Limiting the bearing capacity

[killswitchengage]

Hello
Do you ever limit the allowable bearing capacity to a lower value? For ex from 10 bars to 4 bars

 

[Ron]

This is common. Why do you ask and under what conditions?

 

[killswitchengage]

I asked because i find it weird that we should limit a bearing capacity pressure knowing that the computation and tests were done correctly . This question relates to another one i posted elsewhere in this forum about raft foundations , but my company uses this procedure a lot . We as engineers if we make sure everything was done correctly its all to the benefit of the client , he is the lucky person to get a good bearing soil !
the project i am referring to is a 9 storey building with 4 underground parkning lot

 

[Okiryu]

killswitchengage, you did not provide subsurface information and unless your bearing layer is the bedrock, I would not be worried about bearing capacity, I think that settlements may control the foundation design.

 

[killswitchengage]

well the layer bearing the foundations is a pure marl with Cc about 0.036 i believe and Cs 0.01 (need to check this in my office) Ip about 25 % with Wl 50 %, this layer goes on til the end of the borehole thus its about 14 m deep (from 6.00 m - 20.00 m ), minus the depth of anchoring at 12 m the thickness of the layer underneath the foundations becomes 8 m .

 

[darthsoilsguy2]

what you're describing only really works when there is a good relationship between Structural and Geotech and they communicate often about what each other needs and are comfortable designing to. example of good intentions causing confusion and delay below.

What if the structural egr is comfortable designing in the 2 to 4 bar region because he/she accepts there will be construction defects and wishes to minimize those impacts? Or the structural has a tight budget and never signed up to redo their details and check them against higher stress concentrations? Or they've already gone down the road with the design and don't see a big enough benefit to the project to optimize. Don't expect the structural engineer will say they took this approach to the project team, they will just put the number down on the project plans and sleep super-well at night knowing the "secret safety factor" protects them alone if forensic review were ever needed and since it will be hard to mess it up that much the Owner is protected in the process. Meanwhile, the testing/inspection firm is out there recommending undercuts and freaking out left and right anytime something doesn't exactly match the expected conditions from the soils report.

 

[darthsoilsguy2]

fyi. i'm not implying you aren't communicating well with structural. on a project like you described, there should be some back and forth. just giving a good reason why geotechs might ratchet down the recommendations to what they expect the structural will make good use of based on the building size

 

[killswitchengage]

Just a reminder , i am calling here from the third world so brace yourselves for wild stories.
Anyway, i know the bearing capacity will influence the foundation design but it doesn't effect the structural design right ? only the settlement especially the differential type is important in that regard .
And let me tell you i work for a laboratory that conduct only tests and gives the allowable bearing capacity just for the record . We don't get in touch with the structural firm often.

 

[fattdad]

allowable bearing pressure is dependent on foundation size. When writing geotechnical recommendations, we may have some idea of column loads - but the structural engineer may be up to something we haven't thought about. In light of what's known/expected, sure I'd limit the allowable bearing pressure to what I anticipate, by what formed the basis of my field exploration program.

I'll limit allowable bearing pressure also based on serviceability considerations - i.e., settlement criteria.

There's also the entire realm of reliability - I mean how often do we actually know the full story. . . ?

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

I agree with fattdad. Serviceability will normally control. Since the geotechnical investigations are done during the preliminary design stage, we do not get too much information from the structural, so I state in my reports the assumptions for the column/wall loads and then recommend allowable bearing pressures for a certain range of sizes and depths of foundation penetration (Df). BTW, why don't you check what are the typical allowable bearing pressures for marl soils in your area and compare it against what you are recommending?

 

[killswitchengage]

So i get that its wisdom or experience dependent . But here we usually limit it because it sounds way to scary to put forth high bearing capacity , they offer even 3-4 bars for rigid rocks !
Could you guys reply to my last question about the impact of bearing pressure limitation on structural design ?
@Okiryu the philosophy here where i live doesn't and probably will never raise to that level of professionalism , many people ignore what is true geotechnical engineering

 

[fattdad]

a testing firm should not give allowable bearing pressures, "Just for the record!" How do you know? How do you know how thick the layer of the tested soil is in the field? You are not providing service in extrapolating laboratory data to the field when you don't really know the field conditions, you don't know the column loads and you don't know the expectations on settlement.

I work for the U.S.'s third largest DOT. I manage the geotechnical program for our statewide program and I also manage the consultants that work on our projects. I receive all the design-build geotechnical submittals (well with few exceptions). You have no idea how often I see laboratories submit certificates that include extraneous information, "As a service." I reject all these certificates! If I allow any design engineer to rely on the free service, then I get dragged into the liability. I won't do that! So, if a laboratory certificate for a consolidated-undrained triaxial strength test (with pore pressures) returns a cohesion intercept and a friction angle, I reject that or I require that a professional engineer stamp the certificate. ASTM considers the Mohr-Coulomb failure envelope a professional service. We all acknowledge that firms that only provide laboratory testing are not providing a professional service. It's a very important service, but it's not engineering. The assignation of failure criteria is related to strain compatibility and soil-structure compatibility. Now, if folks don't know that, that's o.k. . . we do. If folks try to make it like it's not relevant, I don't buy it!

I let the engineers do what the engineers are supposed to do and I let the labs do what the labs are supposed to do. We procure engineering services on a time and materials basis and we procure testing on a unit cost basis. These imply different levels of service.

I have no problem limiting bearing pressure. I just can't write up all the reasons an engineer may elect to do that; however. Mostly because engineering is complicated!

f-d

ípapß gordo ainÆt no madre flaca!

 

[killswitchengage]

Well i should clarify that we usually write down : Since the client did not provide any value for the total vertical load per column , we have computed the following bearing capacity as a guideline only and the engineering firm is the only one responsible for the final value adopted for the design .

We then go on by assuming a strip footing in most cases with dimensions 1x1 m and conduct the analysis with any available method . And mind you we do know field conditions since we conduct the borehole tests

 

[fattdad]

so, you provide professional engineering services then? You carry professional engineering insurance for engineering negligence? I'm just a bit confused by the business model. You drill by the foot and provide testing by the unit and just give away professional engineering advice?

Wow!

f-d

ípapß gordo ainÆt no madre flaca!

 

[darthsoilsguy2]

this thread has lost me completely. who is the 'engineering firm' and what do they do?

 

[killswitchengage]

Well there are what we call engineering bureaus that design the structural part as well as the foundations with the help of the results we give them. But we conduct stability analysis and embankment ourselves , we are not intitled as an engineering firm that is

 

[Okiryu]

Engineering firm or not, you are responsible for the information you provide to somebody who will use it for design.

Also you wrote: "the philosophy here where i live doesn't and probably will never raise to that level of professionalism , many people ignore what is true geotechnical engineering" I see you have passion for what you are doing, and I think that you now have chances to make people know what is true geotechnical engineering, it will be a slow process, but failure/costly repairs/expensive designs happen, so at the end people will understand you. Good luck!!