soil report procedures 2

[COEngineeer]

I used to work for a geotech company. I was a tech guy that did the soil tests (I dont miss it but glad I had the experince). Anyway now I do structural engineering on residential houses. We get soil report for each project and I was wondering what are the things you guys do to generate that report.

1. Soil bearing pressure
2. lateral pressure

I understand the soil classification part and uplift for expansive clayey soil (minimum dead load pressure and stuff). How do you determine soil bearing pressure and lateral pressure? I did California bearing ratio test once but I dont remember anything to determine bearing pressure or lateral pressure. Thanks!

http://www.swijetty.com

Sea Water Intake and Jetty Construction

 

[fattdad]

I'd ask these critical questions to the geotechnical engineer in your office. There is no simple answer from afar, but hopefully for the types of projects that your firm is working on the in-house engineer that's signing the reports can provide proper advice.

f-d

¡papá gordo ain’t no madre flaca!

 

[COEngineeer]

We dont have a geotech. We get the soil report from other companies. How do you determine the lateral pressure and the soil bearing pressure?

http://www.swijetty.com

Sea Water Intake and Jetty Construction

 

[GeoPaveTraffic]

You spend several years in school learning the background theory, then several years learning from engineers that have been doing it for the career, then you look at the all the data avaiable and, using professional judgement, determine the values.

No one here can or should try to explain to you all that goes into answering your question.

If you need the lateral and bearing pressure, ask the professionals that wrote the report to provide it.

 

[COEngineeer]

OK, so the geotech does not use a specific test? Just base on the soil classification and engineering judgement? I just thought there was a test to determine all this.

http://www.swijetty.com

Sea Water Intake and Jetty Construction

 

[DRC1]

Generally for simple jobs the phi angle for sands is estimated from the stanard penetration test (SPT). For clays, c should be derived from laboratory testing of thin wall (Shelby) sampling tubes, but more often than not, it is also estimated from the SPT. Not a recommended practice.
Once the Phi angle or cohesion, c,is determined, which foir phi, involves the designer's choice of charts, plus consideration of local conditions and site specific conditions, unit wieghts are estimated and bearing capacity and lateral pressure are computed from classic equations. See Boweles Foundation Design or similar text for all the exciting details.
For larger projects, more sophisticated testing can be done.

 

[emmgjld]

COEngineer,

As the above have stated, the geotech needs to answer the question.
Assuming you are still in the Denver Area, the usual means for Bearing Capacity is the Laboratory Swell/Consolidation Test, and the SPT during drilling. An enormous amount of 'judgement' is applied. I put 'judgement' in quotes for a good reason. Some of the reasoning for the numbers go back to trial and error in the late 50's and 60's. The majority of the present crop of Geotechs do not always have a good handle on how the local judgment came about.

The question of lateral pressure is even harder to get a good answer for. The direct shear test is the simplest and most economical method to obtain Shear Strength data and then compute the Equivalent Fluid Pressure. The at rest EFP and the friction angle are then derived, in an approximate manner. The real problem comes as to reporting these values, which for clayey soils are high to very high, and taking the flack from:
a----Structurals who cannot compute what has been standing for years and continue to ask questions on this forum, trying to get defensible criteria and
b----Builders who believe the Engineering Community has lost it's collective mind.

Add in the problem of what additional pressure can be expected from expansive clays and the problem has the appearances of a crap shoot.

I won't even address the problems associated with litigation and some apparent 'hire guns' which really makes everyone closed mouth as to where the answers come from.

Sorry, the answer is not a good answer.

 

[COEngineeer]

emmgjld, thanks for the answer. I understand what you are saying. I just wanted to get a good understanding on how one generates a soil report. I always trust the numbers that the geotech gave me and I design it base on that.

http://www.swijetty.com

Sea Water Intake and Jetty Construction

 

[fattdad]

edit my earlier response: Ask the geotechnical engineer that issued the report. I'm with the others that say this is just not a one-dimensional answer. For some residential foundations folks use a "presumptive" bearing pressure. It's just based on judgement. For other projects there may be site-specific complications that require a more detailed analysis. Drilling, laboratory and structrural data can be critical to coming up with the design parameters that you've inquired about. There are too many possible methods to just say there is "one" test for this and another test for that. Geotechnical engineering differs from structural engineering as the material properties are not that well defined.

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

i started to try to answer and then realized i was going off talking about shelby tubes, pitcher barrel sampling, consolidation testing, auto vs. R&C, strength testing, shallow vs. deep foundation support, load tests, etc. etc. and deleted what i wrote and decided to let it go.
just as is mentioned above, for light loads, we generally have good idea of what is "reasonable" in our area. however,that even gets twisted when you hit certain geologies or when you're dealing with more special scenerios. so no...there's not a single, simple astm method to determine the bearing capacity or lateral pressure.

 

[Patgeotech]

COEngineer,

I think that the main aspect of all of the above threads comes down to the judgement and experience of your geotechnical engineer.

For typical light structures, the allowable movement that the particular building can handle can be predicted by visual confirmation of the soils, possibly backed up by in-situ testing if there is a degree of variability in the ground.

For heavier structures, a geotech engineer should undertake geotechnical laboratory testing of the soils to back up what his initial thoughts (regarding the ground) were. An experienced geotech engineer always knows the area and the soils he is going to encounter before he sets foot on the site (or he should at least have a good idea).

In South Africa and probably in all other parts of the world as a geotechnical engineer I see a lot of structural engineers doing the in-situ tests to determine the bearing capacity (without geotech advice) and then doing their design based on what they think is the answer - here in South Africa the common dynamic cone penetrometer (DCP) is 'abused' this way - the result is that I am OFTEN called in to investigate cracked houses due to such practices by structural engineers - in a nutshell, I am trying to say that do not look at just the testing for solutions, as the tests can be very misleading if something is not understood or has been missed in the ground - if there is any doubt in your head regarding the soil conditions or something that was unclear in your head, ASK the geotech engineer on why he decided on a particular bearing capacity or lateral pressure. More than often, there will be a good reason for his decision.

Hope this assists with your understanding.