dries out. For the exterior sandy dirt is fine.

 

[truckdiagnostics]

Under the slab straight sand is your best option for compaction dampen it good but don't soak it or the ground under will swell then settle as it dries out. For the exterior sandy dirt is fine.

times 2:thumbsup:

 

[EddieWalker]

Sand is the only materials that self compacts by just adding water. Back in the 90's I ran a water truck and "jetting" the sand was one of my regular jobs. They would fill a trench with sand and then I would put a two inch pipe into the sand and fill it with water until the sand settled. You can see it happen when you have enough water in there, it just liquefies the sand and removes all voids. You absolutely have to use a lot of water when using sand as fill. The negatives of using sand is that its weight bearing load is a lot less then just about anything else. It also holds water longer then the surrounding soil, so if you get a lot of rain, the water will seek the path of least resistance and go into the sand, instead of flow over the top of compacted soil away from your building.

You didn't mention putting in a drain, so I'm thinking that's not part of the plan. I wouldn't want any more moisture next to a building then I had to deal with. I would use clean clay as my first choice and compact it as I built it up. If clay isn't available, whatever you have that will shed water once it's compacted.

 

[truckdiagnostics]

Sand is the only materials that self compacts by just adding water. Back in the 90's I ran a water truck and "jetting" the sand was one of my regular jobs. They would fill a trench with sand and then I would put a two inch pipe into the sand and fill it with water until the sand settled. You can see it happen when you have enough water in there, it just liquefies the sand and removes all voids. You absolutely have to use a lot of water when using sand as fill. The negatives of using sand is that its weight bearing load is a lot less then just about anything else. It also holds water longer then the surrounding soil, so if you get a lot of rain, the water will seek the path of least resistance and go into the sand, instead of flow over the top of compacted soil away from your building.

You didn't mention putting in a drain, so I'm thinking that's not part of the plan. I wouldn't want any more moisture next to a building then I had to deal with. I would use clean clay as my first choice and compact it as I built it up. If clay isn't available, whatever you have that will shed water once it's compacted.

well my guess is you don't see the frost. Where I am at we would never pour directly on clay for a monolithic slab. Any amount of moisture in the clay when it freezes it expands and will shift a building in a heart beat. I won't even pour a side walk directly on clay. I have cut corners and not put enough stone under the drive in some spots and it moves, all over the place.

 

[EddieWalker]

That is why compaction and drainage are so important. All soil holds water. When compacted, clay sheds it better then loamy type soils, but since there are well over two dozen types of clay just here in TX, there is an exception to every rule. Which is why best known building practices are so important. Compact and grade a slope to drain water away from the building. That will solve just about every foundation issue if the foundation is done properly.

 

[Billrog]

Contained sand is very load bearing eg. in side foundations. Brick layers are able to go up many story's high in one day because of it's good compaction the mortar is a binder that holds it in place.
As for exterior back fill clay is a good choice at least for the top couple ft. to help keep water from the footing.

 

[Cord]

Sand does not "self compact" when wetted. It will settle, but it won't compact. When it comes to bearing capacity, there is a big difference between the two. That said, to obtain the required 95% compaction the sand will need to be wet enough for the grains can stick together. When the material is loose and will make a ball in your closed hand, the moisture level is about right for compaction.

I've done sand beds under buildings with mixed results. The sand is pretty easily displaced when the area is being compacted, the bars are set and again when the slab is poured. I've also found that the sand wicks moisture very well and can result in a damp slab unless a quality vapor barrier is installed. 6mil poly, is not a "quality" vapor barrier. A good under slab VB would be a Steowrap or something similar. The other option is to use 3/4" crushed stone. Compacts just as well, but tends to stay in place better than sand.

Clay can have excellent bearing capacity, but certain types of clay expand more than others. I've seen the expansion rate range from 1/4" all the way up to 4". As mentioned above, expansion is bad because it will literally rip a building apart. Putting stone on top of clay does not cause it to magically stop expanding. In fact, I've seen stone catch water and direct it to the clay under a building. I know of one building which had a buried water pipe bedded in stone. The water entered the trench and ran towards the building where it soaked in. This cause the clay around the trench to expand and damaged the building. Generally with clay, if it's wet you want to keep it wet, if it's dry you don't let it get wet. This approach will help ensure the expansion between the wetted and dry states are minimized.

Question: if it's a slab on grade, then what are the "blocks" doing?

 

[topbuilder]

I'm building a house for a client. I knew the soil was pretty sandy before the 15' borings were analyzed. I was looking forward to a job with no clearing, very little dirt work. Wrong. The natural soil had a PI of 1! I had to excavate 3 1/2 feet 3' larger than the foot print and bring clay back in. 110 truck loads. Fortunately I was able to use what we took out of the hole as final grade material. So, no sand does not compact. It does not hold a dug beam very well either.

 

[Billrog]

Good explanation Cord far more to soils than can be written in a couple words like I try and do. Difficult to give real sound advice when some one says they have clay, sandy soil, or anything else unless you are to know the soils total characteristics. Texture: gravely sand, coarse to medium sandy/loamy sand, fine sand loam, silt loam, clay loam silty clay loam & many others .Structure: shape massive, platy prismatic blocky to name a few & grade : single grain , structureless, weak, moderate strong eg. This is the reason it makes it very difficult to make blanket statements as to what is the best coarse of action to take answering some of the questions individuals ask here.

 

[EddieWalker]

Sand does not "self compact" when wetted. It will settle, but it won't compact.

Maybe I wasn't clear in my comments. When jetted, or saturated, sand will self compact enough to meet code in CA when installing water utility lines for cities. It was my job to jet the sand. When I put enough water into the trench, the water would force it to settle and become flat. Generally there was at least a foot of sand on all four sides of the pipes per code. For really large pipes, it was a little more, but not a lot. The inspector would measure the thickness of the sand after I had jetted it. When we failed, it was because there wasn't enough sand on top of the pipe because too much had settled.

After that, I would add water to the dirt that was put into the trench that was usually compacted with a jumping jack. When that was done, the inspector would test that for compaction. Most of the trenches where 4 feet deep and would get about 2 feet of sand and 2 feet of dirt. The only thing done to the sand was the jetting, which I was told self compacted from the water.

If you are saying that they inspectors and contractors I subbed for are wrong, then I'll take your word for it. I've always thought they where right from seeing how solid the sand became after adding all that water.

 

[orezok]

6mil poly, is not a "quality" vapor barrier. A good under slab VB would be a Steowrap or something similar.

A small spelling error, but I'm pretty sure you mean "Stegowrap". When I was building schools, the only vapor barrier specified was Stegowrap.

 

[Billrog]

Eddie jetting the sand would definitely have it fill all the voids around the pipe and settle it enough that the pipe would be stable and not pull apart from any future settling. I had to replace a 6" main that at some point someone had thought it would be a good idea to use hog fuel. After 10 years it settled so bad the joints were coming apart in places.

 

[Egon]

Consolidating sand using water does not give the same results as a properly compacted sand.

 

[EddieWalker]

Consolidating sand using water does not give the same results as a properly compacted sand.

How do you compact sand?

 

[Streetcar]

How do you compact sand?

I have used vibratory roller, as long as sand was at proper moisture content

 

[Egon]

How do you compact sand?

Proper water content. ( described in a previous post ). And a vibratory packer with smooth rollers or plate. Air on the go will also work.

There are places where water jetting sand in a trench is not allowed.

 

[Cord]

A small spelling error, but I'm pretty sure you mean "Stegowrap". When I was building schools, the only vapor barrier specified was Stegowrap.

You are absolutely correct. I will say the one project that I had move to litigation was specified as being built on a sand bed using Stego for the under slab vapor barrier. The contractor replaced the Stego with poly and placed it between the sand and crushed stone. When they installed the sand bed they did so by driving over the sand that had been previously installed. This compacted the sand and caused the stone to puncture the vapor barrier. With VB full of holes the sand now wicked the moisture up to the slab where is caused a host of flooring adhesive problems.