Bearing Capacity

[ar9]

I am building a slab and I will be using 12"x12" piles. I need to know the required length (depth) of the piles but I do not have a bearing capacity. I was given a Geotech "report" that was performed years ago. It is really not a report as I was only provided with the Boring Logs.

I was wondering if there is any way to calculate the bearing capacity with only the boring logs. The boring logs have a SPT for the deeper soils. Please advise, I need an estimate on this so that I do not have to overlengthen the piles.

Thanks in advance.

 

[Ron]

Since you are asking these questions, it is clear you need a local geotechnical engineer involved. He can review the logs and provide some guidance, assuming the logs are good and he's comfortable with doing so.

A pile supported slab is not for the faint of heart! You need good geotechnical and structural direction. If you have no expertise in either, steer clear.

 

[ar9]

Ron, thanks for your reply. You are absolutely correct about this, however, we are in the preliminary stage of the project and will definitely get a geotechnical engineer and new boring logs for the project further. For right now, I just wanted to come up with an approximation for the design.

I would appreciate any other responses.

Thanks

 

[Ron]

ar9...you have lots of variables to consider! First, you are using 12"x12" presumably precast/prestressed concrete piles. In order to determine the final tip elevation of the pile, you will need to determine the capacity you need from each pile, its spacing from other piles, the strata through which it will be driven so that you can determine if the pile support will come from skin friction or end bearing or both. Start with the loading and then get a geotech with local experience to give you a preliminary on pile capacity and length. Unless you are comfortable with evaluating pile capacity from the soil profile you have, you'll have to get a local geotech involved.

 

[ukengineer58]

20m . yes i just made that up completly. you may be able to determine a preliminary idea from spts in logs.if you have an understanding what you are looking at. interesting you have already decided on pile size wiyhout establishing the load on them or the depth required and how the soils conditions in the upper layers affect the design.
without being to critical if you do not know what to do and have no one to guide you you need a proffessional to advise.

 

[brandonbw]

Have you tried calling the company who did the original report? Maybe they can throw you some general numbers for your purposes?

B+W Engineering and Design
Los Angeles Civil Engineer and Structural Engineer

http://bwengr.com

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http://bwstructuralengineer.com

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http://bwcivilengineer.com

 

[ironaddict]

Hi guys-

I am not a geotech engineer by far, but have been involved in a few geotechnical recomendations in Florida. I would like to emphasize that if piles are needed? Typically when we performed soil borings, we already were given a loading criteria for our foundation footings and slabs. But first we took the worst case scenerio (boring log with the lowest blowcounts). Then we calculated a settlement (inches) for that particular loading. Very often we never needed to go with piles and the soil conditions were adequate for a conventional spread footing layout and rarely needed piles. Just trying to think outside the box for you. First, see if you actually need piles.

 

[ar9]

brandonbw and ironaddict, thanks for your replies.

brandonbw- The company that did the original testing and sampled the soils is no longer in business. That is one of the first things I tried and never could get a hold on anthing or anyone that could help me.

ironaddict- This is for a project with really thick layers of soft clay, there is no sand layer near (90') and therefore we are going with piles. This is very common for the area.

As I said before, this project will NOT be constructed without a geotechnical engineers professional assistance, I was just trying to get an estimate of the bearing capacity to see what length of piles to use and to start a construction budget. I have been researching and found that Meyerhoff proposed a correlation to determine bearing capacity depending on the N number from the SPT test. I do not know how accurate this is and dont seem to find much theory behind what he based himself.

 

[fattdad]

the end bearing is 4N (3N for non-plastic silt). Units are TSF. Use the N value in proximity to the tip grade.
the side shear is N-bar/50 (N-bar is the average N-value for the penetration into the bearing stratum). Units are TSF.
Use a safety factor of 3.

These Meyerhoff simplifications are not very precise, so you will really need a geotechnical engineer. You should get some ideas though.

f-d

¡papá gordo ain’t no madre flaca!

 

[ar9]

fattdad, thank you so much. I will give this a try.