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    Geotechnical Testing

    Geotechnical Testing

    New postby aaronm on Thu Apr 09, 2009 12:29 pm

    Codeman:

    IRC defers to IBC when the foundation is built on expansive soil. Both IRC and IBC leave and out for the builder as regards geotechnical testing and put this call in the lap of the AHJ. I the case of a "structurally supported" residential slab foundation, i.e. a PT slab with poured piles beneath it, how do you interpret the PTI, WRI, and ACI requirements for geotechnical testing? Do these requirements on an engineered foundation system override the AHJ's decision not to require soil testing?

    Thanks,

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    Re: Geotechnical Testing

    New postby Jerry Peck - Codeman on Thu Apr 09, 2009 2:43 pm

    Hi Aaron,

    aaronm wrote:IRC defers to IBC when the foundation is built on expansive soil. Both IRC and IBC leave and out for the builder as regards geotechnical testing and put this call in the lap of the AHJ.


    The following is from the 2006 IRC regarding soil testing. (underlining and bold are mine)
    - R401.4 Soil tests. In areas likely to have expansive, compressible, shifting or other unknown soil characteristics, the building official shall determine whether to require a soil test to determine the soil’s characteristics at a particular location. This test shall be made by an approved agency using an approved method. (Jerry's note: That leaves it up to the building official to determine whether or not soil testing is required, so, yes, the IRC leaves that to the building official for their call.)
    - - R401.4.1 Geotechnical evaluation. In lieu of a complete geotechnical evaluation, the load-bearing values in Table R401.4.1 shall be assumed.
    - - R401.4.2 Compressible or shifting soil. Instead of a complete geotechnical evaluation, when top or subsoils are compressible or shifting, they shall be removed to a depth and width sufficient to assure stable moisture content in each active zone and shall not be used as fill or stabilized within each active zone by chemical, dewatering or presaturation.

    The following is from the 2006 IBC regarding soil testing. (underlining and bold are mine)
    - 1802.1 General. Foundation and soils investigations shall be conducted in conformance with Sections 1802.2 through 1802.6. Where required by the building official, the classification and investigation of the soil shall be made by a registered design professional. (Jerry's note: At first reading it may appear as though the requirement in 1802.1 is "shall be conducted", and 'when conducted' shall be conducted "in conformance with" Sections 1802.2 through 1802.6, however, all that is really stating is 'when conducted' the soil testing shall be in conformance with those sections. The last sentence clarifies that with "Where required by", leaving all other 'where not required by' not addressed. So, yes, the building official has the say so as to when it is required.)
    - 1802.2 Where required. The owner or applicant shall submit a foundation and soils investigation to the building official where required in Sections 1802.2.1 through 1802.2.7.
    - - Exception: The building official need not require a foundation or soils investigation where satisfactory data from adjacent areas is available that demonstrates an investigation is not necessary for any of the conditions in Sections 1802.2.1 through 1802.2.6. (Jerry's note: This further confirms your statement that the building official has the power to decide to have the soil tested or not tested, and clarifies the main defensible reason for the building official to 'not require the soil to be tested', with that reason being "where satisfactory data from adjacent areas is available that demonstrates an investigation is not necessary". Once the building official steps outside that defensible and code stated reason and chooses to 'not require soil tests', then that building official has stepped out onto "Questionable Soil" and may well regret it at a later date should legal action be taken when a problem arises. When those problems arise and the building official has stepped outside his given authority, the building official is usually advised that city/county representation will not be available to the building official and the building official's defense is on their own dime.)
    - - 1802.2.1 Questionable soil. Where the classification, strength or compressibility of the soil are in doubt or where a load-bearing value superior to that specified in this code is claimed, the building official shall require that the necessary investigation be made. Such investigation shall comply with the provisions of Sections 1802.4 through 1802.6. (Jerry's note: When the building official steps onto this "Questionable Soil", the soil may well give way under that weight because of the load-bearing value being placed upon it is superior to the load bearing value provided by the soil. In those cases it may be said that the building official has stepped into 'deep doo-doo'.)
    - - (continues on 1802.2.2 through 1802.2.6)

    I the case of a "structurally supported" residential slab foundation, i.e. a PT slab with poured piles beneath it, how do you interpret the PTI, WRI, and ACI requirements for geotechnical testing? Do these requirements on an engineered foundation system override the AHJ's decision not to require soil testing?


    They use of piles and pilings is always an engineering decision and is done, supposed to be done, based on engineering designs, which can only be accomplished with know soil bearing values, which are obtained from soil testing, however, the extent of that soil testing is up to the engineer and the building official. Pilings are supported in two basic manners: 1) end bearing; 2) skin friction. Without knowing the soil bearing values the engineer cannot calculate either end bearing capacity nor skin friction capacity for any given pile. If a pile is driven to bedrock, and end bearing only is used, then the piling would necessarily be quite large in diameter to provide the required end bearing area. Even in those cases, though, without some support from the soil which surrounds the piles, the pile, pile cap, and connection to the slab would need to be engineered to stand on its own without considering any lateral support provided by the soil, thus knowing the soils lateral load resistance is also necessary, which gets back to the engineer needing to have soil tests sufficient to know the load capacities of the soil.

    It is one thing to build slab on ground, with or without being post-tensioned, but in both regards distributing the weight and all loads through the slab and its footing to the earth, it is quite another thing to building a post-tensioned structural slab on pilings or piers, and yet another to build post-tension on slab on grade beam and mid-point pilings or piers. Once the slab on grade is elevated above grade, even when placing the slab on grade and using mid-point pilings or piers, the entire loading pattern of the slab changes, as does all of the reinforcing and post-tensioning cables within a post-tensioned structural slab, which is what that now becomes.

    It is one thing to use post-tensioning to keep a slab on ground together as a raft and allow it to move with the soil, it is a completely different thing to make it a structural slab spanning piling-to-piling-to-piling/grade beam.

    Not sure if that answered your question, but I think it did, or at least provided additional information for you to answer it. If it did not answer your question, please let me know.
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    Re: Geotechnical Testing

    New postby aaronm on Thu Apr 09, 2009 2:55 pm

    Codeman:

    In this instance the AHJ says that soil testing is required for either slab-on-grade, or structurally supported slab. But, and here is the catch, it is nowhere in writing. He also will not put it in writing. The foundation was permitted, i.e. green tagged, but no soil testing was performed.

    My contention is, and it may certainly be far afield, that this sort of hybrid foundation is an engineered system that (according to the design engineer's own plans) must adhere to both the most current issues of PTI and WRI (which require soils testing) must have soil testing. Am I living in the real world here?
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    Re: Geotechnical Testing

    New postby Jerry Peck - Codeman on Thu Apr 09, 2009 3:11 pm

    Hi Aaron,

    aaronm wrote:In this instance the AHJ says that soil testing is required for either slab-on-grade, or structurally supported slab. But, and here is the catch, it is nowhere in writing. He also will not put it in writing.


    If the building official did not, and will not, put it in writing it is the same as "NOT saying"it. That is like waking up in the middle of the night and listening to the echo of what you said earlier in the day ... just sound waves going through the air with no substance, except that the building official's sound causing the echo did not actually happen (not if not in writing) making it a figment of your imagination.

    My contention is, and it may certainly be far afield, that this sort of hybrid foundation is an engineered system that (according to the design engineer's own plans) must adhere to both the most current issues of PTI and WRI (which require soils testing) must have soil testing. Am I living in the real world here?


    Yes, you are in the real world, but only when you can no longer hear that building official's non-echo echo.

    Sounds like the building official slipped up not putting that in writing, but, in all actualities, the building official *did* put that in writing ... if that is on the "approved plans" it is *part of* any and all requirements approved with, and referred to on, those plans, and thus become "enforceable" ... by that same building official.
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