for the calculation of peak dilation angle of irregular rock joints, which angle is to be taken into consideration. the maximum dilation angle, which may or may not come at peak shear strength or the angle that comes at peak shear strength, which may or may not be the maximum one.
please suggest...
Both of them have a physical meaning and play a role in the response. Ideally you could use a model in which the dilation angle varies with the relative displacement across the interface. In the absence of that, take into account that the higher the dilation angle, the higher the confinement generated during shear; this will allow you to determine how best to approach the problema in a conservative fashion.
calculate dilation angle considering both points (1. maximum during test, and 2.point of maximum shear strength) separately. In case of rock mass, 1st point is important, while in case of rock joint or rock having single joint second point is important.
thank u all for your suggestions..hope to get some more suggestions/ disscussions
This is explained in some detail in Hencher & Richards 2015 Rock Mechanics & Rock Engineering and in Hencher 2015 Practical Rock Mechanics.
Dilation in a rock shear test on a piece of core or such-like will be sample- and direction- variable as well as stress-dependent so does not help much. To make use of test data you need to correct the stresses using measured dilation at very small increments. This is not easy but is essential. After correction, the data reveal the shear strength of an essentially planar surface with the correct polish/texture/mineral coating. Most commercial labs do not do this properly and as a result the data are simply useless. The ISRM guidelines are simplistic and again essentially mean that the direct shear test on discontinuities is regarded by many as unreliable (correctly) - essentially an unrepeatable index test.
For a series of tests on rough discontinuities through silicate rock like granite or sandstone, done properly, the corrected dilation-free friction angle is typically about 38 to 40 degrees from my experience (even where coated with FeO or MnO2). Other coatings such as green, hard chlorite can give much lower 'basic' friction angles. [Do not confuse this with the 'basic' friction angle from a saw cut surface].
By observation of sample performance you can judge whether or not roughness features will survive in the field situation and contribute to shear strength (assuming that tests have been carried out at the correct, representative, engineering stress levels, mimicking the field situation). Note that stresses calculated on gross area are much lower than they will be at actual roughness interactions - but that is true of both lab and field. Byerlee did famous work where at very high stresses dilation was completely suppressed (i.e. roughness sheared through) and again his resulting value of friction was ATAN 0.85.
You then have the problem of what allowance to make for field roughness. Obviously large waviness will help (if resisting direction of shear). Smaller scale roughness of the type that survived in the lab can be relied upon if pervasive and the joint has not already dilated. It may well be justifiable to add on of the order of 5 to 10 degrees for small scale roughness but it really is a matter of careful measurement and judgement. An alternative for design is just to use the basic friction (non-dilational) in the analysis and allow a lower FoS for this part of the equation. Other unknowns such as water pressure (negative) and rock-bridge cohesion (positive) will probably be even less well established than friction/roughness dilation.
Any dilation angle that has the most negative impact on final performance of the bearing capacity or...........of tested rock is suggested to be considered.
I read all the comments above and they are very good and helpful. What I suggest is to be aware of the inclination of your joints and make sure to put them as horizontal as possible, as inclination definitely affects the measured dilation. Hope this can be of help.
@yanrong Li ...thanks for your suggestion. During encapsulation of samples we took care about this issue and kept the joint plane as horizontal as possible.
Dear Prof. Steve Hencher
Thank you on explaining the query in detail. I already gone through your papers and they are very much helpful for me.
Dear Behzad_Kalantari
Thank you for your suggestion. But I have a doubt, if time permits then, please clarify it.
my query is
As far i know both the positive and negative dilation angles have some significance .so how to differentiate which one have the more negative impact on bearing capacity from the test data. if any article is there, then please suggest that
Can I suggest that for those wanting to understand how dilation at lab scale translates to the field scale (in a realistic rather than an empirical/theoretical way), Professor Goodman in his Introduction to Rock Mechanics (1989) gives a very good discussion, incorporating the works of Patton, Schneider, Fecker & Rengers, (all of which is very good in my opinion) from pages 164 to 171. Well worth going through in detail.
Bikash, regarding bearing capacity, of course roughness will play a part in the way that the rock mass deforms but generally Bearing Capacity is defined broadly and simply with reference to UCS and degree of jointing (British / EU Standard) or percentage weathered rock (HK standard). It is too tricky to think about positive/negative dilation within a confined rock mass other than in numerical modelling perhaps.
Roughness is accounted for (by some) with respect to rock sockets for bored piles, but again rather more empirically than analytically as one might do (should do) for rock slab failure in a rock slope.
Dear Prof. Hencher
Thanks a lot for a quick response. I will definitely go through the book, you suggested.
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