Hello all.
According to the AFTES guide, schisitosity is a type of discontinuity.
However, this schisitosity may not be an interruption of the continuity of the rock matrix but rather a rearrangement of minerals (with certainly a weak zone where detachment is easier).
For the geomechanical classification of a shist (eg. RMR), what is the rating to take for spacing discontinuities?
it will always be the lowest rating?
Rock mechanics
Tunnel engineering
Engineering geology
Spacing obtained by geostructural survey or by non contact technicques is based on actual joints.The occurance of foliation or schistosity can easily induce new fractures in the rock mass, but this fact can be interpreted in terms of strenght reduction (first term of the RMR) or by means of a care determination of RQD (second term of the RMR), taking into account that a scistohsity is connected to anisotropic rock masses.
In general terms, technical classifications put in evidence some limits for anisotrophic rock masses, so it is relevant to understand in advance which could be the expected behaviour under investigation to properly apply the RMR procedure, so to be able to decide to take the lower value of spacing or to work of RQD.
Some interesting papers are the following
Article Rock Mass Rating and Geological Strength Index of rock masse...
or
Determination and applications of rock quality designation (RQD) LianyangZhang
Journal of Rock Mechanics and Geotechnical Engineering Volume 8, Issue 3, June 2016, Pages 389-39
Regards
Hello, Thank you for both answers, Indeed, the rating can be quite difficult. As mentioned by dear sandra, wethering is favorable in areas of schistosity, where shearing often occurs. However, I also agree with Mr. Claudio. In this second case, can we, in term of foundation (behavior in compression) consider a favorable rating (the schistosity remains closed), and in term of stability (like the tunnels) considered an unfavorable rating since the schisitosity tends to be opened. respects. Rafik HEBIB
Hi Rafik Hebib,I would like to advise you that not always, in the problems of foundation and in any case of compressive stresses on schist rocks, can be considered as favorable, it depends on the orientation of the schistosity with respect to that of the solicitation. Regarding the use of classification indexes I suggest you use the GSI and consider an equivalent continuous rock mass. However, if you want to use RMR, you should calculate the compressive strength parameter, either in the laboratory or with the Point Load test, taking into account the orientation of the load with respect to the position of the schistosity.
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