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.