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Design allowable bearing capacity based on Overburden pressures5

Design allowable bearing capacity based on Overburden pressures

(OP)
Engineers,

I am working on a project that will consist of a 20 story building + 45 ft of excavation for 5 levels of underground garage.

Due the loads, and after performing all the geotechnical calculations, the best solution to support the building is on a mat foundation. After running all the bearing capacity and settlement calcs, we concluded that using an allowable bearing capacity of 8000 psf will be ok as it will provide settlements within the range provided by the local code.

A structural engineer questioned our recommendations and suggested to review the design bearing capacity as he says it needs to be 8000 psf + the Overburden pressure, in this case will be 8000 psf + 45 ft x 120 pcf = 13,400 psf.

My question is, what does the structural engineer will do with that "design bearing capacity" of 13,400 psf? as clearly, from the geotechnical point of view, the mat will settle within the limits provided by the code at 8000 psf.

From the geotechnical point of view, I am not sure what this 13,400 psf will be useful for.

Replies continue below

RE: Design allowable bearing capacity based on Overburden pressures

Does your calculation account for the overburden pressure, meaning a net allowable increase? Or, does the 8,000 psf include the pressure from the 45 ft of material to be removed?

A higher bearing capacity usually means smaller foundation elements for the structural design team.

You say settlement is a factor in the design. That is another consideration, again dependent on whether your calculations include the overburden pressure.

You probably understand this, but I'll say it for those that may not. The removed material was exerting a pressure of 45 x 120 = 5,400 psf on the foundation supporting material. So foundations exerting the same 5,400 psf should experience zero settlement, because the underlying soil does not feel a difference.

RE: Design allowable bearing capacity based on Overburden pressures

(OP)

No, I got the 8,000 psf from "local" bearing capacity analysis, such as, at bearing capacity analysis at the bottom of the excavation. Therefore the 8000 psf does not consider the Overburden pressure.

In this case, settlement is the critical part.

What I don;t understand what will be useful the 8,000 psf + OB pressure (13,400 psf) to the structural engineer if I already specified in the report that at 8,000 psf the mat will settle within the allowed limits.

From my geotech point of view, he is not supposed to do anything with 13,400 psf. If I run again a settlement analyses using 13,400 psf, settlement value will be way higher than the allowed values.

RE: Design allowable bearing capacity based on Overburden pressures

What is the soil at the level of the slab and say, 1-2 time the slab width below it, and what are it's properties?

RE: Design allowable bearing capacity based on Overburden pressures

(OP)
I am not sure how relevant this is.

All is the same. Clayey Sands (SC).

RE: Design allowable bearing capacity based on Overburden pressures

It's very alarming that you don't think the strength and stiffness of the soil beneath the foundation is important.

RE: Design allowable bearing capacity based on Overburden pressures

What is the consistency of the clayey sands below the 45 feet level?
How deep did your exploration go?
What method did you use to determine that a mat slab would be the best foundation option?
What will your final contact pressure under the mat slab be?
Tell us what steps you took to get to this point.

RE: Design allowable bearing capacity based on Overburden pressures

You are mixing up terminologies. You should only say 'bearing capacity' when you are talking about shear failure of the soil. This is youre Terzaghi bearing capacity calculation. At 45ft deep and for a large mat you will have a huge bearing capacity. As such, bearing capacity failure is irrelevant.

The only thing you should be focusing on is settlement. The 8000psf, is an allowable bearing pressure as it is based on your settlement criteria. But in saying that I am not sure what you have done is correct since you said ' you are not sure how relevant the soils to 1-2B below your foundation are'. This is the only thing that matters.

The structural engineer needs to give you a contact pressure beneath the mat foundation (say 15000psf). That is a gross pressure, you need to work out the net allowable by remove the overburden stress (120pcf x 45 = 5400psf). Your net is 9,600psf.

Do a settlement calc based on that net applied bearing pressure.

Also, if you have clayey SAND, that will act like an elastic material, then you dont really have a settlement problem. Most engineers like to make a drama out of elastic settlement, and this is probably an unpopular opinion, but in reality elastic settlement is not an issue. Since most of your load is dead load, the building will settle as construction progresses. As such, the settlement during the lifetime of the structure, after construction will be minimal.

(Now there is such a thing as time dependent creep settlement in SAND, which is applied in the Schmertmann method and Burland and Burbridge method. It depends on where you are in the world if you apply it or not. I am not sure if the US does. A lot of people just use a standard elastic theory settlement calc with no creep settlement applied/ But this is a topic for another day)

RE: Design allowable bearing capacity based on Overburden pressures

As someone who designs retaining walls, a big frustration of mine is when there is no clarity on what the "allowable bearing capacity" is actually referring to.

A too often scenario: I will get a geotechnical report that states the allowable bearing capacity is 2,000 psf for the retaining wall. Except the project is to be designed per AASHTO LRFD and the wall is 20-ft tall, meaning the weight of soil (assuming 120 pcf) is more than the allowable bearing capacity before we even design the wall. So, I submit an RFI asking if the "allowable bearing capacity" is really a "net allowable" (since the footing will be placed 5-ft below existing grade) and whether the allowable bearing capacity can be compared to the serviceability limit state (since the project is required to be designed in LRFD). The response I usually get is "design per the geotechnical report and lengthen the footing/reinforced zone as required."

Back to your scenario. You should state that either the 8,000 psf is "net allowable" or the contact pressure is 13,400 psf (this says the same thing, but showing 13,400 psf will lead to less confusion as the contact pressure presents it in the language the structural engineer and any reviewing authority speaks). If this is not what you are trying to communicate, and 8,000 psf is the maximum pressure allowed under the foundation at elevation -45ft, you are implying that 25 stories of building can only increase the pressure by 2,600 psf compared to 45-ft of soil. That seems an impossible task.

RE: Design allowable bearing capacity based on Overburden pressures

If it doesn't make sense to you from a geotechnical point of view, maybe you should be asking in the structural engineering forum if it makes sense from a structural point of view.

Maybe it's related to the structural design of the mat itself (thickness, rebar placement, etc) , maybe not.

RE: Design allowable bearing capacity based on Overburden pressures

@pelelo:
To give you some more context from the structural side here is how the design process would go on our end.

1. Get Geotechnical report - see recommendation for mat foundation, allowable bearing pressure, and spring constant (hopefully you also provided this as without it we can't really design the mat foundation as our software requires a spring constant)

2. We then generate our structural model get our foundation loading and jump into our mat foundation software package of preference.

3. Model the mat foundation and support it with an area spring and run the model

4. Generally the software will post-process the spring reactions into a bearing pressure map, we will then review this against the allowable bearing pressure in the geotechnical report. It is here where the engineer is likely getting pressure results in excess of 8,000 psf and did a quick napkin calc to see if it is reasonable and determined that the existing 45 ft of overburden nearly exceeds this 8,000 psf value and is now questioning if they should be checking their contact pressure in the software against 8,000 psf or 8,000 psf + 5,400 psf (existing overburden) = 13,400 psf.

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