Allowable Soil pressure in New Mexico

[NqChan]

Hi All,
I’m designing foundation (drilled pier) for some pipe supports in New Mexico. At this stage, we don’t have soil report yet. As I look at drawings done by other, there are soil bearing assumptions that I am not sure about:
1. Allowable end bearing pressure: 12.5 ksf.
2. Allowable downward skin friction: 250 psf.
3. allowable uplift: 200 psf.
Question: are these assumption reasonable (conservative) in New Mexico. On the detail of pier depth, they stated: “8’-0” or to refusal”. Does this mean that the bedrock this shallow?
Any advices or direction to web site in New Mexico would be appreciated. Please note that I am not familiar to this area.
Thanks

 

[jdonville]

NqChan,

I don't know what the presumptive soil strengths "officially" are in New Mexico, but based on the values you've quoted, the end-bearing pressure indicates an "equivalent" su of about 3500 psf (cohesive) or "equivalent" uncorrected SPT N value of about 25 to 30 (cohesionless).

The downward skin friction is 25% higher than uplift, which looks OK. "Equivalent" su for side friction is about 400 to 500 psf, assuming full development of ultimate capacity.

As for the detail dimension of 8 ft or refusal, it could be that bedrock does get that shallow. You should check with the NM Dept of Natural Resources or State Geological Survey, etc. to see if they have any freebie downloads for soil and bedrock data in your project area, if you're that concerned about it.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[BigH]

NqChan - these sure wouldn't be the values in areas of firm to stiff clays - and I am sure that NM has some of these areas. I would suggest you get an US Conservation Authority surficial soils map and get a handle on the type of soils/rock you would likely encounter. Then add this to this post. We might be able to help you. It is common, as jdonville says, to have uplift be slightly less that download. A lot less if the piles are tapered!

 

[DRC1]

12.5 ksf sounds like end bearing on rock. Check with local building department. Usually the code has presumptive bearing capacities and the building officals may be familar with soil and rock conditions.

 

[BigH]

Sure it does - that is why he needs to do some kind of soil assessment first - he really needs his geotechnical engineer to be involved - if he has one.

 

[NqChan]

Thanks to all of you for your fast replies. I really appreciated your helps. I'm really not that good with soil. most of the times, I would get a report from Geotechnical engineer to design fdn. This time, it's just preliminary phase so I have to asssume something conservative (reasonable) that I don't have to change my design much in the future.

Jdonville: what's the su meaning? I couldn't find it in my soil book (Essentials of Soil Mechanics & Foundation by McCathy)
BigH: can you direct me to where I can find US Conservation Authority surficial soils map?

Thanks to everyone again.
Nqchan

 

[BigH]

I would look up their county office in your area in the phone book. At least in Indiana, all counties have them. They would have the appropriate maps. They can be quite detailed. My mum lives in farm country and it is amazing the details (and different surficial soils) of the farmer's fields around her. Same field, 3 or 4 types of soils - it helps on fertilizer requirements. I have always found them useful to give ideas of soil types - then you at least have an idea of what you are dealing with.

 

[NqChan]

BigH, I live in Denver, Colorado. This project is in New Mexico, that's why I Don't know about this area.

 

[jdonville]

NqChan,

"su" is undrained shear strength: su = qu/2
"qu" is unconfined compressive strength
"c" values are commonly taken to equal su

Note: " c' " is effective stress cohesive strength and is not necessarily equal to su.

The propoer name of the US Conservation Authority is the Natural Resorces Conservation Service. A link to state offices is here:

http://www.nrcs.usda.gov/about/organization/regions.html

To find contact information for local soil and water conservation agencies (usually by county) try here:

http://www.nacdnet.org/resources/NM.htm

Good Luck!

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[BigH]

Jeff - knew I could count on you for filling in the details!! If NqChan had problems living in Denver, how about me in West Bengal India!!??? for you!

 

[NqChan]

Thank you all for your great help. I feel very lucky that I found this site.

Best regards,
Nqchan

 

[Panars]

The New Mexico Soil Survey appears to have soil survey information online. Look for the "Web Soil Survey" on this page.

http://www.nm.nrcs.usda.gov/soils/

Another good source for geological information is the geological survey for the state. New Mexico's is called the New Mexico Bureau of Geology and Mineral Resources

http://geoinfo.nmt.edu/index.html

They seem to have extensive geological maps online. You may be able to find depth to bedrock on these maps.

 

[tocoadog]

I work in Albuquerque, and unless you are in the extreme eastern part of the city, shallow rock shouldn't be encountered. We have the Sandia mountains that border the eastern part and I've had borings refuse at 3 feet in decomposed granite.

As for the bearing values, those seem reasonable to me. Low skin friction, meaning the overburden soils above the auger refusal materials are weak and a fairly conservative end bearing pressure indicating fractured rock which is a reasonable assumption for Alb. Do you know if the geotech company has attempted coring the material? Also, where in town is the development?

 

[tocoadog]

Another view to look at, if the rock is that shallow, a mat foundation may be more reasonable.