Design of Residential Slab-on-Ground Foundation without soil report
Design of Residential Slab-on-Ground Foundation without soil report
(OP)
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.
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.






RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil report
https://websoilsurvey.sc.egov.usda.gov/App/HomePag...
I would assume a worst case scenario for any data that is on there. If there is any clay at all, then I would assume a PI of about 40. The WRI method looks at the top 15ft of earth and does a weighted average. Just assume an overall PI for all the regions.
You can also recommend that they remove 2-3ft of existing earth and replace with select fill.
When the client complains about the cost, you can say "A geotech report with 2 borings will run you less than $2k"
RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil report
ASCE Texas Section has a publication "Recommended Practice for Design of Residential Foundation" which outlines acceptable methods to analyze and design SOG. They include and make modifications to BRAB, WRI, PTI and finite element methods. They insist that a geotechnical report is required.
So BRAB, WRI, PTI, ASCE etc. etc. all say that a geotechnical report is not optional.
I have felt the same pressure that I assume you are getting to proceed without a report. In those situations, I have drafted up a letter to the owner which essentially states that they are waiving the geotechnical report and in effect taking upon the risk that the assumed values we are using are not adequate. So far they have always ended up getting the report.
Speaking of the WRI method. Does anyone have example calculations that they would not mind comparing? I have found the document to be riddled with errors and the example does not even check shear or deflection.
RE: Design of Residential Slab-on-Ground Foundation without soil report
If you replace the swelling potential soil with granular, the depth needed maybe very deep and make that option too expensive, so then you live with the movements. A partial undercut of course helps.
RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil report
Let's assume you design the slab as you've done without a geotechnical investigation. Further, let's assume that two years from now the slab heaves from expansion and cracks the house all to hell. You get sued. Do you have a defense? NO! You have not met the engineering standard of care because you didn't require or seek site-specific soils information. Bottom line....you're screwed.
Think about it. No single client is EVER worth compromising your engineering standard of care. It is the measure by which we, as engineers, are judged with respect to professional negligence. How much do you value your license and your livelihood to feed yourself and your family?
RE: Design of Residential Slab-on-Ground Foundation without soil report
BA
RE: Design of Residential Slab-on-Ground Foundation without soil report
I think you should try to convince the clients that spending a small sum (2k-3k) now is well worth it to ensure the most important part of the structure performs adequately. Otherwise they might find themselves spending tens of thousands on underpinning in the future with no guarantee that it will actually work.
RE: Design of Residential Slab-on-Ground Foundation without soil report
I worked through the WRI examples about a year ago and was able to derive their moment formula and shear formula, but not their deflection formula (i think there is a deflection formula). I do remember their being errors in the example also.
Additionally, the design example is worked off of the original 1981 WRI document. The 1996 document is a little different and the example was never updated.
Our office has used the WRI method for what we classify as low risk slabs such as generator pads and odd buildings at water treatment plants. We sometimes have imperfect geotech, or the nearest boring is 3 miles away in another county.
It is an accepted design methodology, but I think it is purely an empirical method. We often joke that it is the 'witch doctor' method of designing slabs.
The only inputs are the climate zone, plasticity, and unconfined compressive strength. (and you really don't need the unconfined compressive strength. The table value is near 1 for .5 TSF to 4 TSF, which covers a lot.
I'll look tomorrow at the office to see if I still have my attempted math proofs.
RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil report
I am designing this foundation for a log cabin in Goliad,TX. I figured that if I assume a very poor soil it would be okay. Also like WARose mentioned,I planed on putting a note like: "foundation designed for X psf as per IBC Table [such and such], owner/contractor to verify capacity with licensed geotechnical engineer". If soil reports are not required in Goliad,TX then I believe this should suffice.
I will check to see if the City of Goliad requires a geotech for their residential foundations.
RE: Design of Residential Slab-on-Ground Foundation without soil report
The problem with your approach is that "X psf" is only part of the geotechnical story, and when it comes to single family dwellings, a very small part. Shrink-swell potential of the soil is much more important, as others have advised.
RE: Design of Residential Slab-on-Ground Foundation without soil report
I tend to agree. In fact I am the one typically screaming for a geotechnical report. But at this point in time, you don't get one on every project. I haven't done that much work in Texas (and its been mainly industrial when I have), but I find it hard to believe that people are getting geotechnical reports for every residential project (that just involved shallow foundations)......although it would be a pleasant surprise (if so).
I'd check with a local code official.
RE: Design of Residential Slab-on-Ground Foundation without soil report
I figured that if the governing city does not require a soil report and the client does not provide one, then as long as the engineer makes reasonable assumptions that they are not legally liable for damages as long as you have designed the foundation per code according to your reasonable assumptions. Let me know if this thinking is correct. I know that the city of Pasadena,TX does not require a geotechnical report for designing foundations.
RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil report
We have a duty of care to the customer, and have a responsibility to adhere to a standard of care.
The overwhelming majority of homes built in Texas have engineered slabs based upon geotechnical reports. (Sidenote: the large national warranty providers are driving this. Since Texas has such a problem with expansive clays, they will not sell warranties to the builders unless there is a soil report within xxx ft of the project and an engineered slab.)
Regardless of the building code requirements, the very first thing out of an attorney's mouth would be "Why did you design a slab without knowing the site specific soil conditions? We have talked to local engineers A, B & C; and they all use soil reports for their slab designs"
Our office sometimes will do slabs without a specific soil reports, but they are not for structures that people will be living in where cosmetic issues and minor settlement will be a problem.
RE: Design of Residential Slab-on-Ground Foundation without soil report
Generally (in the absence of a soils report) you would cut it off at the limits of a settlement analysis. (I.e. the influence zone.) I.e. about 2 times the least width of a spread footing/mat, or 4 times the width of "infinite" strips.
RE: Design of Residential Slab-on-Ground Foundation without soil report
It is my understanding that by not obtaining a geotechnical report you are not following the code. §1808.6.2 of IBC 2012 requires that for foundations on expansive soils the "Moments, shears and deflections for use in designing slab-on-ground, mat or raft foundations on expansive soils shall be determined in accordance with WRl/CRSI Design of Slab-on-Ground Foundations or PTI Standard Requirements for Analysis of Shallow Concrete Foundations on Expansive Soils."
As you stated, you will be using WRI. WRI requires a geotechnical report.
As JoeTXCive mentioned, in Texas especially in areas of expansive clay, obtaining a geotechnical report is common and thus I believe it would required to meet the "standard of care".
Explain to the owners that you may be able to save more money in material for the foundation than the cost of the geotechnical report since you will not need to assume conservative values. Not to mention, the savings that will be experienced if they were to have foundation problems.
RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil 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.
RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil report
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.
RE: Design of Residential Slab-on-Ground Foundation without soil report
"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.
RE: Design of Residential Slab-on-Ground Foundation without soil report
Without a geotechnical report, you really don't know if your assumed values really are conservative.
CF
RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil report
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'.
RE: Design of Residential Slab-on-Ground Foundation without soil report
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/2014/06/200...
RE: Design of Residential Slab-on-Ground Foundation without soil report
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!
RE: Design of Residential Slab-on-Ground Foundation without soil report
It is easy to see the downside of working without a soil report. Is there an upside?
BA
RE: Design of Residential Slab-on-Ground Foundation without soil report
RE: Design of Residential Slab-on-Ground Foundation without soil report
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.