From: zmyd02@hou.amoco.com (Denton Meredith)
Newsgroups: sci.geo.geology
Subject: Re: Unconfined shear strength of common roc
Date: 28 Dec 92 20:03:51 GMT
Reply-To: zmyd02@hou.amoco.com
Organization: Amoco

In article 23478@monu6.cc.monash.edu.au, simon@earth.monash.edu.au (Simon Cox) writes:
>In article 15158@hou.amoco.com, zmyd02@hou.amoco.com (Denton Meredith) writes:
>>
>>>In article 9oF@mach1.wlu.ca, jbeebe6@mach1.wlu.ca (john t beebe 9208 G) writes:
>>>>Hi there. I am looking for information on the unconfined shear strength of
>>>>common rocks and soils. If anyone can help, I would be most greatful. For
>>>>example, I need to find the unconfined shear strength of 
>>>>	limestone?
>>>>	granite?
>>>>	sandstone?
>>>>	quartz?
>>>>	topsoil?...etc.
>>>>
>>>
>>In article 23924@monu6.cc.monash.edu.au, simon@earth.monash.edu.au (Simon Cox) writes:
>>>
>>>The problem is that rocks, like many other pressure sensitive brittle 
>>>materials, do NOT usually fail in shear when unconfined - rather, a sample 
>>>will split with "tensile" fractures perpendicular to the zero stress direction.
>>>Who put you onto this?
>>>
>>>SImon Cox
>>>
>>
>>
>>Not quite true.  While they theoretically should (and sometimes do) fail with tensile fractures parallel to the loading axis, the majority of failures of unconfined materials are fractures inclined at some small angle (usually less than the "common" ~30 degrees in triaxial tests) to the load axis.  This is generally agreed to be due to virtually unavoidable end effects;  in order to split in a tensile fashion, the specimen must have the ability to slide against the loading pistons transverse to the 
>>
>>
>>is, and most rigs simply don't have a low enough coefficient of friction to allow this at the stresses required to fracture a given rock.  Thus, the specimens generally fail in a shear manner.
>>
>>
>>Meredith Denton
>>Amoco Production Company
>>Houston, Texas
>>
>>(formerly of the Texas A&M University Center for Tectonophysics)
>
>
>OK - I can be a pedant too  ;-) - if you consider the real state of stress in the case that you 
>describe (with platen friction) then it is strictly NOT a uniaxial test any more. In the 
>CENTRE of the sample, where the assumption of uniaxial conditions is best satisfied, 
>the sample splits.  Or, if you use a longer sample, which has the uniform stress state 
>over a greater proportion of its length, then you will find that the failure is tensile.
>
>In any case, it is very difficult to assign a "shear strength" to the results of any of these 
>tests.
>
>
>Simon Cox


Sorry, didn't mean to be a pedant.  

All I was trying to say was that I've done a lot of uniaxial stress tests, all of them complying with ASTM standards and done in the manner generally approved by the rock mechanics community, and I'm here to tell you, very few of them split longitudinally.  They DID fail in a shear manner, as do most unconfined specimens.  So, technically, you're right, it isn't REALLY a uniaxial stress test, but (A) it's the closest we've been able to get in this topsy-turvy imperfect world, and (B) the magnitude of the s







tresses generated at the platens are negligible (or nearly so, depending on how anal retentive you are!) compared to the magnitude of the compressional stresses.

Another point;  if you use a longer specimen (longer than about 2.5:1 length:diameter) your sample will buckle and all bets are off!

Anyway, while it may be difficult to assign a shear strength to the results of these tests, it has been done.  Check the CRC book entitled something like "Properties of common rocks and minerals" or any of the rock mechanics books published by the AGU, including the Handin volume, etcetera.

Meredith