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Weight of soil acting on a shallow foundation

  • 1.  Weight of soil acting on a shallow foundation

    Posted 02-20-2018 11:43 AM
    Edited by Krishna Ghimire 02-20-2018 04:40 PM
    Hello, I am seeking your insights on how to calculate the weight of soil acting on a foundation. There are different opinions on what portion of the soil above the footing should be accounted, for example, to calculate uplift resistance. Fig. (a) through (c) show different boundaries of soil assumed to exert weight on the footing. Fig. (c) is consistent with the weight of a wall considered when designing window lintels (i.e, not all wall weight is assumed to go on the lintel). Please suggest the angle of inclination as appropriate. Your valuable thoughts are highly appreciated. 

    Schematics for the weight of soil acting on the footing

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    Krishna Ghimire
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  • 2.  RE: Weight of soil acting on a shallow foundation

    Posted 02-21-2018 04:33 PM
    Normally (b) would be used.

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    [Bruce [Montgomerie] M.ASCE
    Design Engr
    GLOUCESTER [AUSTRALIA]
    Bruce
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  • 3.  RE: Weight of soil acting on a shallow foundation

    Posted 02-22-2018 08:26 PM
    (a) is logical (opposite of punching) provided the angle of friction is suitably reduced for presence of ground water.

    (b) is conservative. However, if there are constraints on depth or plan dimensions e.g. in an existing industrial layout then (a) may need to be adopted.

    (c) is normally not applicable.

    Pendharkar 





  • 4.  RE: Weight of soil acting on a shallow foundation

    Posted 02-22-2018 08:28 PM
    Edited by Tirza Austin 02-22-2018 08:27 PM
    To take into account the load on a footing, usually after computing live loads for a multi storied building including seismic considerations, dead load consisting of footings self weight and soil over footing is taken 10% of the sub total mentioned above. I find it works.
    Some text books adopt the same.

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    Peter Singh M.ASCE
    Managing Director
    Kingdom Designers
    Birganj
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  • 5.  RE: Weight of soil acting on a shallow foundation

    Posted 02-23-2018 11:29 AM
    I consider the (b) is the correct one and normally used. The (a) could be, but you have to consider that the foundation is executed in a excavation and will be surrounded by a artificial backfill. To consider an angle you have to know the cohesion of the backfill and this is unknown during the design process.
    (c)  could be the most unfavorable for uplift consideration, but it's strange.

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    Serban Tiparescu P.E., M.ASCE
    Construction Procedures Manager
    The Woodlands TX
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  • 6.  RE: Weight of soil acting on a shallow foundation

    Posted 02-25-2018 08:48 AM
    Failure surface at ultimate uplift looks parabolic as in option (A).
    However, reducing embedment depth is always on the conservative side.
    Factors affecting uplift capacity are depth of embedment (Df), width of footing, length of footing, friction angle for granular soil and undrained shear strength. The angle of inclination may be developed from the angle of friction based on the type of soil.






  • 7.  RE: Weight of soil acting on a shallow foundation

    Posted 02-24-2018 10:05 AM
    I typically use (b).  Choice (a) would be acceptable if you have a rational basis for your selection.

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    Andrew Holmes P.E., L.S., M.ASCE
    Consulting Engineer
    Melbourne FL
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  • 8.  RE: Weight of soil acting on a shallow foundation

    Posted 02-25-2018 08:40 AM

    I'd say that b) is the simple and natural way to approach this, basically because we are talking about weight, no soil particles friction, in many cases like passive pressure, soil might be taken off the equation thinking in the worst case scenario so actually in this situation I usually use case b) for soil bearing capacity and no soil for uplift and sliding.

    LUIS A. SANCHEZ, P.E., M.ASCE
    33154, Miami. Florida.


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    Luis Sanchez P.E., M.ASCE
    Civil Engineer/Structural Engineer
    Bay Harbor Islands FL
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  • 9.  RE: Weight of soil acting on a shallow foundation

    Posted 02-21-2018 04:37 PM
    My opinion: C can't happen because in order to pull out of the ground all of the soil above the footing would need to be displaced. If the footing is pulled out of the ground the soil may then fall off the footing like this, but it has already failed in uplift. A may be appropriate due to the friction angle of the soil, but I use B because it is conservative.

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    Carrington McVeigh P.E., M.ASCE

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  • 10.  RE: Weight of soil acting on a shallow foundation

    Posted 02-21-2018 04:38 PM
    ​I choose figure A and the angle shown from horizontal would be 45-Phi/2

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    Matthew Beck P.E., M.ASCE
    Engr
    Beck Engineering Pc
    Bondville VT
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  • 11.  RE: Weight of soil acting on a shallow foundation

    Posted 02-21-2018 05:16 PM
    ​B is the traditional choice for both gravity loads and uplift resistance.  Depending on soil conditions and type, there might be an argument for A for uplift considerations, but B is conservative.  In the case of gravity loads, in many instances the allowable soil pressure given by a geotechnical engineering report is "net" allowable.  In that case, the weight of soil can generally be neglected.  The "net" weight of the foundation, building and lateral loads are then considered in the design of the footing since the weight of the soil is accounted for in the determination of the "net" allowable bearing pressure.

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    Michael Shelton P.E., M.ASCE
    Associate Vice President / Office Manager
    HNTB
    Lenexa KS
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  • 12.  RE: Weight of soil acting on a shallow foundation

    Posted 02-21-2018 05:18 PM
    Edited by Tirza Austin 02-21-2018 05:17 PM
    b) Is the most universal soil ballast pattern for most soils, especially for linear footings. Other issues are moisture content of granular soils above the footing (saturation above bottom of footing reduces gravity on footing and soils by SG of water) and moment from wall to footing impacts overlying soil ballast pattern.

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    Robert Bullard P.E., M.ASCE
    President
    Ahimsa Technic Inc
    Ponce Inlet FL
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  • 13.  RE: Weight of soil acting on a shallow foundation

    Posted 02-21-2018 05:20 PM
    Edited by Tirza Austin 02-21-2018 05:19 PM
    ِDear Krishna
    For designing you can calculate soil weight similar to the Fig. b. Although, actually it isn't true because of soil discrete material.
    But it is better that use pile or another method to uplift controlling.

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    Hadi Valizadeh S.M.ASCE
    Urmia
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  • 14.  RE: Weight of soil acting on a shallow foundation

    Posted 02-22-2018 08:30 PM
    for telecommunication tower foiting uplift desing
    _ soil to be well compacted engineering fill to Astm
    _ we are using figure A with angle of repose around
    30
    _related sections of TIA_EIA standard G to be used


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    Ammar Fakhoury P.E., M.ASCE
    Riyadh
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  • 15.  RE: Weight of soil acting on a shallow foundation

    Posted 02-22-2018 08:31 PM
    Edited by Tirza Austin 02-22-2018 08:31 PM
    In a previous company that I worked with, we designed a lot of buried tanks that could be subject to floating.  One group of senior engineers advocated section B while another thought section A was less conservative.  We actually performed a small scale test with an embedded "T" in compacted soil an found that the pullout was more like A than B, using Rankine theory the failure angle would be about 30 degrees.  However, to be more conservative we agreed to 20 degrees.  This can make a major impact on the thickness of the mat and the width of the toe.

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    David Kittridge P.E., M.ASCE
    Principal Eng
    Maitland FL
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  • 16.  RE: Weight of soil acting on a shallow foundation
    Best Answer

    Posted 02-22-2018 08:33 PM
    Edited by Krishna Ghimire 03-09-2018 04:09 PM
    It is common practice to use "b".

    "a" looks attractive given the
    contribution of friction, but that is an unknown.

    "c" applies to lintels because of the deflection of lintels relative to the rigid (and brittle) wall. The seams break in tension there possibly following a zig-zag line along the seams. Here, in our case, the soil is not more rigid than the foundation, actually less, and can follow its settlement with ease. So it doesn't apply here.

    Neil Kazen

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    Neil Kazen F.ASCE
    Former Structural Design Manager Transportation Division
    Toronto ON
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  • 17.  RE: Weight of soil acting on a shallow foundation

    Posted 02-22-2018 08:37 PM
    (c) is not a possible uplifting failure mechanism.

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    Ziad Elias M.ASCE
    Prof Emeritus
    Univ Of Washington
    Bothell WA
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  • 18.  RE: Weight of soil acting on a shallow foundation

    Posted 02-22-2018 08:38 PM
    Edited by Tirza Austin 02-22-2018 08:38 PM
    I think figure 1 is appropriate. If I'm wrong, please correct me.

    Thank you...


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    Emre Erdogan EIT, A.M.ASCE
    Mus
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  • 19.  RE: Weight of soil acting on a shallow foundation

    Posted 02-23-2018 11:30 AM
    "B" is the most usual method for calculating the weight of soil in the foundation for the design of footing in associate with gravity loads from a structure. I believe the condition such "A" can happen when the shear capacity of soil is considered, when there are uplift forces from a structure, although, in such case, you may not be permitted (based on many codes) to consider weight and other components of soil, at all, in such case weight of footing can be the only resisting load or you may use pile/s to bear the loads.
    "<g class="gr_ gr_807 gr-alert gr_tiny gr_gramm gr_inline_cards gr_run_anim Punctuation only-ins replaceWithoutSep" id="807" data-gr-id="807">C</g>" I think, is not applicable for a buried foundation.

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    Mazhar Kamangar C.Eng, M.ASCE
    CPECC
    Beijing
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  • 20.  RE: Weight of soil acting on a shallow foundation

    Posted 02-23-2018 11:31 AM
    Option (b) is considered to determine the weight of the soil because for calculating the safe bearing capacity  of soil we consider in same fashion as b but while determining the settlement of soil we consider as option c. Hence my opinion is to use option b

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    [supriya [hv] S.M.ASCE
    [Student]
    [Mysore] [India]
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  • 21.  RE: Weight of soil acting on a shallow foundation

    Posted 02-26-2018 09:48 AM
    Option b.

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    Dikaios Psarogiannis A.M.ASCE
    Consulting Engineer
    OMETE SA
    Holargos
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  • 22.  RE: Weight of soil acting on a shallow foundation

    Posted 02-26-2018 09:56 AM
    We usually use case "b" in shallow foundations. However, case "A" can be critical in deep foundations and in cases where the friction of soil is considered. I believe that case "C" is not correct, I don't see such a case before.

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    Ahmed Abdeldayem Mohamed A.M.ASCE
    Sherbrooke QC
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  • 23.  RE: Weight of soil acting on a shallow foundation

    Posted 02-27-2018 02:19 PM
    ​Option A represents the accurate model of soil failure for the problem. 

    Option B would be an adequate model if you added soil friction along the vertical failure line. Even so the soil can't fail like this unless there is a surcharge beyond the failure line to hold the soil down. In this case the limiting friction would be the weight of the soil wedge that's captured in A. 

    I can understand using B as the safe and conservative option. I choose the more accurate option A as the prediction of soil force and then vary the factor of safety to add more conservatism if that's what I want. For example we do this regularly with seismic vs. wind uplift where the FS for wind might be 1.5 but only 1.1 for seismic.

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