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dumb question about soil testing for engineered foundation

Posted by breakingground08 (My Page) on
Tue, Feb 26, 08 at 20:06

hello everyone,
So the only criteria for building in the city of selma, texas is to have the foundation stamped by an engineer. The engineer we found charges .12 cents a square foot but charges almost $200 for the soil testing because he contracts it out. My local nursery does free soil testing. Do you think its the same thing or are they looking for something completely differant? He asked me if I had any info on the soil, I told him no and he then asked me to dig about one foot into the group and bring him about a quart of dirt so he can send it out. The impression that I got from the conversation is that he simply needs to know what kind of soil we are building on...but does any one know if there is more scientific mumbo jumbo to it then that???


Follow-Up Postings:

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RE: dumb question about soil testing for engineered foundation

First off there is no dumb question in building world.

The soils testing is for compaction requirements to see if the base or grade will meet those requirements.


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RE: dumb question about soil testing for engineered foundation

i think soil testing means something completely different in the world of horticulture. i have over 18 holes on my property that are approx 16 feet deep from our engineers' soil test. i don't think there's a plant guy in the universe that is going to that extreme! GL!


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RE: dumb question about soil testing for engineered foundation

Wow, you guys are right. Im going to call him tomorrow and just have him take care of it. I did some research of old posts on here and found that other people in texas were paying about $600+ so $200 just doesnt sound that bad at all. Although, all he is having us do is dig a hole 12 inches deep into the ground and gather about a quart of dirt from there on so i dont know how extensive his test will be either.


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RE: dumb question about soil testing for engineered foundation

The soils compaction testing is generally onsite. The hole size you mention is standard, but the test is performed in the dug hole adding a sand to see how long it takes to perc after filled with water. Its generally a timed test to see if the pad or grade meets compaction. That's how it's done in our area, they dont send out samples that im aware of.

It could be the engineer is familiar with the soils in your area and is testing to confirm, but i've never heard of that method.


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RE: dumb question about soil testing for engineered foundation

I am not an authority by any means, but I can't see what a disturbed scoop of dirt would reveal once sent to a lab.

The on-site perc test that Sierraeast describes would tell far more, I would think, than some jumble of dirt.

Could all this testing be the result of engineering trade groups lobbying for laws requiring mandatory testing that provides income for the members of their profession... with rarely any benefit for the home owner?

Just asking....


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RE: dumb question about soil testing for engineered foundation

I'm building near Austin so I think we're probably in the same geologic zone so maybe I can shed some light.

We decided to have an engineering firm design our foundation even though this not required in our community because I knew that here in central Texas (east of the hill country) we have expansive clay soils that can wreck havoc on improperly designed foundations. The engineering firm we hired required a GEOTECHNICAL soil study upon which to base their design.

The geotechnical soil study included drilling a number "core samples" fifteen feet deep and then performing a whole series of laboratory tests on those samples. They tested to determine the "plasticity index" of the soil at various levels, the natural moisture level of the soil at various levels, the maximum amount of liquid the soil could absorb, etc. The whole purpose of all the tests was to determine just how expansive the soil is, how much vertical and/or horizontal movement is likely to occur as it drys out during droughts or becomes saturated with water after heavy rains, and basically just how deep our foundation piers needed to be to ensure that the soil was strong enough to support the structure we proposed to put on it.

As you can tell, all of this is a very different kind of soil testing than that done by a nursery. A nursery's tests are designed to find out what kinds of nutrients are available for plants to grow in. While soils can be so clayey that plants can't get air, most plants will grow just fine in expansive clay soils that will wreck a foundation! Mostly the nursery will run some chemical tests that are very similar to the ones you probably did in high school chemistry classes and then tell you what kinds of fertilizers to add or whether you need to mix in some sand. I'm sure you recall using litmus paper to determine if something was acidic or basic and maybe doing some other tests where you saw color changes if certain minerals such as calcium or iron were present.

Geotechnical soil testing is also different from "perc testing" and I think that is what Sierraeast is describing. Perc tests determine how fast water will "perculate" thru soil and they are only needed if you have to put in a septic system. Liquid sewer wastes have to perculate thru soil in order for the septic system to work. It is the movement of the waste thru soil where bacteria can eat up the bad stuff that cleanses the waste out of the water. The lower the perc rate, the larger your septic absorbtion field has to be and if the perc rate is too low, you can't put in a septic system at all. So, if you were building in the hill country where you might have basically solid rock with a half inch layer of soil over it, you might not pass a perc test at all. But, if you could hook into a city sewer line, you could build a house there without much worry about foundation problems because the rock would not expand/contract with the seasons.

Perc testing is very low tech and cheap. Dig a hole, pour some water in, time how long it takes for the water level to drop one inch. Run some calculations and you're done.

Nursery testing is also easy and pretty inexpensive tho it does require a few chemicals. You can buy soil test kits to do it yourself if you want to.

Geotechnical soil testing however requires some heavy equipment and some pretty high tech knowledge. (Frankly, I didn't understand 3/4th of the information that was in the 20 page geotech report I got even though I used to teach chemistry and physics!) Geotech soil testing is not cheap. But, when you live in an area with expansive soils, it is a whole lot cheaper than having major foundation issues a couple of years down the road!

Please be careful that the person you are hiring is a professional geotechnical engineer who knows what he is doing. I'm sorry but if he is asking you to bring in a quart of dirt from the top 12 inches of your soil, I rather doubt he does.

Getting a good foundation is, IMHO, the single most important thing you have to do when building a home. In other areas of the country where the soils are more stable, a geotech soil study might not be necessary but around here, this is not a place to skimp!

Good luck.


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RE: dumb question about soil testing for engineered foundation

If you get your foundation engineered for $0.12/sf and a geotechnical report for $200 in TX, you are getting a real steal!

I shopped around in Dallas for a soil analysis (3 holes, 30ft deep). It was almost impossible to find a geotechnical engineer interested to do that for a _single_ single family home (the market here is geared towards huge developments, both commercial and residential). Quotes ran from $1200 to $3000.

Are you sure the $200 are for a _geotechnical_ soil analysis (for a foundation), not a horticultural soil analysis (for landscaping)?

Also, $0.12/sf for the structural engineer sounds way, way cheap. $360 for 3000sf ?!? What a deal !!!


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RE: dumb question about soil testing for engineered foundation

"The only stupid question is one you dont ask."

Geotechnical tests are definitely different than horticultural ones. Based on the price and the amount of soil he wants, I would hazard a guess that your engineer plans on having the soil hydrometer tested - which would tell him how much sand/clay/silt is in the soil. That is a good thing to know.

However, having you sample it, and only going to 12 inches is not so good. They should have a couple of hand augers done at least and then decide on testing based on what they find.

That said if this is an area that he is very familiar with he could be pretty sure of what is there already. A well-established geotechnical engineer gets to know their area pretty well, but most of the ones I know would still want to see whats there anyway.


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RE: dumb question about soil testing for engineered foundation

If legal, I would be tempted to put the $1200 to $3000 into a thicker slab with bigger footings in the first place... and skip the visit from the engineer all together.


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RE: dumb question about soil testing for engineered foundation

The perc test i referred to is similar to a septic perc test only it is for compaction of the pad pre foundation.

It sounds like bevangel is on track to what is going on w/ the op's project


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RE: dumb question about soil testing for engineered foundation

Sierra, I've never heard of the test you are refering to. Could you mean a sand cone? There is no water involved, but you do dig a hole and pour sand into it. Now-a-days most people use a nuke gage rather than a sand cone (thank God). The only people we still do cones for is the Army Corps of Engineers (I asked them why once and was told it was because they developed the sand cone method!)


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sand cone

I might be off on how it's done these days, but back when i was involved in new work, it was the sand cone method but water was used as a timed materials due to the natural sand beds here in the desert. It's only for compactions of the foundation pad (pre slab) that is generally built up to a required elevation. No special fills are used, just the natural sand base involving the pad, as well as the upper 12" or so of native soil. In most areas in our valley, (mojave desert), produced from dried up washes and lake beds, not too far below the surface is a material known as coleche which is almost concrete hardness and makes for mighty difficult digging going deeper than the upper layer of sand, which is mainly broken down granite and sands from the sierras.The closer to the sierra foothills, the deeper the coleche and the looser the native soils, making trenching difficult from constant cave-ins. The coleche isn't a concern and would make an awesome base if it were at the surface, it's the upper and built up soils that are in need of compaction to meet requirements for grade or a pad. On builds close to dry lake beds, compaction comes easy.


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how?

Sierra, Im confused as to how you could determine density by pouring water into a hole. The sand in a sand cone is not timed, it is weighed before and after in order to determine the volume of the hole. Im intrigued by your description. Is it an ASTM method? Do still do a proctor?


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im the confused one!

chiroptera, i called a buddy i worked w/ back in those days, (he's retired, im sorta retired), and he stated i was wrong in what i saw, it wasn't water but sand only the lot prep engineesr was doing. I was reflecting back to a built up pad next to a house we were framing and saw this action where i thought water was being used. Appearently it was just the sand cone method. A small hole was bored and i saw something that i thought was water. Im not familiar w/ the process and didn't think much of it only to be explained that it is a test for compaction. Sorry for the confusion. I should have simply walked over and saw what was going on at that time, but at that time really didn't have much of an interest, just assumed it was some sort of water perc test similar to septic. Dont really know how it's done these days concerning sand cones, but the lot prepers have it relitivaley easy obtaining compaction here in the desert valley, harder as you get closer to the hills, but im told most strive for 90% compaction which is pretty solid, ( why pour a slab)! Build it on dirt w/ 90% although it makes it hard to clean the floor! :-)


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RE: dumb question about soil testing for engineered foundation

Wow, thanks so much for all the input. I asked the city inspector about the perc test and he said it wasnt needed since we arent needing a septic system. I called around to differant engineers but they all seem to be in the same price range .10 to .30 a sqaure foot. I chose the engineer we are going with because he is familiar with the city of selma and has done some work for other people in my neighborhood. We arent dealing with rock (luckily). My future neighboor is a general contractor who built his home and the neighbor's to our other side and he said that around here it is general practice to just figure out what kind of soil we are building on...nothing to technical. Hmm...??? I dont know it seems like we do things differant here in texas or maybe just in this small city. Doesnt seem like its a good thing though... On the upside, pricing is great around here. I have gotten some pretty in depth quotes from differant builders ranging from $118,000 to $148,000 for about a 2300 sq ft home. Our lot costs $21,000 (1/2 acre). Its not anything to impressing but its our first home and our dream home. The only upgrades are travertine tile through out the house and granite counter tops, those were some of my most important wants (not needs by any means).


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RE: dumb question about soil testing for engineered foundation

If an engineered foundation is required then the soil conditions in the area are obviously a problem so don't try to second guess the engineer.


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