soil properties in Phoenix area 2

[tvar]

I am working on an estimate for a client who is looking to put a facility somewhere in the southwest. One site is within a 70 mile radius of Phoenix (don't know the exact location due to confidentiality agreement) They had contacted a geotech in the area and were told that in general the allowable bearing is 1500 psf @ -1.5'. There is a layer of sand/silt 10'-20' transitioning into a layer of sand/gravel/clay (S/G/C) with an allowable bearing capacity of 7-8 ksf. He said for heavy loads caissons are used. My questions are: For caisson design what values can be used to determine the skin friction in the (S/G/C) layer? Any other information would be helpful as I am from the northeast and not familar with the soil conditions in the southwest. Thanks

 

[unsrtn]

That's a very general question.
How much load can I place on a steel I-beam?
10,000 pounds ?

 

[tvar]

unsrtn
Because of the confidentiality and preliminary nature of the project this is all the information I was given and all I will have for this stage of the project (estimate)

 

[whyun]

Is there a geotechnical report that addresses the recommended design values in that area? If there isn't a report available, ask the client to hire one.

 

[tvar]

Because a site hasn't been selected yet that is hard to do. There will be more than one site considered once the client has determined that a building a plant in the area will be profitable. This phase is a feasability study to determine the costs and returns of location in the southwest. Engineering judgement along with what ever information I can get on the area and not a specific site will be all I have to determine the estimated foundation costs. Thanks for your response

 

[JedClampett]

Most soils in the Phoenix area are pretty good. Due to this, if we need caissons they usually are end bearing only. If you need friction values, you'll need a specific Geotechnical Report. Assume end bearing for budget purposes.
The 1500 psf number seems kind of low. I would expect 2000 psf near the surface increasing to 3000 psf a few feet down.
We do have some expansive clays in the valley. While they are not as bad as Texas expansive clays, they are a nuisance and can cause problems in residential construction. As the region expands, there is more and more construction on these expansive clays.

 

[tvar]

JedClampett,
Thanks, I heard they were expansive but didn't require void spaces under like are required in Texas

 

[JedClampett]

I'm not really into residential construction, but I think they're going to more post-tensioned slabs to avoid problems.
For industrial and/or commercial we can usually just over excavate a little and replace with good dirt. When I mention void forms I get a lot of blank looks.

 

[tvar]

Thanks again, I was going to put 12" structural fill but may bump it up to 18" for the estimate

 

[tvar]

JedClampett,
You mentioned that you design the drilled piers counting on end bearing only, do you have a typical bearing value and depth? Thanks

 

[JedClampett]

The depth wasn't a big issue for the last project I used them on (we were just trying to get below some old fill) and we used 7500 psf end bearing.

 

[tvar]

Thanks, The inforamtion won't be used for final design but it has been very helpful.

 

[GabeB]

I know a very good geotech in Tuscon who knows the Phoenix area and southwest very well, he has 40+ years experience and can probably tell you what you need to do, his name is Bob Booth, his number is (520) 219-7503. Most of Phoenix and that alot of the area in and around Phoenix is in the Salt River formation which is extremly hard and cemented sand and gravel (not very good for caisson drilling), may not have to put your building on piers if you locate to where the Salt River formation is close to the surface which would save you some money. I suggest you contact Bob and pick his brain, tell him Gabe told you to call.

 

[tvar]

Thanks GabeB. I will call him.