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.

 

[Rabbit12]

Is swelling of the soil a potential issue that could cause problems?

 

[JoelTXCive]

You can go to the US Department of Agriculture and look up soil in the area. This general information only and certainly not enough to legally hang your hat on.

https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm

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"

 

[WARose]

You can use a presumptive value (as you have) from the code or a local building code official. In cases where the customer does not want to get a geotechnical report, I always put some type of CYA note on the drawing that puts it on the contractor/owner to verify bearing capacity. (Something like: "foundation designed for X psf as per IBC Table [such and such], owner/contractor to verify capacity with licensed geotechnical engineer".)

 

[TXEng-USA]

WRI TF 700-07 ( Design of Slab-on-Ground Foundations) states "It is considered imperative that a soils investigation be made on any site on which a design is to prepared". They reiterated this point in the 1996 update.

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.

 

[oldestguy]

If you do find that it is a clay soil with high swelling potential (very common in Texas), the design of the slab should take into account that the exterior support likely will move up and down more with seasonal changes. Nearby trees can accentuate these things so no trees within about 30 feet or more of the house. Lacking advice from geotechs in the area, I'd design the slab so that it will support the house even if there is no interior support and maybe support is only within 10 feet of exterior walls. Or that only the interior rectangle of 1/3 area supports the house and outside walls do not have soil contact under the slab there.. These are rough only, lacking better site knowledge. Likely a series of "ribs" (trenches) will be cheapest foundation cost due to high bending moments. Soil bearing pressure is likely not a factor for design. Utilities need too be very flexible and sloped for drainage regardless of slab position (up or down).

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.

 

[oengineer]

Clay, sandy clay, silty clay, clayey silt, silt and sandy silt (CL, ML, MH and CH) is what IBC says for the 1500 psf allowable foundation pressure> It does contain clay.

 

[Ron]

oengineer....clay is a generalization in some respects. As others have noted, areas of Texas are noted for shrink/swell potential in the near surface clays. Just because IBC says that 1500 is an allowable bearing capacity does not take into account localized conditions. With the exception of a few states that have specific statutory building codes, the IBC is generalized and intended to be a guide only. Localized conditions will prevail and particularly so in areas of known issues such as Texas.

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?

 

[BAretired]

I would insist on a soil report before providing a foundation design.

BA

 

[txeng91]

I agree with Ron and TX, even if you did assume a high PI value like 40, there is no guarantee that the soils on the site won't exceed that value. Being that your site borders on an area with some of the most expansive soils in the country, it very well might not even be suitable for a slab on grade foundation at all.

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.

 

[JoelTXCive]

@TXEng-USA

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.

 

[TXEng-USA]

Thanks @JoelTXCive. I have come to a similar realization as you. Even if we are not doing post-tension I still prefer to determine the moment, shear and deflection with the PTI method.

 

[oengineer]

@JoelTXCive _ I have been having issues with the deflection formula as well.


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.

 

[hokie66]

oengineer,
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.

 

[WARose]

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.

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.

 

[oengineer]

I call the City of Goliad,TX and apparently they do not have an engineering department. I spoke with the city secretary who said that City of Goliad does not require a soil report for foundation designs. I am working on getting a conformation through their building inspector as well. Also, Goliad shows on their website that the adhere to IBC 2006. I was told that the city just adopted IBC 2012. So I believe if I base my assumptions on IBC I should be okay.

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.

 

[oengineer]

What is the recommended depth of compacted fill to be used for such a situation. I know this is usually called out in the soil report, but since one has not been provided i am either going to put a note saying that " COMPACTED FILL AS REQUIRED BY CODE" or " 48 INCHES OF COMPACTED FILL". If anyone has some suggestions/comments, please let me know.

 

[JoelTXCive]

I am sure you are correct on the Goliad/Tx building code requirements, but I don't think that will protect you if have an unhappy homeowner and they decide to sue.

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.

 

[WARose]

What is the recommended depth of compacted fill to be used for such a situation. I know this is usually called out in the soil report, but since one has not been provided i am either going to put a note saying that " COMPACTED FILL AS REQUIRED BY CODE" or " 48 INCHES OF COMPACTED FILL".

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.

 

[TXEng-USA]

"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" (oengineer)

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.