I am working on natural rock discontinuity, and found peak friction angle less than basic friction angle. Is it possible?
Can peak friction angle less than basic friction angle in case of natural rock discontinuity?
Obviously not. However, the observable friction angle can be smaller tan the basic friction angle due to weathering or polishing of the discontinuity.
Also rememeber that the friction angle introduced in the (not-so reliable) Barton's formula is the residual one fi_sub_r this is eqaul to basic frictoion angle minus 20º, + 20 multiplied by r/R. Where r is the Schmidt rebound count on the actual discontinuity to be analysed and R is the same value on sound rock.
Interesting examples by Hencher are availabele in the literatura with firction angle around 10º associated to previous shear movements (polishing) of the joint.
The right answer is: NO!
The basic friction angle is approximately equal to the residual friction angle.
The basic friction angle is the angle of friction of smooth planar sliding surface. Barton -1973 - suggested that the second order play an important role in Patton equation at low normal stress. Finally Barton proposed the values of Phib + i for limestone, shale, quartzite, gneiss granite and amphibolite discontinuitiesat conditions of low nomal stress. The value of Phib+i represents the peak frition angle of discontinuities surface roughness. After Barton the peak friction angle it is estimated from JRC.
Thanks to all of you for valuable suggestion. That might be due to weathering of discontinuity surface.
If the rock material is the same obviously no! But if the natural discontinuity is weathered it is possible that the basic friction angle be higher than the peak one. However, we have to commwent that in the case of wethered discontinuity the comparison of respective friction angle is not correct as they refer to different material: the weathered on one side and the non weathered rock on the other side. Finally I am a little bit puzled by the equivalence of basic friction angle (the angle obtaine on a fresh and polished surface) with a residual one (obtained after a complete shear).
Never. Peak friction is usually greatest friction angle in both peak and residual conditions.
Theoretically yes, but it is practically possible. As in generally discontinuity is composed of uneven, the walls roughness, and filling, it can be considered that the base friction is approximately equal to the residual fraction, obtained in the shear experiments. However, in the shear tests there is possible to obtain a negative dilatance in the field of shear fracture. In this case, the peak shear angle may be less than the residual friction angle or basic friction angle.
If basic friction is a property of the rock, and peak friction is a property of the discontinuity, and the discontinuity is *different* than the rock (due to polish, weathering, clay infill, etc), than yes.
In addition to, if you sheared rock samples containing high moisture content. Peak friction can be varied low than residual friction angle.
Actually there are common misunderstandings in that people use the terms 'basic friction' or 'residual friction' angle in empirical relationships where 'basic' is anything but basic (in the sense of lowest) and 'residual' is specific to the starting surface and sliding conditions.
True 'basic friction' (lowest achievable) is actually that of the minerals making up the rock, with no textural roughness - for silicates that can be about 6 or 7 degrees (see for example Lambe & Whitman's textbook on Soil Mechanics). For a typical flat saw-cut surface through rock the frictional resistance (non-dilational) might be about 30 degrees but much of that resistance is derived from the surface finish textural interactions. That textural interaction (like sandpaper) can be reduced by preparation or polished away experimentally by repeated sliding approaching the 6 or 7 degrees minimum. For natural silicate joints such as granite (rough) the non-dilational 'basic' friction (angle it would slide if texturally rough but not sufficiently rough to dilate) is typically about 38 to 40 degrees (This is also the value Byerlee (1967) found when testing at very high stress levels so that dilation was suppressed). Interestingly it is also a typical critical friction angle for weathered granite (soil-like). If coated with chlorite or, of course, clay the friction of natural rock joints can be much lower (I have investigated rock slope failure where the friction angle was much lower than that of a saw-cut surface through the parent rock).
Where the joint is rough there will be additional strength due to dilation (stress dependent)
If the joint is impersistent (with rock bridges or healed sections) then for shear strength there is, in addition, true cohesion which is very difficult to predict and generally ignored in analysis. Apparent cohesion is often mis-interpreted for persistent, open joints, from lab test data where proper attention has not been paid to the dilatational contribution that varies throughout testing. Any set of lab data presented with c/phi should be treated with extreme caution for design.
I completely agree with the answer of Steve Hencher. The nature of the debating problem originates from confusing basic and residual friction angles.
I already answered to this question almost 2 years ago. In principle, peak friction cannot be less than residual or basic one
I changed my replied before this. Belonging my investigation of sheared rock samples. Peak friction angles are higher or nearly same as residual friction angles.
The peak friction angle of a discontinuity is mostly greatest. This is followed by the residual friction angle and then basic friction angle. All these friction angles however, can be equal when discontinuities are weathered.
You should look at anisotropy and difference fabrication between field and laboratory.