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longisland (Geotechnical) (OP)
20 Nov 02 4:45
Hi all,
I'm looking for SPT N correlation to phi for rocks. The samples I collected ranges from very dense sand, shale & rocks with SPT N 50. Is there any reference online I can refer to ? Bowles has a pretty good correlation table for sand up to SPT 45.

Thanks in advance
Focht3 (Geotechnical)
20 Nov 02 12:00
Sorry, longisland - no specific reference to cite.  Have you checked out any rock mechanics texts?

If the material (under a microscope) is angular to subangular, composed of "durable" materials, and is quite dense, then the phi angles are probably quite high (say 40+).  Remember that phi is dependent on density, and is not an intrinsic property.  Do you "need" phi angles higher than 40?

Be aware that soils with high mica content (10% or more) may not behave as expected.  The mica particles are compressible, and one of our most fundamental assumptions about soils and their behavior is that soil grains are incompressible.  Also some non-quartz materials may not be as strong, resulting in grain breakage and unexpected settlement if some threshhold bearing pressure is exceeded.
touma (Civil/Environmental)
8 Jan 03 1:56
I believe you will have a hard time finding such a correlation as you do not learn much once you hit refusal even if you go to 100 blows.  However, if you are looking for C and Phi values in rock check Hoek and Bray (Rock Slope Engineering) and Thomlinson.  As ageneral guide, I have rarely seen Phi values above 35, generally between 27 and 35, and C estimated at about UCS/10
pigdog (Geotechnical)
28 Jan 03 10:44
longisland
Here in NC, our definition of weathered rock is N=100+. Rock is 1 inch penetration for 50 blows. Either one is much more resistance than N=50. At N=50 though, you are probably going to have residual or primary rock texture that is going to control or influence the failure. (Two samples of N=50 could yield wildly different results)

How did you get the samples for testing?  This is sort of the range that is difficult to core, and too hard for Shelby tubes.
pigdog
longisland (Geotechnical) (OP)
30 Jan 03 9:44
Hi all,
thanks for the input.
I believed most civil engineers will agree that it's pretty rare to get straight forward answer for any geotechnical problem. It's site specific; one has to visit the site or to be involved in the process to get the full picture. I may have missed out some fine details that may be the most important factor to my problem. I'd like to take this opportunity to thank all you folks out there who took the time and effort to chip in.

p/s, my company uses mazier sampling which is similar to christiensen sampler. I have problems with samples containing losts of fines or gravel. I can't get UD & the recovery is pretty bad.
DRC1 (Civil/Environmental)
3 Feb 03 12:41
SPT test is highly emperical test for soil. ASTM limits blows to 50 blows for 6" increment or 100 blows total. At high blow counts damage to the sampler begings to occur. I don't think you can corralate SPT count in rock to Phi value. Typacally, once rock is encountered, it is cored with a dimond bit core barrel and the percentage of the length pieces over 4" to the total core is known as the RQD and emperical coralations can be made from that. If you need C and Phi information, I would suggest laboratory testing of quality field samples with an appropriate adjustment forfull scale field conditions
geonet (Geotechnical)
2 Mar 03 23:00
I believe the Shaft program (ensoft)manual references a soft rock  correlation for soft rock/shale (N and qu).  I believe developed by Reese or O'Neil although not absolutely certain.  Also I have a reference for intermediate geomaterial from a Greek author between SPT and qu values.  I will look up if you desire.  Note that the SPT to qu correlation has a tremendous amount of scatter.  Just a good first estimate.
jdmm (Geotechnical)
6 Apr 03 21:15
longisland,

You suggest that you don't get straight answers from geotechnical engineers so I will be straightforward.  The answer to your question is really very simple.  There is no relationship between the standard penetration test and the friction angle of rock and from soil mechanics or rock mechanics theory there should not be any relationship.  I think we expect too much of banging a hammer on the ground.

jim
Focht3 (Geotechnical)
7 Apr 03 17:33
jdmm:

Can we talk about p-y curves now?  

ruwanraj (Geotechnical)
27 Apr 03 9:27
when you hit the rock, SPT spoon will just be bouncing for hours.  How could some one make any correlation,  Only way to find the phi angle is to do a unconfined test
BigH (Geotechnical)
27 Apr 03 14:46
aw . . . ruwanraj - you do realize, though, that "soon" someone will put the pile driving analyzer to the SPT test and get the said correlation!!  (agreed, I've bounced many a hammer on rock, and cobbles/boulders, and . . .)

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