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Lateral Bearing vs. Passive Earth Pressure

  • 1.  Lateral Bearing vs. Passive Earth Pressure

    Posted 07-26-2017 02:57 PM
    When calculating the depth of embedment of a pole type footing using IBC 1807.3.2, would it be correct to use passive lateral earth pressure from the soils report for allowable lateral bearing pressures (S1)?

    Any ideas as to the difference between the allowable lateral bearing pressure and the passive lateral earth pressure?

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    Jordan Jones EIT, A.M.ASCE
    Tacoma WA
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  • 2.  RE: Lateral Bearing vs. Passive Earth Pressure

    Posted 07-28-2017 03:03 PM

    Hi Jordan,

    I have often wondered the same thing. I used to use the passive soil pressure from the soil type or geotech report, but now I use the allowable lateral bearing pressure.  The definition of S1 states to use the values in 1806.2 and uses the phrase "as set forth in Section 1806.2." This leads you to Table 1806.2 and values that range from 100 - 1200 psf/ft.  You are able to use a higher value if they are "submitted and approved."  Also, you can increase the value with depth according to 1806.3.3 and multiply by 2 if you meet 1806.3.4.  

    I'd be interested to see how others have used this part of the code. 



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    Jason Herrman P.E., M.ASCE
    Professional Engineer
    Oldcastle Precast
    Littleton CO
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  • 3.  RE: Lateral Bearing vs. Passive Earth Pressure

    Posted 07-28-2017 03:03 PM
    ​The values are the same. If you have a geotech report that prescribes the passive allowable earth pressure this is the same as allowable lateral bearing pressure. It should be given in the same units as the table value (pounds per cubic foot or psf/ft, same thing).  The value from the geotechnical report supersedes the value from the code table and should be used as required in the post embed formula.

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    Brett King P.E., M.ASCE
    Senior Structural Engineer
    GHD Inc.
    Lake Oswego OR
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  • 4.  RE: Lateral Bearing vs. Passive Earth Pressure

    Posted 07-28-2017 03:04 PM
    Yes they are the same thing.

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    Thomas Kelley P.E., M.ASCE
    Associate Engineer
    Kelley Engineering and Surveying
    Hollister CA
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  • 5.  RE: Lateral Bearing vs. Passive Earth Pressure

    Posted 07-28-2017 03:04 PM
    Great Question, Jordan.  I have always wondered the same .  Id like to hear what an experienced geotechnical engineer would say.  

    The lateral pole bearing equation is an old empirical equation that has been in the code for many code cycles.  I believe that the two soils parameters of Passive and lateral bearing pressure just may be the same.  However, the code lateral bearing values are lower and may be more appropriate to use with that empirical equation.  Therefore, I don't use the higher passive pressures published in the geotechnical report when using that equation.

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    Gregory Meyer P.E., S.E., M.ASCE
    Las Vegas NV
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  • 6.  RE: Lateral Bearing vs. Passive Earth Pressure
    Best Answer

    Posted 07-28-2017 03:04 PM
    Jordan,
    The allowable lateral bearing capacity and the passive earth pressure are very different things. The passive pressure is the maximum pressure that can be developed on an earth-retaining structure before failing by shearing a displacing a wedge of soil. It can be adapted to a pile but the theories were developed for walls. More importantly, it represents a limit state condition, so it is not an allowable pressure.

    For the IBC post and pole method you want to use, there are presumptive lateral bearing pressure values in Table 1806.2. However, what you need to understand is that allowable bearing pressure is not a physical property of the soil. It is a lumped parameter that tries to account for shear strength, deformation, and, to an extent, foundation geometry. The presumptive values in the code are lower bound values of what is already a conservative approximation and should not be used for significant structures because they are excessively conservative.  

    A better approach would be to seek guidance from the project geotechnical engineer, assuming the has adequate foundation engineering capabilities (as opposed to just sampling and testing) to field this sort of question. A soil-structure interaction approach (p-y curves and such) will typically provide a more rational prediction of soil behavior and forces internal to the foundation.

    I hope this is helpful.

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    Richard J. Driscoll P.E., M.ASCE
    Consulting Engineer
    Lebanon, NH / Washington, DC
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  • 7.  RE: Lateral Bearing vs. Passive Earth Pressure

    Posted 07-28-2017 03:04 PM
    I believe the Lateral Bearing Pressures in Table 1806.2 come pretty close to matching the typical allowable passive pressures when the 2X multiplier of Section 1806.3.4 is applied.

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    Mark Myers P.E., M.ASCE
    Gig Harbor, WA
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  • 8.  RE: Lateral Bearing vs. Passive Earth Pressure

    Posted 07-28-2017 03:04 PM
    You would actually use the difference between the active and passive lateral earth pressures to calculate your allowable sliding force and overturning moment.  The active lateral earth pressure is responsible for causing the structure such as a retaining wall or a pole foundation to move away from the soil whereas the passive earth pressure is responsible for causing the structure to move or lean toward the soil.  Since a pole foundation can move laterally in any direction by virtue of its shear strength, the intersection of the active and passive zones determine the net force created by these lateral earth pressures; that is, passive minus active.  Notice that the earth pressure coefficient for passive resistance is roughly the reciprocal of the active earth pressure coefficient, for any internal friction angle.  Hopefully these hot geotech facts can give you more insight as to how the allowable bearing pressure of a pole foundation is calculated.

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    Richard Grow P.E., M.ASCE
    Principal Structural Engineer
    South Plainfield NJ
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  • 9.  RE: Lateral Bearing vs. Passive Earth Pressure

    Posted 08-03-2017 01:24 PM
    Lateral bearing pressure accounts for the geometry (diameter) of the pole.  Imagine the Terzaghi bearing capacity, having a triangle under the footing followed by a transitional zone, and then followed by the passive zone.  The passive resistance assumes that the pole is infinitely wide and, as a result, misses the benefits from the two- or three- dimensional failure of the soil around the pole.  Lateral bearing capacity is typicaly two to three times larger than the passive resistance.  A potential  problem either way is that neither approach provides you with the deformation required to activate the resistance.   Use of p-y curves resolves this issue.

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    Panos Kiousis A.M.ASCE
    Associate Prof
    Colorado School Of Mines
    Golden CO
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  • 10.  RE: Lateral Bearing vs. Passive Earth Pressure

    Posted 08-04-2017 06:23 PM

    In an academic sense Mr. Driscoll and Mr. Kiousis give answers that may be technically correct.  

    While comments regarding a 2D vs. 3D problem are all technically part of the issue in determining "allowable pressure" for an element of a small width it should be observed that table values in 1806.2 are per foot of width values which cannot account for pole diameter since it's not part of establishing the table. In fact the table values for lateral pressure can be used in retaining wall design or lateral resistance of footings where no geotech data is available.

    Based on recollection, I believe the pole depth formulas are based on limiting deflection to a reasonable value given assumed capacity of the soil and footing diameter. Capacity is controlled by the allowable bearing input.

    However, I doubt you are trying to address an academic problem.  No one develops p-y curves for the design of structures (light poles, pole barns, fences, signs on poles, etc.) that are conceivably covered by section 1807.3 Embedded posts and poles or the allowable bearing tables.

    I believe many designs for these types of posts and poles have been done using the passive pressure values for footings and walls provided in a geotechnical report as a direct substitute for Table 1806.2 values.

    I predict that your designs using the geotech values, should you choose to use them, will also be successful.



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    Brett King P.E., M.ASCE
    Senior Structural Engineer
    GHD Inc.
    Lake Oswego OR
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