Discussion: View Thread

Original Message------

hello, 

Is it necessary to consider surcharge height when designing for pad foundation 

Yes.  Surcharge can be helpful on the perimeter of the foundation as it increases the bearing capacity of the soil beneath the foundation.  If you are considering surcharge on top of the foundation, yes it must be considered as an additional load to the foundation.

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Ronald Woods P.E., M.ASCE
President
Woods Engineering, Inc.
Jacksonville FL
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Original Message:
Sent: 10-31-2017 15:28
From: Akintunde Alade
Subject: Surcharge

hello, 

Is it necessary to consider surcharge height when designing for pad foundation 

When you say "surcharge height", do you mean the backfill that is placed around and above the pad  (the term qNq in Terzaghi's equation)?

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Panos Kiousis A.M.ASCE
Associate Prof
Colorado School Of Mines
Golden CO
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Original Message:
Sent: 10-31-2017 15:28
From: Akintunde Alade
Subject: Surcharge

hello, 

Is it necessary to consider surcharge height when designing for pad foundation 

The surcharge (backfill load) contributes to the bearing capacity (Terzaghi's equation) with the term qNq, where q is the surcharge.  This is a major contribution to the bearing capacity, especially for frictional soils with little or no cohesion.
Ignoring this term is conservative for cohesive soils, and VERY conservative for cohesionless soils.

Be careful however, on how you  use it.  A foundation at the perimeter of a house may have surcharge on the one side, but not on the other.  In that case, you should NOT use it.

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Panos Kiousis A.M.ASCE
Associate Prof
Colorado School Of Mines
Golden CO
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Original Message:
Sent: 11-01-2017 09:41
From: Panagiotis Kiousis
Subject: Surcharge

When you say "surcharge height", do you mean the backfill that is placed around and above the pad  (the term qNq in Terzaghi's equation)?

------------------------------
Panos Kiousis A.M.ASCE
Associate Prof
Colorado School Of Mines
Golden CO

Original Message:
Sent: 10-31-2017 15:28
From: Akintunde Alade
Subject: Surcharge

hello, 

Is it necessary to consider surcharge height when designing for pad foundation 

I assume that you mean the term in some form of the Tergaghi bearing capacity equation that accounts for the overburden or backfill above the footing bearing surface. If you are using some other method, the surcharge term may not apply. 

As mentioned by others, considering the depth of the foundation below ground will increase the bearing capacity by accounting for the confinement the surcharge provides. However, a few caveats are in order. First, if your depth of "pad" foundation, measured from the bearing surface to the ground surface, is small relative to the width of the foundation, this calculation may not be worth the effort. The surcharge also must be present on each side of the foundation during loading. If <g class="gr_ gr_2559 gr-alert gr_gramm gr_inline_cards gr_disable_anim_appear Grammar only-ins doubleReplace replaceWithoutSep" id="2559" data-gr-id="2559">load</g> is present when the surcharge is not, you may have to consider that condition, as well. Unless the surcharge is seriously unbalanced, resulting in a potential stability failure, it is conservative to neglect it. In that sense, it is typically not "necessary" to consider.


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Richard J. Driscoll P.E., M.ASCE
Lebanon NH
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Original Message:
Sent: 10-31-2017 15:28
From: Akintunde Alade
Subject: Surcharge

hello, 

Is it necessary to consider surcharge height when designing for pad foundation 

Original Message------

I assume that you mean the term in some form of the Tergaghi bearing capacity equation that accounts for the overburden or backfill above the footing bearing surface. If you are using some other method, the surcharge term may not apply. 

As mentioned by others, considering the depth of the foundation below ground will increase the bearing capacity by accounting for the confinement the surcharge provides. However, a few caveats are in order. First, if your depth of "pad" foundation, measured from the bearing surface to the ground surface, is small relative to the width of the foundation, this calculation may not be worth the effort. The surcharge also must be present on each side of the foundation during loading. If <g class="gr_ gr_2559 gr-alert gr_gramm gr_inline_cards gr_disable_anim_appear Grammar only-ins doubleReplace replaceWithoutSep" id="2559" data-gr-id="2559">load</g> is present when the surcharge is not, you may have to consider that condition, as well. Unless the surcharge is seriously unbalanced, resulting in a potential stability failure, it is conservative to neglect it. In that sense, it is typically not "necessary" to consider.


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Richard J. Driscoll P.E., M.ASCE
Lebanon NH
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If your pad is bearing at grade you don't need to add any surcharge dead or live load around it, if the bottom of your pad is below grade then you have to add the surcharge effect on you bearing stress; I hope it clarify your doubt.

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Roberto Nava Cuellar P.E., M.ASCE
KBR MONTERREY S. A. DE C. V.
Monterrey, Nuevo Leon
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Original Message:
Sent: 10-31-2017 15:28
From: Akintunde Alade
Subject: Surcharge

hello, 

Is it necessary to consider surcharge height when designing for pad foundation