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packing the fill dirt

cutawooda's picture

This is the third post I have made about my slab settling..I hope you will bare with me. I went on the concrete network website and noted that they spoke specifcally about packing the fill dirt with a compactor helps in preventing cracks, and more importantly helps in stabalizing the slab form settling.

well i spoke to my concrete guy, and he says that he ran a tractor over it and ran the back tire over the outside edges. He said that that is alot better than a compactor. I find this hard to believe and was wondering what your thoughts are about it. Seems to me an "impact" would compact better than a push. I sure don't mean to push THIS issue, but my slab is settling more than it should for a 1 year old house. I have cracking bricks as well as mortar joints and that is alarming me. Doors are getting racked  and I am worried it will get worse. I would hate to think this is all coming to pass because of one missed step. I am in the construction business, but I am more of a remodeler, but I am still required to follow protocol. If I dont , I am subject to the repercussions, like everyone else. Let me know yor opinions please.

(post #94480, reply #1 of 28)

Any slab I ve ever poured was on #2 crushed stone brought up in 6 in. lifts and compacted with a vibratory plate tamper or roller.  I've even added water to maximize the compaction.  A tractor tire is not enough in my opinion, esp. with dirt.  good luck

(post #94480, reply #2 of 28)

Don't take anyone's word for the condition of a dirt pad.

It needs to be tested properly and recompacted if not up to standard.

We had our's tested when it was built and retested before starting the house because it had been some 3 months and the builder wanted to be sure it had held well. I watered it for two days every week to keep it at the optimal moisture and it tested exactly like it did when built.

They used a grader, water truck and sheepsfeet to mix and compact the dirt (caliche screenings road base that we provided from our pit) a layer at the time.

That packing of the dirt for a slab is too important to mess with.

Who will be responsible for the repairs? I think that he is, if he built the dirt pad.

(post #94480, reply #3 of 28)

I had compaction questions once and consulted a civil engineer who specialized in soils.  (He even owned and used his own drill rig -- kind of a small town guy.)  He said a jumping jack tamper is good.  A vibratory plate compactor is not sufficient to compact soil for construction.  I would imagine driving a tractor over the soil is even worse.  And of course it matters what type of soil it is.  Some soils are not compactable and must be replaced with different material.

You're not going to settle any dispute with him with internet feedback.  If it means trying to recover damages for improper work, you'll need a licensed soils engineer.  He can order testing of the actual soil from a certified lab to find out (and prove) whether or not the soil was compacted properly for the job.  It really won't matter what method the contractor used (only to prove that his method could not have worked, if that is the case), only that his results were unsatisfactory to support the load of the structure he constructed.

(post #94480, reply #5 of 28)

my soil hee is clay..clay clay clay..So hard you cannot get a post auger in the ground more than 2 inches, ( I am not lying). then you add water and wait a day,  you get another eight inches....It is amzing stuff. and when it gets wet....dont get me started!  what a mess!!!

code here is that you must dig into 6 inches of undisturbed soil for you footings...we went about 12 to 14. I told the concrete guy to beef up everything because I was new to the clay we were building on, ( I am from Houston). Currently I am in North central Texas and clay is the local terra firma.  So, he went 24 inch footings and #5 rebar in them. I was doing another job the few days befor the pour so I was not aware that he didnt compact it. I later found out from his lead guy, (ex-lead guy) that he owns a compactor but does not use it.  (huh?) This whole scenario may not be his fault at all. It might be the soil.  I hate this clay!  maybe too wet..maybe too dry...maybe drying too fast in one spot.  I back filled against the hous to try and slope it away but I just read that you should use CLAY as your fill, not a sandy soil...whoops!  I read that drying clay could settle so you need to water it to keep it moderate. The cracks began happening in a drout to I used a soaker hose...WRONG! Turns out you should shed water AWAY fromthe house....I dont freakin know anymore!! I always hated gutters because they rob you foundation of moisture and that they rot facsia board...turns out I need them..

every day I am a wiser man...this day just happens to have 8760 hours.

(post #94480, reply #4 of 28)

Hey Cutawooda,

Sorry to hear about your trouble.

Most of the responses so far range from pretty close to spot on, with some allowance for regional variations. <G>

You've certainly taught me some things I didn't know, I hope this is my chance to return the favor.

Here's the straight scoop on dirt.

All soil can be compacted. With the exception of expansive clay and soils with too much organic matter, most soils are suitable for residential construction if they are handled properly. Some really nasty soil can be made to work if your dirt dude knows what he's doing. If your concrete guy also knows what he's doing, you can build successfully in some really weird places. I have built successfully on soil that you can't get a 4WD truck through when it's dry, and 70% of it passes a #200 sieve.

As you know, site prep is crucial, especially so with backfill and compaction. What we are shooting for is "bearing competence", and when backfill is involved, that means compaction.

Compaction is expressed as a percentage of maximum density at optimal moisture content. 95% to 98% is usually what we're shooting for. It's not a linear curve, so 88% or 90% is really not too good. Going for 100% isn't a very good idea, either. If you overcompact the soil you shatter it, which means you have to rip it up, reprocess it to optimal moisture, and start over again.

Moisture content is key; not enough moisture and you will have a very tough time hitting your compaction values (like you probably won't), too much moisture and you will get mush that will be useless to build on. If your soil is too wet, you need to rip it up and let it dry or carefully blend in dry soil and reprocess it to a homogenous state before you compact it again.

For some soils, that optimal moisture curve is pretty narrow.

While it is true that you'll need a geotechnical specialist to tell you with scientific certainty whether or not this objective has been achieved (especially after the fact), it is also true that an experienced and competent excavation contractor can hit this by feel during the construction process.

(Note to naysayers -- I do this all day. I also have geotechnical engineers check on it, and we do get it right. Kinda like the way you guys can work to a 128th of an inch in a cabinet shop. <g>)

Regarding the difference between jumping jacks, vibrating plates, sheepsfoot rollers, and smooth drum rollers; none of these are "better" or "worse" -- they are made for different types of soil, different kinds of conditions, and different applications.

A jumping jack is made for cohesive soils. This means soils that exhibit some plasticity. A vibrating plate is made for granular soils, like sand or soils with high sand content and not much clay.

Plasticity is an important concept. If you try to use a vibratory plate on heavy, cohesive soils, the previous poster is then correct -- not much happens in the way of compaction. Thes soils need to be pounded, and in some cases "kneaded" -- hence the sheepsfoot drum.

If you have granular soils, the vibratory plate "rattles" them into place. The jumping jack just makes a mess, especially if the soil is not captured or contained.

A vibratory drum is useful on well-graded soils like base course.

All of these tools have some depth to which they work. As you might expect, bigger tools hit harder. But in general, the rule of thumb for residential construction is to work in 8" lifts that are typically placed fairly level to avoid shear planes. In other words, you fill 8", compact it, wet it slightly if need be, add 8 more inches, compact it, and so on.

"Wheel-rolling" as you described certainly has its place in dirt work, it's very common to see dirt wheel-rolled into place and then properly compacted with whatever tool is appropriate. It saves a lot of time and money, and there is nothing wrong with it.

But I seriously doubt if you could hit 95% to 98% compaction by wheel-rolling alone. There might be some kind of dirt at some moisture content under some rubber tire in some very short lift that this might work, but I haven't seen it yet.

It could also be that your dirt work actually did come out well enough, and that the problem is elsewhere. I would in no way whatsoever blame another person's work, especially from this perspective (the computer screen), without hard proof. Cracks in concrete and mortar can be caused by many things besides soil settling, as can racked doors.

If you think you have a dirt problem (and it sounds like you might), this would be a really good time to consult a geotechnical engineer. They really are not that expensive, and then you have some hard data to work with that is not subjective. I haven't told you anything at all about your specific problem, nor can anyone else without a site visit and some testing.

I know this was a little long, but now you have the information you need to pursue this matter from a more informed position.

Very best of luck to you.


(post #94480, reply #6 of 28)

very informative Dave...thank you very much.

Now the 100,000 dollar question: If it is my soil..regardless of whose fault it is...can it be fixed and locked so this wont continue to happen.  On a side note.....I debated on hiring an engineer to design my slab but thought it would be a waste of and learn

(post #94480, reply #7 of 28)

Are you anywhere around Denton or in that stretch from Denton to Wichita Falls? That clay is legendary in its ugliness.

I believe that some of the clays in the north central Texas area are expansive, I know that in drought conditions you guys get cracks that will swallow small pets and look like they should have magma coming out of them.

If you are in the kind of clay that I think you are describing, then I have to say that I don't know what the answer is, and it would be best to find a geotech engineer with local experience. This is a regional problem that is best handled with regional expertise. I've also had builders tell me about the soaker hose that keeps the soil "pumped up" so the house doesn't move. It doesn't sound good to me, either.

The good news for you is that all we have really been discussing is bearing competence for what amounts to fairly light loads, and there is always a way to achieve that.

It sounds like you did a good job of specifying some measures that could be helpful in the long run, particularly the #5 bar.

If it turns out that this really is a soils problem, you might look into helical piers. It's a relatively inexpensive and highly reliable repair, but you really would need an engineer. A.B. Chance has one version, I'll see if I can find a web site. These can work in truly terrible soil conditions. I know of engineers signing off on them for houses built on uncompacted mine tailings piles (the neighbors had an 18" crack in the footing -- OK, I guess 18" isn't really a crack anymore <g>), and the owners of every house in the subdivision were suing the builder. Apparently the helical piers were plenty good enough of a fix.

They also were specified for a house (slab construction) that sunk so much it sheared off the shower drain line. Now that's a settling problem.

It seems that the fill was probably not very well compacted in the first place, then a copper hydronic heat line leaked about 30,000 to 40,000 gallons of water under the slab. The landlady was both absent and somewhat inattentive to her water bill, with expensive results.

Hang in there, get some good local recommendations for a structural engineer and a geotechnical engineer (it's worth shopping around), and I bet this comes out fine.


(post #94480, reply #21 of 28)

yes Weeeechita Falls

(post #94480, reply #22 of 28)

Are there any rules of thumb for using a plate compactor vs a jumping jack - short of hiring a soils engineer.  (the only soils engineer in our region told me that the soils were completely stabilized on a house I bought, only they were still sliding and were tearing apart the foundation, so I would rather not use that guy)  

I have both a plate compactor and a jumping jack.  I was told by a concrete guy that plate compactors were the only thing to use as jumping jacks tore the soil up rather than compacting it down.  In my younger days, I worked as a laborer on an earth filled dam and the only thing we used on both the clay and cover dirt was a jumping jack. 

My steel 30'x40' shop building will have a 5" reinforced slab with 2' depth frost requirement in our area.  I had figured on just using the plate compactor to compact a few layers of gravel, probably about 6" total gravel depth, and then just dig the trench for the footings trying to do minimal disturbance to the underlying ground. 

The ground drains fairly well and even in our few rainy months never causes a problem when driving over it with my pickup.  In the summer, it gets fairly hard but my pneumatic tired forklift can bury itself pretty quickly nonetheless. 

So, any reason not to just tamp the ground and then the gravel fairly well with the plate compactor?  Any reason to use the jumping jack.  Any reason to use the jumping jack in the ditches before I pour the footers? 

This is along the Columbia River about 75 miles east of Portland, OR.  It is great soil for raising poison oak, scrub oak, pines, and ticks - and holds enough small water pools in the spring for a great crop of mosquitos.... 


(post #94480, reply #23 of 28)

plate tamps are only use for making something look pretty. They do not have enough force to compact over two inchs, clay forget it. Pure sand and making asphalt look good. They be ok for gravel because gravel is uncompactable. For deep down compaction jumping jack the only way.

For footing, jumping jack about four passes then plate tamp to smooth bottom and grade.

Edited 2/7/2004 10:55:26 AM ET by BROWNBAGG

(post #94480, reply #24 of 28)

Brownbagg specializes in soils.  The advice he gave matches what a licensed P.E. soils engineer told me a few years ago, that a plate compactor might be fine for touching up the apperance, but will not produce the compaction necessary for sturctural support or to prevent settling.

A plate compactor is for settling pavers into their bedding, not for structural compaction.

If you are using crushed stone (rather than gravel) you can expect about 25 to 30% compaction, so to get 6" finished you'll need to place about 8", which would be better done in two lifts, compacted in between.  I don't know the compaction on gravel because it's not as good a base and I haven't used it.

(post #94480, reply #25 of 28)


Here's an unscientific but firly reliable three-part way to get a sense of what you are dealing with;

Scrape the organics off the top of the site.

Get a few glass jars with big lids, like mayonaise jars. How many depends upon whether you think you have a lot of variation in the site conditions. I've seen sites that were uniform, I've seen sites that were cobble on one side and clay on the other.

From a few different area on the site, get enough dirt to half-fill each jar. Add water to the 3/4 level, shake well, and let it settle.

You will get at least three different layers, maybe more.

The bottom layer will go from gravel at the bottom up to fine sand.

This will settle out in less than a minute.

Then the silt, then the clay, and the organics on the top.

That part will settle out over the next day.

The less granular the soil, the more it will need to be rammed (jumping jack) or kneaded (sheepsfoot)

Now take a dry handfull of a fairly representative sample. Wet it slowly with clean water and work it until it is just moist enough to hold together in a ball when you squeeze it hard, but it won't leave any water on your hand. Now hold your arm out and drop it on a hard surface from shoulder height.

If the ball you have formed holds together, it's pretty cohesive. If it cracks and falls apart, it's not quite so cohesive. If it splats you got it too wet.

Next, take a small handful about half the size of a golf ball. Wet it enough that it holds together well and try to roll it between your hands to form a thin ribbon about the thickness of an ordinary pencil.

If it keeps cracking and breaking off it's not very plastic. If you can get a roll about as big around as a pencil and about as long, it's more plastic. If the roll of dirt starts getting really thin and long before it breaks it may be a problem, it may not.

In a perfect world you will have soil that is about 70% to 80% aggregate, the balance being silt and/or non-expansive clay with no organics, fairly good cohesion and low plasticity.

Bear in mind that you have a real lot of latitude here. We really can build on almost anything if we're careful.

The result of these three tests will tell you a lot about your compaction requirements.

The more cohesive or plastic soils will require low-frequency compaction. This means a jumping jack and very, very careful attention to your moisture content, or a small sheepsfoot roller if it's really got a lot of silt and clay. If the moisture content of the soil is too high, it is so easy to liquefy these kinds of soils (you'll know you screwed up if the ground around the jumping jack also starts jumping). This is known as "pumping." You will also see evidence of pumping if a heavy truck drives over the ground and the dirt you compacted gives a little.If this happens you need to start over. Don't let it happen.

When the jumping jack on the dirt sounds like you are being chased around by a Jolly Green Giant (the H-3, not the vegetable guy), you're about there.

More granular soils will benefit from higher-frequency compaction. This means a plate or a vibratory drum. You can rent big ones, they are worth it.

Here's a finer point on the discussion of plates. I would not disagree with Johnnie (Brownbagg) in any way, the man knows his dirt. The vibratory plates that two guys can unload from a truch are exactly as he says -- worthless.

However, you can get reversible plates (self-propelled in both directions, about 800 pounds or more) that really do work well in short (6" to 8") lifts if the soil is granular and the moisture content is right. They will save you a real lot of time. I use a mixture of base course and crusher fines processed to optimal moisture for structural fill when we really need to fill a lot of volume, and a reversible plate followed by a jumping jack will turn this stuff into something that resembles metamorphic rock.

As he points out, unless you are building on almost pure sand or pit-run gravel, it's still a good idea to hit it with a jumping jack when you are done.

OK, one last thing -- do not mistake hardness for compaction. While most soils do get hard when compacted, there is no necessary relationship. You can have hard soils that are not compacted and (in unusual circumstances) compacted soils that are not hard.

Keep the moisture content right, work in 6" to 8" lifts,use compaction equipment big enopugh for the job, be patient, don't overcompact, and you'll be in good shape.


(post #94480, reply #26 of 28)

take that jar fill with gravel, see all the ait void, fill those with sand, you still got small air void, fill those with silt/clay. That is the best soil right there. But nobody has perfect soil. Gravel will not compact because of no fines, it just rolls around, but it real good for removing water. Clays pump to easy, They do not remove water, they swell. so a sand /clay mixture is good. we build here on pure sand. when the water run through the sand it will pull the fines with it and lock the sand aggregate together.

On equipment weight over vibration. The vibration breaks it loose and the weight pushes it back together. Vibration alone, nothing but smooth top. Force, jumping jack, no pretty but it compacts.

(post #94480, reply #27 of 28)

Cool. Thanks, Johnnie.


(post #94480, reply #20 of 28)

It's probably time for an engineer.


(post #94480, reply #9 of 28)


Re your post about compaction, very interesting stuff.  I know that I myself have been in situations where a vibrating plate seemed to do nothing, and others where a jumping jack just seemed to pump the clay.  Now, I have at least some idea why.

I have a few questions but will first tell you about our local soil condition here in Central NC.  Generally, we have clay with several " of topsoil on top of that.   We always excavate away any organic brown soil before placing concrete.  Clay here, generally comes in several different colors, which is very significant.  Red is good, redish brown/grey marginal, and light grey to white always requires a soils engineer.  South and east of here, the soil is more like loam - brownish, and more granular, but if you dig down a foot or 2 you can usually find clay, or a sandy clay.

When you say 95% compaction - 95% compared to what?   I believe I have heard a dirt guy I know talk about > 100% compaction.   How does virgin (never excavated) soil fit into this?  Is it considered 100% compacted?  I have seen soil engineers use only a probe rod to evaluate soils - is this an adequate method?  I've also had them show me about the feel of the rod - I consider this a basic building skill.  I have also seen a small (~6" long) tool, kinda pencil shaped with kind of a spring loaded dull point and some kind of scale that indicates soil compaction - but this tool, seems to me to only check maybe the top 1" of the soil.  I gotta think that on interstate highways, etc, they use more sophisticated methods...  and I would suspect that every lift must be checked...

And, re soil moisture content, I guess that is an acquired art of knowing when it is right.... to get proper compaction.   I do know that if our footers get rained in too much before you pour, you will be $crewed.   I guess you touched on this when you talked about plasticity.

Sorry I have nothing to offer the original poster, but I have heard nightmare stories about those unstable soils in Texas, post tensioned slabs, etc.  Man, I'm glad we don't have to deal with that stuff!


(post #94480, reply #10 of 28)

A tractor will not do it, not enough weight. He was just too cheap to get a compactor. A plate tamp is good for making asphalt pretty and smooth, thats about it. It works marginal on sand.

Your question 95 % of what. That 95 % of ASTM D 1557 or 100% of ASTM D 698. Basically I take a sample of your dirt and beat the hell out of it and get a known weight of maximum compaction. Then I compare your slab to my results, you need 95%

(post #94480, reply #11 of 28)

So, you are saying that compaction is measured in terms of the weight of a given volume of soil.  So, for easy numbers, if 1 cu ft of fully compacted soil weighs 100#s then a cubic foot of 95% compacted soil weighs 95#s.  Is that correct?



(post #94480, reply #14 of 28)

yes thats correct. sand, pure, weight about 100 lbs compacted, sand clays 112- 125 clays about 130. Now this is in my area.

(post #94480, reply #13 of 28)


Dave has supplied some excellent information and I will hopefully shed a little light on your question about the 100% compaction and optimum moiture content.

About 10 years ago I worked at Toronto Pearson Airport testing soils for their then 2 new taxi ways and new runway.  Basically several "representable" samples were taken from the supply quarries, placed and compacted in controlled lifts in a test cylinder with water added.  From this they determine the material's (weight of material/volume of cylinder) density and the percentage of water required to acheive this maximum density ie. 100%.  For crushed granular material its optimum moiture is around 6-7% and on highways the goal is 98% of this maximum density. For native soil about 95% (I can't remember optimal moisture).  The lifts range from 12" for Granular 'B' (natural sand/stone mix) down to 6" for crushed granular.

3 important things to remember when placing and compacting:

1. Never add too much water because either it will have to be removed and replaced or let sit until it dries out, which could be weeks if it is clay.

2. When it comes to granular material never overcompact.  Once you reach its optimum density if you keep compacting you could start crushing the stone and from there on in your density will start decreasing.

3. Don't try to place too thick a lift because you will never be able to compact the lower portion.

For clay soils the basic test was to do a roller test.  On our runway job after the subbase (clay) was graded we would have a scrapper drive over it and watch the soil under the tires.  A good base would have no give, a poor base would sink under the weight.

As for testing densities we used Nuclear Densometers. It has a rod with a radioactive material at its tip sunk into the material of choice and the base of the unit would then receive its readings from the rod then calculate the density. Not rocket science but the devices can cost $4000+ used. And yes every lift must be checked. 

Just remember as mentioned, every soil has its own characteristics and every source or location must be treated as different.

Hope this answers a few of your questions.

Freddy S


(post #94480, reply #17 of 28)

They tested the caliche road base, that has plenty of red clay, that we presented to them for the 66' by 100' dirt pad, in the lab first.

Then they told the dirt contractor what he needed to do and that the moisture should be 13%. He did a great job, removing topsoil first, hauling, wetting and packing the material every 6" that he leveled with a laser. He said that if he did it right, it would be fine. He has been doing it over ten years. The tests showed 98% compaction and 12.9% moisture, excellent.

Then they came with the nuclear densimeter to test in several places at a 6" depth. That instrument has to be kept locked when not in use and every user certified, because of the radioactive material in it.

I kept wetting it with a sprinkler to a certain superficial moisture content, to keep it from drying, two days a week, as per their instructions. They said too much or not enough would let the pad start to deteriorate, so to consider any other moisture or extra drying windy days on how much to apply.

Thanks for all the information on the how and why. Now it makes more sense why they had to do all that. It is not cheap, but it seems a reasonable insurance, when a house is built over it.

Edited 2/4/2004 1:57:01 PM ET by Ruby

(post #94480, reply #15 of 28)

You guys are smart!  Just a quick thought on your compaction percentage question.  The lab tests some of the guys mentioned will give a "maximum dry density" when compacted with a certain amount of "effort".  The "effort" is based on weight of the device used to compact the soil and the height from which it is dropped onto the soil.  As you would expect, bigger hammer falling from greater height yields more "effort" and thus a greater "maximum dry density". 

All densities measured in the field would be compared to the maximum dry density measured for that soil type in the lab and are expressed as a percentage of the max dry density.  It is possible to get relative compactions in excess of 100% if the compaction methods in the field produce more "effort" than the laboratory compaction equipment.

(post #94480, reply #16 of 28)

Now that is an interesting bit of info that I never knew -- thanks.

What else can you tell us about dry density versus density at optimal moisture?

I think you are on to something very interesting about when water gets under a house.


(post #94480, reply #18 of 28)

"What else can you tell us about dry density versus density at optimal moisture?

I think you are on to something very interesting about when water gets under a house. "

Well, just so you know, I'm not a geotechnical engineer- just a civil engineer with some geotech courses under my belt and now doing a lot of grading and stormwater design.  So I'm not going to gaurantee everything here. 

I can tell you from seeing quite a few laboratory compaction curves that when plotting dry density against moisture content, the relationship is an upside down parabola.  That is, the dry density increases up to a point and then begins to drop off.  The point of maximum dry density is the "optimum moisture content" and that's what the earthworks contractor would shoot for to get the best compaction.  The nuclear density meter one of the others mentioned reports both moisture content and dry density (it has to measure moisture content because dry density is the density of the soil after the water is driven off). 

As far as what happens when water infiltrates into the soil beneath a house would probably depend on the type of soil.  A non-cohesive soil would (I think) not swell so you woudn't lose the compaction but it would pick up water and lose some strength.  A cohesive soil (i.e., clayey) would swell and so would lose the compaction and would also lose some strength.  That's why clay is generally bad news for buildings.  The thing is, if this guy's slab was on an expansive soil (clayey) and the soil was subjected to a lot of water, the soil would expand, then probably slowly release the water and would remain in an expanded form (i.e., not as compacted as it was when installed).  I don't think that would yield settlement problems- just heave problems, right?

One thing to consider is that the original post mentioned settlement.  That may not have anything to do with water getting under the house.  If the soil was not compacted well to begin with (and driving over it with a tractor generally doesn't do a good job, especially for structural applications), the settlement may just be due to the increased load of the slab.  Another thing that could cause settlement is washing out of the underlying soil.  I wonder if the site has groundwater issues that could cause that to happen.

Edit: One more thing.  I just saw that the house was backfilled with sand.  This may be reaching a little but could the clay under the slab be intermingling with the sand backfill?  I know for a fact that soils with radically different grain size distributions will sink into each other.  If this is happening, the settlement may be due to loss of clay from underneath the slab.  This is just a wild [JOBSITE WORD] guess really.

Edited 2/4/2004 1:26:06 PM ET by stonebm

(post #94480, reply #19 of 28)

You said:

"A cohesive soil (i.e., clayey) would swell and so would lose the compaction and would also lose some strength.  That's why clay is generally bad news for buildings. "

I have only practical experience to rely on, and not the engineering education you do, but where I live we like nice red clay, and I don't worry about a foundation on undisturbed soil of that type provided it is not too wet.  I have heard about expansive clay in TX, but I don't think that applies to where I live, or at least not with red clay.  We do have some lighter color clay that is bad news, in which case a soils engineer would generally be used.  Really though, I can tell a lot with just my probe rod.


(post #94480, reply #12 of 28)

"...he ran a tractor over it and ran the back tire over the outside edges."

Rear tractor tires are made for traction and floatation. They typically only have between 8 and 15 PSI in them. So IMHO using a tractor for compaction is ludicrous.

You know you're getting old when you stoop to tie your shoes and wonder what else you can do while you're down there.

While tractor tires are (post #94480, reply #28 of 28)

While tractor tires are typically inflated to fairly low pressures (My manual recommends 16 PSI for rear and 24 for front) the heavy lug treads are only about 1/4 of the area of the footprint, so the net effect is about 60 psi. So it acts a bit like a way too light sheepsfoot roller.

You can likely get the same effect by driving your pickup back and forth.

You can also fill tractor tires with calcium chloride solution to give them more weight.  However this doesn't translate into greater pressure, but rather a flatter foot foot print.  Whatever the PSI in the tire is going to be the average pressure on the ground. More weight will give you more square inches on the ground.