Design of Residential Slab-on-Ground Foundation without soil report 1

[oengineer]

I am designing a Residential Slab-on-Ground Foundation without a soil report. It is located in Goliad,TX. Since I don't have a soil report,I am assuming 1500 psf for my allowable foundation pressure. This assumption is based on Table 1804.2 from IBC.

I am using the Wire Reinforcement Institute - Design of Slab-on-Ground Foundations guides.

One of the issues I am having is determining a usable P.I. value.

If any one has some suggestions/comments of other things to consider, please let me know.

 

[oldestguy]

Don't dismiss the post by Ron. I'd love to have the job of expert witness when your case comes to court, for the owner of course. Quote all the references you can find, but that didn't stop all those cracks.

 

[oengineer]

I spoke with the client and he said that he would look into obtaining some information regarding the soil for his property. The client said that the top 1 ft layer is sand, then the next 1 ft to 2 ft of layer is sand and clay. He said from then on you have caliche soil. This was based on a pond he created a while back on the property. He was going to send me an email with the soil information. I will have something regarding the soil that I can document.

 

[txeng91]

It's not really going to help unless you know the plasticity index of the soils. Also you need more data then the top 3', you could have a bad layer anywhere in the top 5' or 10' and it can greatly influence the effective PI of the subgrade. Typically residentional borings go down to about 20' and the geotechnical engineer will provide multiple remove and replace options with corresponding effective PI and PVR values.

 

[oengineer]

I have been informed by the governing county that they do not require a building permit nor do they require a soil report. I have also informed the client, in writing, of the benefits of obtaining a soil report for their residential foundation and the problems that my arise without having one.

 

[oengineer]

The client informed me in writing that there is about one foot of sand and then about 6-7 feet of clay with some caliche mix then it turns to mainly caliche.

 

[TXEng-USA]

It sounds like you have done what you can as far as covering yourself, however lets say that for some reason the owner at a later date gets upset with you or turns around and sells it to another owner that does not care what the previous owner told you. If brought into court, as others have stated it would very easy to establish that the standard of care is to obtain a site specific geotechnical report.

If the owner is so hell bent on not getting a geotechnical report then why does he want an "engineered" foundation. I use "engineered" is quotations because it is my belief that in order to be called engineered, it would have to based off of a site-specific report.

 

[oengineer]

I guess my question there would be that if I have recommended that he obtains a soil report (in writing), checked with the governing county to see if they require one (which they do not, I mean they don't even require building permits), informed him of potential issues, state all my assumptions on my drawings, put a note like: "foundation designed for X psf as per IBC Table [such and such], owner/contractor to verify capacity with licensed geotechnical engineer", what more could I do to show in court that I have met the standard of care? What more could be said to show that I did not meet the standard of care? My assumptions are based on what he has written to me himself as being the soil on his property.

 

[oldestguy]

If you must go ahead without the geotech report, at least be more conservative with designing for spanning no-support zones of 8'x 8' and none under say 8 feet along the wall at any place. I've given that recommendation when no footing is done and some question as to uniform support is present and never had it come back as a problem.

 

[Ron]

oengineer....please get off the premise that the county doesn't require a geotechnical report. Who cares if they do or do not. YOU need the geotechnical report for YOUR design. They don't need it.

 

[serrojo]

Hello, as I see it you have two ways:
1- You give a provisional solution on the assumption of any similar soil nearby (with a report) and go thru. Later review the design up on receiving the report.
2- you plan a soil replacement with a soil, known for you, with a bearing capacity, and a minimum thickness. And go thru without the soil report here you will be less dependent on the report data but it will cost more money for the client.

Regards.

MSc. Eng. Serguei Joa
Structural Engineer
Bouygues Batiment International, Cuba.

 

[oengineer]

I appreciate all the information provided from all of you who have posted. The message below is what I have told the client:

"I recommended that you obtain a geotechnical investigation report to determine the site specific soil conditions for your property where the residential building will be located because you may be able to save more money in material for the foundation than the cost of the geotechnical investigation report. Without a geotechnical investigation report, the foundation design will be based on conservative assumed values. Obtaining a geotechnical report will also help to avoid surprises during construction. Not to mention, the cost savings that will be experienced if you were to have foundation difficulties arising later on due to not knowing the site specific soil conditions.

Without a geotechnical investigation report of your property where the log cabin foundation will be located, assumptions will be made in order to design the foundation for your log cabin based on the information you have provided"

I have assumed a PI = 40 and adhered to the 15'-0" spacing for my grade beams stated in WRI. I am specifing 3'-0" of select fill to be placed under the foundation. The select fill properties I specify are 36" select fill should consist of a clean sand & clay with a liquid limit less than 35 & P.I. 10 to 20, to be placed in 8" lifts, max & compacted to 95% proctor density per ASTM D-698. Moisture content within -1% to +3% of optimum moisture.

I believe that I have informed the client that obtaining a geotechnical report is the route to go. I don't believe they will do it though. Also, the 36" of select fill should resolve settlement issues.

 

[26sig]

Without a geotechnical report, you really don't know if your assumed values really are conservative.


CF

 

[oldestguy]

Who would ever specify Select Fill like this "fill should consist of a clean sand & clay"? It is obvious no knowledgeable contractor would know what the heck that means. If it is clean it has no clay in it. I'd not want any plasticity of the "Select Fill" either. Why not call for granular fill relatively well graded with sizes between 2" and #200 sieve, but not over 10 percent P-200. No point in asking for Atterberg limit info. Leave the geotech stuff to the geotechs.

 

[JoelTXCive]

oengineer-

This is just my two-cents but I would get your grade beam spacing down much lower. I believe that is what oldestguy was referring to with the 'no support zones' of 8'x8'. He is saying make your pad dimensions 8x8 which would be 9ft center to center spacing on your grade beams if you do 12inch beams.

Your proposed 15ft spacing seems large. That would be 14ft pads. It has been a few years since I was in the residential world, but I believe the tract home builders keep their pad sizes under 12'x12'.

 

[JoelTXCive]

The large national warranty companies have a lot of experience with slabs, so they dictate many design parameters to builders. Obviously, warranty companies are not engineering firms, but they do know what minimum design standards will keep them out of court.

Here is a link to an OLDER HWB Corporation 2-10 design guideline. All home warranties in the state of Texas are essentially the same, and the large providers all require basically have the same standards for builders.

They have max pad size of 12ft and minimumn beam depth of 26" for most of their slabs (it varies by region)

It's worth perusing this book to make sure you design is in the norm:

http://www.2-10.com/wp-content/uploads/2014/06/2005_Texas_Risk_Management_Manual.pdf

 

[kmart30]

Sorry to chime in on this afterwards but adding 3 feet of fill and the new load of a building will actually Increase your settlement. They need to excavate and replace with clean (no clay) fill.

If you are going to do this without a geotech you need to cover your arse...Tell them they need to have at least 3 feet of clean fill beneath the bottom of all foundations and they need to be compacted to 95% in lifts no greater than 12 inches. Put all these notes in your plans and require that they submit these field density test reports to the building department and the EOR. They will cry about this for sure but if they don't build it according to your specifications as the EOR then it's their problem. Doesn't mean you won't get sued later down the road if something pops up but at least you will have a good leg to stand on and you used standard engineering practices. You could argue standard engineering practice is to hire a geotech but if the owner doesn't want to pay for it then what do you do? What if you were designing a 10 story building without geotech? Drive piles to China and tell them too bad...

Here in Florida the soils are different and we have other things to worry about but I have notes in my residential plans that it's the contractors responsibily to verify a bearing capacity of xxxpsf and that no expansive or deleterious soils are present beneath foundations. No one gets geotech on residential lots here unless it's a big track development. I've even seen the civil engineer, structural engineer, and architect on a large commercial project totally miss a recommendation from a geotech to remove the buried trees and trash debris they found in every boring across the site. Slabs crack to hell all the time because contractors never prep/compact correctly and the building departments don't care as long as it's less than 12 inch of fill. If it's not in the plans the contractors don't care or never see it because no on told them.

Sorry for the long post but it irritates me when engineers, architects, contractors, and owners don't care about geotech or think it not needed. Good luck!

 

[BAretired]

In the absence of a soil report, is it fair to say that the EOR becomes responsible for any soil conditions affecting his design when he visits the site during construction? If so, it might be a good idea to take along an auger to check soil conditions down to a reasonable depth, perhaps eight or ten feet or as otherwise determined by the EOR.

It is easy to see the downside of working without a soil report. Is there an upside?

BA

 

[oengineer]

Thank you all for your input. I greatly appreciate it. I have revised my grade beam spacing to create 8'x8' grids. I have gone smaller is some areas.

 

[oldestguy]

OG here gain.

My recommendation for sizing a slab, grid or footings is to check any area as capable of spanning various very weak zones. As at a corner for this beam and slab design assume the 8' x 8' zone here has nothing under it and the beams and slab have to cantilever over that zone. Likewise any such area within the building would have the structure span that zone and carry any columns sitting on as load. This doesn't automatically space those grids at 8' x 8'. You may well have that weak area with beams meeting at the middle of that weak zone. The principle is only a means for designer to size things, given loads from above and support below. weak zone area and grids may well be different size. I would think that some judgement is needed for setting each dimension. Make the assumed weak area very large and you run into an impractical load and strucvture situation for the beam and slab combination.