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Fill under slab?

davidmeiland's picture

Last week we dug out for our shop, and this week I got the footing and wall forms in. Pour is on Tuesday.


Next step is to prep for the slab. We had to dig out quite a bit of topsoil--up to 24" in some places--so quite a bit of fill needs to go back in. The usual fill here is crushed rock. I figure I need approx. 60 yards, and it costs $25 per yard, so I'm looking for ways to save money if possible. Sand is somewhat cheaper, and in places I read that sand is a good slab base and in others I read that it's not. I also have a major pile of fieldstone--our soil is full of it--that I could mix back in, if appropriate. That pile ranges from stones the size of grapefruits up to suitcases and steamer trunks.


I'll bite the bullet and go with all crushed stone if that's the best thing, but I'm looking for options. According to my excavator, and what I see locally, other folks use all sorts of stuff, including old bricks, old cars, etc.

(post #100441, reply #1 of 30)

I would think that using SOME fill (i.e. rocks, bricks,etc.) should be acceptable as long as you are able to get crushed rock, pea stone, etc. to get into the "voids" around the fill areas. Not sure if compacting is an option...but you want to limit the chances of air pockets forming in the concrete which would probably eventually crack

(post #100441, reply #2 of 30)

We have a plate compactor and will fill in 6" lifts. The excavator will use the flat bottom of his bucket to 'smear' the fill into place, which compacts the h&ll out of it.

(post #100441, reply #3 of 30)

that makes sense ! is it necessary to vibrate the concrete as well as its poured?

(post #100441, reply #4 of 30)

Not the concrete for the slab... but definitely the walls and footings.

(post #100441, reply #5 of 30)

we use sand for everything.

. 2+3=7

(post #100441, reply #6 of 30)

In Michigan (lots of bad roads) we have an abundance of crushed concrete (21AA).  This is 3/4" down to dust.  Some call it crush and run.  It packs hard due to all the differnet sized pieces but still drains.  It's really cheap too.  I brought in 48 yards of the sutff for my driveway.  I think I paid about $160 for a 16 yd truck or $10/yd delivered.  If you can pick it up yourself (borrow a truck from a buddy) it is really cheap.


Just a thought.


You buy a cheap tool twice and then you're still stuck with a cheap tool!
You buy a cheap tool twice and then you're still stuck with a cheap tool!

(post #100441, reply #7 of 30)

I would try the raod base(recycled concrete) it is made to pack down for under roads locally and is about 11.00 a yard here. Rent a conpactor if you dont have one for a day or 2 and it will be worth it in the end. I had my 6 year old daughter running mine on my shop gravel until she got bored(5 minutes)

(post #100441, reply #8 of 30)

Pretty much all I do every day is make holes in the ground and fill them back in. Finally a question that doesn't involve crown molding or cabinet hardware or all that other stuff that I know absolutely nothing about. <G>

Here's the story on backfill and compaction;

Since your backfill is restrained by stemwalls, you've got it easy. If you were building an unrestrained pad (like fotr a monopour) you'd want a little bit of cohesive properties to the soil, but in this case, it's no benefit. So any material that is not expansive and has no organic content is good, with two exceptions.

The two exceptions are if you have a drainage problem and the maximum size particle that is consistent with good practice.

What you are after first of all is reliable bearing competence and no chance of a change in volume (shrinkage or swelling) after the slab is down. This is achieved by compaction of suitable soil and good drainage around the building. Typically, the amount of compaction is expressed as a percentage of the theoretical maximum density at optimal moisture content. This maximum density is calculated by a Proctor test. Standard Proctor is adequate for residential use. But I don't think you need that here.

This should also be as inexpensive as possible.

Unless you have serious water problems there is no reason to use crushed stone. As you have discovered, it is expensive.

The other exception I mentioned above is large chunks of anything. I don't like to see anything bigger than about 4" dia. in the backfill in the last couple of feet. Anything bigger than 12" in the last 10 feet (vertical) of fill is a code vioaltion under the UBC.

When the compaction equipment hits the big chunks it tends to skip over it and you end up with soft spots that can turn into voids. So old cars are out. <G>

If it's slab prep, I like to see clean material in the last two lifts -- ideally nothing bigger than about 3/4". Base course or engineered fill work real well, crusher fines for the last thin lift gives the concrete guys a real smooth surface.

In your case, pit-run gravel that is 4"-minus would get you up to the last lift, then you could use base course or e-fill to top it. E-fill is usually cheap -- it's often no more than a mixture of crusher waste and gravel. Pit-run (a.k.a. bank-run) gravel usually costs more to load and carry than it costs per ton to buy. It's cheap, and it's strong.

It sounds like you might be doing this yourself, so here are a few more tips;

The more well-graded a material is, that is, a mixture from fine to coarse particles, the better it's going to work.

The more fractured a material is, the better it's going to work.

The closer your material is to optimal moisture content . . . yep, the better it's going to work. If you want the short discourse on determing optimal moisture content, just ask. If the material is too dry or too wet you will not get your compaction values. This is seriously important.

As for compaction equipment, as Brownbagg has repeatedly and correctly pointed out here, those little 200 pound plate tampers are not for dirt work, they are for patching asphalt. Do not use them, they will not work. They look like they are working, but they don't.

If you want to use a plate, rent a reversible plate, they weigh about 800 pounds and they are intended for this purpose.

The selection of compaction equipment is a function of your soil type. Generally speaking, the more granular a soil is, the more it needs to be vibrated at a high frequency and low amplitude, the more cohesive it is, the more it needs to be pounded at a low frequency and high amplitude. I can go into more detail if you want.

Success is not the key to happiness. Happiness is the key to success.
-Albert Schweitzer, philosopher, physician, musician, Nobel laureate
(1875-1965)

(post #100441, reply #9 of 30)

One afterthought -- you probably know this already, but just in case; any compaction equipment big enough to do the job properly is also big enough to push out your stemwalls. Always backfill the outside first.

Sometimes I wonder whether the world is being run by smart people who are putting us on or by imbeciles who really mean it. -- Mark Twain, author and humorist (1835-1910)

(post #100441, reply #10 of 30)

Thanks for the detailed info. I have questions, of course.


The main thing for me is the lack of availability of material. I live on an island that has one functioning gravel pit. The material is shale and is available in 2"-minus and smaller. There is also apparently some sand available. I do not know what type of sand... whether it's the type used in concrete or something else. Those are the two choices that I know about at the moment. I suppose there's also the possibility of soil excavated from another site, and/or rock hammered from another site.


So... would you go with the sand, whatever it is, or skip that and go with the gravel? Any reason to risk it on possible soil from elsewhere? My excavator knows enough to get material that would compact well. What about rock-hammer spoils?


Also, what would your method of compaction be? There excavator mentioned driving the machine into the hole, over the top of the stemwall. I'm quite leery of this. I need to get going on the filling and slab prep ASAP after Tuesday's pour. So, with the machine he might be limited to reaching in with the boom to spread material and maybe push it down. Other than that I can rent a jumping jack or other compacting equipment and use it in the hole.


There does not appear to be any water issue. We had hard rain immediately after the hole was dug, and there was some water standing in the footing areas but it's gone now, I'd say both by evaporation and soaking in. The soil we're bearing on appears to me to be a mix of clay and sand.


I hear you about not using large chunks. I would like to dispose of a fair amount of small stone, maybe 6" stuff, in the hole, but I certainly don't need to.

(post #100441, reply #11 of 30)

I myself would fill with sand hit with a fire hose and be done with it.

. 2+3=7

(post #100441, reply #13 of 30)

I would be very careful about the shale. Check with someone local who knows for sure what they are talking about, like maybe a geotechnical engineer. Some shales when broken up into dirt are expansive. That would be bad. If there is even a remote chance that this stuff is expansive, stay away from it. If it is definitely not expansive, then it might be an option but I would talk with your excavation sub. He knows the area, I would think.

There is nothing wrong with sand at all, as Brownbagg pointed out. It's a PITA if it's unrestrained, but since you are inside stemwalls, no problem.

If you use sand, you want to work in 6-inch to 8-inch lifts, and you want to use a reversible plate. A jumping jack will be worse than useless, and driving on it with a machine does not get you to the 95% you are looking for. Neither will pushing on it with the heel of the bucket.

Here is why; the more granular a soil is, the more it needs to be vibrated at high frequency and low amplitude to get it to compact. Sand is as granular as it gets. The good news is that sand is very forgiving of moisture content. You can get it to max. density even a little on the dry side. But it needs to be rattled, either with a smooth vibrating drum or a plate.

I don't like working with sand just because it's easier to get a good job if the soil has a little bit of cohesive properties.

If you can find soil excavated from another site and blend it with sand you should be in pretty good shape. If there is a silt/clay component and you can mix it with the sand, you are way ahead of the game. Reason is then you have a more cohesive soil. Driving on it with the machine will definitely help. You can also then use a jumping jack, which is easy, cheap, and reliable. With a slightly cohesive soil and a jumping jack, you will know when it's compacted.

If you go this route, optimal moisture content becomes important. Cohesive (slightly plastic) soils are less forgiving of moisture content than sand. If it is too dry you will not hit your density. If it is too wet, you will not hit your density and you might liquify the soil (that's the "pumping" action you see under truck tires in wet weather). If your soil starts pumping, you need to pull it out and start over.

For soils that are cohesive, the field test for moisture is simple. Pick up a handfull, squeeze it as hard as you can, see if it stays in a ball. If it falls apart, it's too dry. If you have water on your hands, it's too wet.

Now, if it stays in a ball and your hand has no visible moisture, hold your arm out to your shoulder height and drop the ball on a hard surface. If it plops, it's too wet. If it shatters into a bunch of small pieces it's a little dry. If it cracks into 2 or 3 big pieces that stay together, it's just right.

If it's a granular soil like sand or gravel, none of this applies.

In humid climates, the dirt might be OK right out of the pile.

Sounds too simple, I know, but I have been using this a long time, and it is amazingly accurate. I have had geotechs come out to my site with instrumentation and I can usually guess with in about one-half a point of where the dirt reads.

As for rock that has been hammered out, if it is good material, and if they have a crusher at the pit maybe you could have it crushed to 1"-minus. That would be good.

Too many parents make life hard for their children by trying, too
zealously, to make it easy for them. -Johann Wolfgang von Goethe, poet,
dramatist, novelist, and philosopher (1749-1832)

(post #100441, reply #14 of 30)

I don't know if the local rock would be "shale" to a geologist, but it is the local term, it is the available material, and everyone here uses it under slabs. That may mean it works, it may mean it's easy, or both. The only specially trucked in material I see here is the sand used in mount systems, and the sand and gravel used by the batch plant.


Your descriptions (and Pif's and Bagg's) are quite clear. I'll be talking to the excavator tomorrow to find out what he knows about the sand. I read on the JLC forum that the ACI specifically does not recommend 'concrete sand' which is apparently very uniform.


Lemme throw another question out there: vapor barrier placement. The spec from the designer shows concrete over insulation board over gravel layer over Moistop. Is that what y'all would do? I would rather use poly and I'd rather use it directly under the insulation, because it would seem that compacting rock on top of poly will pierce it. The Moistop would be a special order--I can do that if need be. I am aware that the presence of the insulation will mean that bleed water from the bottom of the slab may make it very slow to become ready for finishing. I have not poured concrete over insulation before, just over gravel. A guy I trust here said he would order the mud with hot water. It will probably be 40-50 degrees when we pour, and overcast. The shell will not be erected first, so the slab will be exposed.

(post #100441, reply #16 of 30)

One of the things I have learned here at Breaktime is to defer to local custom. There is usually a reason, and folks usually know their own area. So if the locals are making it work, I would not question it from my perspective.

As for concrete sand, what you read is correct. You don't get the gradation you are looking for as you do with pit-run.

As for concrete and vapor barriers, the answer is also regional. In humid climates puting the insulation and vapor barrier directly under the slab is not as much of a problem as in dry climates. In dry climates you have to put sand or crusher fines over the vapor barrier (that is, directly under the slab)to keep the slab from curling.

I would defer to your concrete guy, but technically there is no reason why you can't put down your VB, your insulation, and then concrete. From a practical perspective, however, I have found that to be a real PITA and I don't like it because the insulation tends to not lay tight to the subgrade and tends to not hold still. I think your concern is well-founded. I do not like seeing concrete going right over insulation, but I know that it can be done. As long as your concrete sub is used to this and knows how to adjust for the bleed and keep it from curling, I guess you would be OK. You might talk with him about water reducers and accelerants. If you will have freezing temps before the heat is turned on you might also look at air-entrained and a little extra PC or high-early.

The compromise I have found to work real well is to insulate the stemwalls with 2" XPS and run 2" thick XPS insulation only 2' (horizontal) around the inside of the building. If you cut a little out of the subgrade this is easy. The reason is from what I have seen of energy analysis and the MEC most of your heat loss is addressed this way and you still get a good slab.

This way you are not working on top of insulation anywhere but 2' at the edge which is pretty easy.

Too many parents make life hard for their children by trying, too
zealously, to make it easy for them. -Johann Wolfgang von Goethe, poet,
dramatist, novelist, and philosopher (1749-1832)

(post #100441, reply #17 of 30)

More great info, many thanks. FWIW, I'm the concrete guy. On a client's house I would hire one of two good subs here, but they are very busy and fairly expensive. This is my building and if I hire a sub this early I'll be scraping at the end to finish it. What I WILL have is a couple of good men on the slab pour, and I'm a pretty good finisher myself. Anyway, I'm the strategist.


Re the slab insulation, the designer spec'd 4" everywhere. The BI said he only needs 2", and mentioned the 2' perimeter that you describe also. My feeling is that I want every bit of insulating power I can get--heat is not getting cheaper. However, I am attracted to the idea of pouring most of the slab over gravel, because it will make the pour easier and maybe more successful. I have also read about a 'blotter' layer of sand over the insulation--that's a possibility too, although it beats me how that sand stays in place while I walk all over it and tie the mat.

(post #100441, reply #18 of 30)

The VB always goes against the concrete slab. reason: if you place concrete on sand or an aggregate, it will wick the moisture out of the concrete and it will crack. The bottom half of concrete layer be dryer than the top which will cure at a different time causing cracking. Beside the sand and aggregate needs to control the ground water under the slab. A VB will block this under the aggregate. As far as insulation. I know its purpose, I,m just afraid of settlement.

On backfill material , we will backill with couple feet of sand in one lift, hit with fire house, flood it good. come back day layer, add a foot of sand clay on top , compact with jumping jack. The clay layer is so you can walk and confine the sand. The purpose of the water: as the water drains through the sand it will pull the particle tighter and tighter. like at the beach.

. 2+3=7

(post #100441, reply #20 of 30)

<<The VB always goes against the concrete slab. reason: if you place concrete on sand or an aggregate, it will wick the moisture out of the concrete and it will crack. The bottom half of concrete layer be dryer than the top which will cure at a different time causing cracking.>>

That's a regional consideration. In humid climates, like yours, that is of course correct. In a hot-dry climate the opposite is true -- placing the concrete right on the VB is the most sure-fire way there is to curl the slab and crack it badly.

The ACI has done a lot of recent research into the behaviour of thin slabs on VB in different climates and released a new set of recommendations, just a few years ago I think.

I think one of the most important things I have learned here at Breaktime (pointed out to me by Gabe Martel) is that the problem in one part of the country is the solution in another part of the country.

Too many parents make life hard for their children by trying, too
zealously, to make it easy for them. -Johann Wolfgang von Goethe, poet,
dramatist, novelist, and philosopher (1749-1832)

(post #100441, reply #21 of 30)

There really doesn't seem to be much agreement about this stuff. Some say the VB should go on the warm side of the insulation, so that the dew point is underneath it. Others put the VB on the bottom to keep water out of the insulation. You're saying that sand or aggregate under the concrete will wick out the water prematurely, but others say without it the bottom will dry very slowly and all the water will have to come out the top.


You use poly or something like Moistop?

(post #100441, reply #29 of 30)

Anyone have any actual data on how good the sand compaction is using the flooding method? 


Seems like it would depend a lot on the makeup of the fill, how good the drainage is under the fill, along with a host of other variables. 


I know my parents' basement/garage was done this way about 25 years ago (back then, the fire department would come out and empty a tanker on it), and it's held up well.


Don


 

(post #100441, reply #30 of 30)

I've been thinking about this. Pouring a fire tanker into my foundation would be like filling a swimming pool. I suppose the sand would stay wet forever, but the rest of the water would have to migrate under the footing to get out. Not sure that's a good thing.

(post #100441, reply #19 of 30)

I'm more than happy to help. For the knowledge (and occasional wisdom) I have gleaned from Breaktime I think I still owe. <G>

Let's talk about insulation some more, because I think that this could turn out to be a problem for you.

My understanding of insulation is that more is better up to a point, after that point the return diminishes rapidly. So the difference between R-nada and R-10 is substantial while the difference between R-40 and R-50 would be hard to justify.

In a 6,001 degree-day climate (much of the country) prescriptive compliance to the MEC gets you a lot of bang for the buck. That's the 2' vertical 2" XPS on the outside of the stemwall and 2' horizontal perimeter.

If you cut about 3 or 4 inches off of your subgrade around the edge (or better said, leave the last lift low) you can put that XPS down, put a little sand over it, and be done. Then the slab has uniform thickness and uniform curing conditions.

Where problems happen is if the slab thickness is not uniform or if the sub-slab conditions result in an uneven draw of the water in the concrete because then your cure rate is not uniform.

As attractive as a fully insulated subgrade might be, it is a lot of work to get it right. More work than necessary, IMHO.

An option to consider if this is still a concern is R-Foil. I will no doubt elicit an avalance of criticism with that one; I've seen all of the arguments here why it is a scam and can't possibly work. Except it does. I've put it under radiant heat systems, and it speeds up the response time dramatically.

Anyway, I would compare the cost of R-foil with insulation. If you overlap and tape the R-Foil seams I don't see why you would also need the VB.

A little bit of sand (no more than an inch) over the R-foil and you are back to finishing an ordinary slab. No fighting with rigid insulation, no finishing concrete under lights in the middle of the night because you waited all day in 41 degree temps for the bleed water to leave, no racing a slab that went off sooner than you thought it was because it was batched hot and placed right on the VB or the rigid insulation, no curled slab . . . and on and on. Sounds to me like you've already been through this one before. <G>

I don't know what the weather is like where you are, but if it's cold those chemical admixtures are worth every dime.

The sand is a PITA to be sure, but it's not very thick and if you keep it slightly damp it's easy enough to work on over a VB or R-foil.

Rigid insulation is another story. Part of the sand ends up under the insulation, the insulation ends up broken and lifted up a little . . . I just wouldn't do it.

Good luck. Holler if I can add anything else.

Too many parents make life hard for their children by trying, too
zealously, to make it easy for them. -Johann Wolfgang von Goethe, poet,
dramatist, novelist, and philosopher (1749-1832)

(post #100441, reply #23 of 30)

"I don't know what the weather is like where you are, but if it's cold those chemical admixtures are worth every dime."


Let's say it's 45 degrees, a little bit of a breeze, alternating overcast and sun. I assume you're talking about an accelerator. What would you use?

(post #100441, reply #25 of 30)

I would talk with the batch plant about hot stone or hot water, however it's done there. That is probably enough. If you want to go faster, I would ask them what they have for non-chloride accelerants. Again, it's a regional thing. The only accelerant I ever used is called "triple nickel" and it is set off by sunshine. You don't want that. I'd be careful, though. If there is no danger of freezing, I think I would stay away from any more than hot stone or hot water. There is no reason to spend the money or take the risk of the slab getting away from you if there is no risk of freezing. Accelerants can backfire on you if the slab starts going off too soon.

If you are going to put the concrete right on the VB and you are not in a humid area, a mid-range water reducer might help, I don't know. Never tried it.

If you are confident of your finishing crew, and it sounds like you are, a super-plasticizer (high-range water reducer) could help a lot. A truckload of 3,000 psi - 3/4" batched at a slump 4 with a couple bags of SP thrown in at the jobsite will give you a slump 6 or better for 30 to 45 minutes. After 45 minutes at the most, the chemical wears off and it goes back to a slump 4. This makes it a lot easier to place.

What I'm getting at here is controlling the water content, because I think if you place directly on a VB or rigid insulation you will have a problem unless it is very humid and the air is still.

I used SP on almost everything. It costs a little more but it's worth it. Less water, stronger concrete, and a much, much happier crew.

Is it going to freeze at night before you have the building up and the heat on?

Too many parents make life hard for their children by trying, too
zealously, to make it easy for them. -Johann Wolfgang von Goethe, poet,
dramatist, novelist, and philosopher (1749-1832)

(post #100441, reply #26 of 30)

An afterthought;

If you do end up working right over a VB or rigid insulation, here is another idea to consider.

As Brownbagg pointed out, if there is a big difference between the moisture at the bottom of the slab and the moisture at the top of the slab there will be trouble.

In dry climates if you put the concrrete right on the VB the top dries faster than the bottom (not "cures" but "dries") and you get a curl.

In humid climates if you DON'T put the concrete right on the VB then the opposite happens, as he pointed out.

One of the ways you can address this is with a curing compound. You can't use a curing compound if you are going to stain the floor or apply a sealer that is incompatable, but check this out;

There is a curing compound called VO-COMP. (It's a VOC-Compliant acrylic based compound, hence the name.)

The VO-COMP 20 can be sprayed right on to the finished surface once you are done trowelling. It seals the the right amount of moisture in and prevents plastic shrinkage cracking. I have seen great results.

The beauty of this stuff is you can also get VO-COMP 30, which is the same thing with higher solids content. It makes a great sealer. Not too shiny, not too slippery, works great. Acrylic based, no odor.

So after you're done with the project you spray (or mop with lambswool applicator) the VO=COMP 30 on, and you have a dust-free floor. Anytime you need to freshen up the finish, you go over it with a scrub pad on a floor buffer and mop more VO-COMP on. No stripping of the old finish, none of the trouble with urethane that sticks in some places but not others.

Too many parents make life hard for their children by trying, too
zealously, to make it easy for them. -Johann Wolfgang von Goethe, poet,
dramatist, novelist, and philosopher (1749-1832)

(post #100441, reply #27 of 30)

OK.... putting this all together. 


I'm likely to put down insulation under the entire slab. I think there will be some thermal benefit and that way I get a consistent 'condition' under the slab, rather than being partly over rock and partly over XPS. I can keep it in place and in good condition since it's a small building and it's just me working on that short phase.


Since this will slow the moisture leaving the bottom, what above covering the top with poly and flooding it after we're done finishing? Leave it that way a couple of days to hopefully promote even drying and curing. I don't have access to curing compounds here and want to acid stain the slab anyway.


The plasticizer sounds like a great idea. The plant here delivers a fairly stiff mix and the driver always asks if we want water added, and we always say no. Being able to get it levelled off and bull floated quickly would be good. Then we can sit around and wait.


It could be anywhere from humid to dry here, depending on the day. Last week was warm and sunny, but we could have colder temps (hi 30s lo 40s) or foggy/misty conditions. Rain cancels. There is very little chance of a freeze. We might get some light frost overnight.

(post #100441, reply #28 of 30)

If you are going to stain the slab, then curing compounds and chloride accelerants are out of the question, for sure. The thing to consider about the poly is that it will leave a mark on the slab anywhere it touches -- just a blemish in the color that will show up in the stain. Anything touching the concrete will do this a little bit, poly clings real well and does it noticeably. When you put the stain on it will be a little different anywhere the poly touches.

That might be OK, it might even look good, depending upon what you want.

Keeping the slab wet for 3 days is a great idea. Even if you don't use poly and just mist it constantly during the day you are ahead of the game. Someone will no doubt suggest flooding the slab, depending upon your soil conditions (I think you mentioned clay) this might not work out so well in the big picture.

Good for you not adding water on site. I guess I don't have to say anything about that. <G>

Like I said, I'm a big fan of SP. It's worth every dime to me just in how much better everyone's back will feel at the end of the day.

If as cold as it's going to get is high 30's I don't think you have another thing to think about. Sounds to me like you've got it covered.

Too many parents make life hard for their children by trying, too
zealously, to make it easy for them. -Johann Wolfgang von Goethe, poet,
dramatist, novelist, and philosopher (1749-1832)

(post #100441, reply #12 of 30)

Any mix of mineral aggregate works as long as you compact it and provide drainage.

Thje trick is to compact in lifts of 6" to 8". Do not fill it all the way and then get the compactor in there. That will leave you with a nicely compacted top layer over looser fill that will eventually settle out leaving non-supporting air pockets

 

 


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(post #100441, reply #15 of 30)

<<Any mix of mineral aggregate works as long as you compact it and provide drainage.>>

Almost any. Some shales are bad news. Some clays are bad news. Some high-plasticity soils that don't necessarily have much clay in them are bad news. But besides that, of course you are correct. The trick is in getting the compaction.

Some materials are a lot easier to work with than others. For someone who does not have a lot of experience with problem soils, there are approaches that can provide more reliable results with less effort.

This is not to be taken lightly. Every year I am called to bid on repairs to failed foundations due to bad soil conditions, or more to the point, because of inadequate site prep.

It's a lot more expensive the second time.

If you have well-graded soil at optimal moisture content with good cohesive properties and low plasticity, sure, almost anything will work.

But there are a lot of other possibilities. I deal with them every day. I've been on sites where over 80% of the soil passes a #200 sieve. That stuff is rough -- it has about 2 points of latitude in the moisture curve, and if you do not get the harmonics right on the compaction equipment you can hit it until you run out of fuel and it will not compact.

But if you get the moisture and the compaction weight, frequency, and travel speed right you can hit the 98% you are looking for.

<<Thje trick is to compact in lifts of 6" to 8". Do not fill it all the way and then get the compactor in there. That will leave you with a nicely compacted top layer over looser fill that will eventually settle out leaving non-supporting air pockets>>

Good point. This cannot be over-emphasized. Proper compaction is dependent upon a reaction against the previously compacted lift. The detailed mechanical explanation in unneccessary, but for residential construction, 8 inches is the maximum thickness of each lift.

Too many parents make life hard for their children by trying, too
zealously, to make it easy for them. -Johann Wolfgang von Goethe, poet,
dramatist, novelist, and philosopher (1749-1832)

(post #100441, reply #22 of 30)

Good point. We have some shale here that is locally referred to as 'magic mud' Use it for a road base and the moisture and traffic agitation will turn it to a slick mud instead of anything resembling stone

 

 


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Oh Well,

We did the best we could...

(post #100441, reply #24 of 30)

Weird stuff, isn't it? I've ripped it out with a dozer -- it starts out as rock at one end of the site, sometimes you can only pull two shanks, by the time you push it to the other end of the site it's dirt.

Hard to explain to the GC why you have to charge him for rock removal when he doesn't see any rock in the pile, even worse, he can't use all that good looking black dirt, we need to export it and bring good stuff. They start getting suspicious and I don't blame them.

Even weirder, the PSR folks at Caterpillar have told me that shale is one of the easiest things there is on the undercarriage of a dozer because the "magic mud" actually has some lubricity to it and it doesn't tear up the track rollers like regular rock.

Too many parents make life hard for their children by trying, too
zealously, to make it easy for them. -Johann Wolfgang von Goethe, poet,
dramatist, novelist, and philosopher (1749-1832)