Integrated Buildings & Structures

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. 



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Krishna Ghimire
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Normally (b) would be used.

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[Bruce [Montgomerie] M.ASCE
Design Engr
GLOUCESTER [AUSTRALIA]
Bruce
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------Original Message------

Normally (b) would be used.

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[Bruce [Montgomerie] M.ASCE
Design Engr
GLOUCESTER [AUSTRALIA]
Bruce
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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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Original Message:
Sent: 02-21-2018 02:08
From: Bruce Montgomerie
Subject: Weight of soil acting on a shallow foundation

Normally (b) would be used.

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[Bruce [Montgomerie] M.ASCE
Design Engr
GLOUCESTER [AUSTRALIA]
Bruce
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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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Original Message:
Sent: 02-21-2018 02:08
From: Bruce Montgomerie
Subject: Weight of soil acting on a shallow foundation

Normally (b) would be used.

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[Bruce [Montgomerie] M.ASCE
Design Engr
GLOUCESTER [AUSTRALIA]
Bruce
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------Original Message------

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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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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Original Message:
Sent: 02-21-2018 02:08
From: Bruce Montgomerie
Subject: Weight of soil acting on a shallow foundation

Normally (b) would be used.

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[Bruce [Montgomerie] M.ASCE
Design Engr
GLOUCESTER [AUSTRALIA]
Bruce
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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.

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Luis Sanchez P.E., M.ASCE
Civil Engineer/Structural Engineer
Bay Harbor Islands FL
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Original Message:
Sent: 02-21-2018 02:08
From: Bruce Montgomerie
Subject: Weight of soil acting on a shallow foundation

Normally (b) would be used.

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[Bruce [Montgomerie] M.ASCE
Design Engr
GLOUCESTER [AUSTRALIA]
Bruce
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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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​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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​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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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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ِ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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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

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Ammar Fakhoury P.E., M.ASCE
Riyadh
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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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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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(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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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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"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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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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Option b.

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Dikaios Psarogiannis A.M.ASCE
Consulting Engineer
OMETE SA
Holargos
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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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​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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