So, just for clarification:

You do have a block of rock that has an open fracture, you perform a shear test and observe, that the surface roughness of the upper part has increased after the experiment? Did I get it right?

Dear Halisch,

Yes, that was question.

I am not that much a rock mechanics expert but maybe the publications I have added with the links might help. ;)

http://www.posiva.fi/files/2854/WR_2012-25web.pdf

http://www.gruppofrattura.it/ocs/index.php/ICF/ICF10/paper/viewFile/4741/6748

http://oro.open.ac.uk/5495/1/JVGRbalme01_corrected_for_publicpdf.pdf

It is important to see the morphological character of the joint. That way,  the placement, application of the load and orientation of granulation may provide you interesting results.

Dear Prof Trivedi,

Thanks for concerning my question and gave valuable information. I am working on granite (grain size equally distributed in upper and lower block) but result was different. Upper block became more rougher than previous roughness of upper block. Till date actual reason is not clear. Could you please clarify your answer.    

Amid the geomechanical processes the importance of I1, J2, J3 is seen to chip off the granulation as they fall to the gravity, hence result in greater roughness of those faces selectively. Repeat the test on yet another DST equipment to see reproducibility keeping entire set of conditions the same. J3 may be controlled by the joint granulation or equipments issues.

Dear Prof Trivedi,

Whatever you are saying that is right and that is other parameter which influence shear behavior but as i mentioned upper block and lower block made by  same material, if granulation cause chip off due to shearing, in that situation lower block should also show same characteristics features and trend but scenario was different lower block found lower roughness than previous roughness while upper block shows different as question was asked.     

I do not remember such a phenomenon from our 130 natural joint samples. On the contrary, JRC reduced due to highly-stressed damage 'islands'.

Dear Prof. Barton,

Thank you very much for your kind reply. I am agreed with the concept “JRC reduced due to highly-stressed damage 'islands'” and in general, I have also observed similar case during my study as I have used unmatched very planar rock joints. However, this phenomenon differs only for few natural joint samples in which I have observed JRC increased with normal stress in the upper block as well as lower block due to chipping off mineral grains from the joint surfaces. Moreover, I have also observed JRC increased with increasing normal stresses in one block only (i.e., sometimes it happened only in lower block or in the upper block), due to chipping off mineral grains from top/bottom block. This observation has been found only in few samples not for all samples.

It is to note that I had asked this question four year ago (during the initial phase of my research) when I faced such unusual results in consecutive two samples of very planar unmatched joints.

Thank you again for your kind reply

Hemant K Singh

Dear Neil,

Thanks a lot for your reply and I am agreed with you that I have observed some rare and unique results (i.e., exceptional results in consecutive two samples of very planar unmatched joints) that’s why I inquired this question four year ago (during the initial phase of my research).

It is true that I have observed such increased roughness (after testing) on a very-planar slickenside sample due to mineral grain chipping off and agreed with you it will occur on both sides of the joint surfaces (also observed in my case). However, in some cases of unmatched and very planar joints, I have seen JRC increased with increasing normal stresses in one block only (i.e., sometimes it happened just in lower block or in the upper block), due to disruptions of mineral grains. This observation has been found only in few samples.

Thank you again

Kind regards

Hemant K Singh

What type of natural material are you testing? The grain size may play a role or the level of normal stress that you are applying

Dear Mr. Hemant K Singh

I developed some shear testing on fractures molded in mortar to control roughness and compression strength of the samples, going from planar to very rough joints and from 25 MPa up to 95 MPa in JCS. The results always showed roughness (JRC) decrease after the shear tests. Are you testing with very low compression strength rock samples? Do you noticed this behavior even for large upper block/lower block displacements?

Anyway your case is very interesting.