What will that formula do when the rock may have cracks or weak seams? How large a diameter was your test specimen? Did it include these possible weak zones?
OG, I just remembered that higher factors of safety may be considered due to the issues you mentioned above. If you look at Das book, while typical FOS of 3 for foundation design is used, a FOS of 5 is recommended for the unconfined compressive strength of the rock. Also, although it depends on the exerted loads in the foundation , I do not give very high bearing capacities for rock. I just tried to check for the common value used in my area, despite the values I get from the calculations (which normally are very high). For example, if you have a small loaded structure and you give high allowable bearing capacities, foundations may be too small and unstable. Of course, the structural engineer will check this and may use a size of footings corresponding to lower allowable bearing pressures than the ones you have provided.
In the 9th edition of "Principles and Practices of Commercial Construction", table 2-1 on p. 9, gives a bearing capacity of 20ksf for sedimentary rock. This, of course, is a generalization and is based on a compilation of various codes and text references.
You can check out D. F. Coates work - I have the book below - but it is in deep storage in Canada . . . I remember that he had a good section on bearing capacity on rock.
I don't know for sure, Ron, but D. F. Coates has been publishing geological papers, as far as I can tell, since 1964 - which would put him up there in the 80s or so. I've never met him - I only have, in storage, the book I referenced.
Unfortunately, there is really no way to calculate bearing capacity in a jointed rock mass, without knowing a priori or estimating the relevant properties of the jointed volume.
Furthermore, if the rock mass is anisothropic, the single joint planes will govern failure conditions and you will need the orientation of the joint sets and the Mohr parameters along the governing joint sets.
I'm curious about the q/10 rule though, I may want to go and check that in an isothropic rock mass at which level of jointing it applies.
Hello
I do have Goodman's book on rock mechanics , he suggests qu= UCS x ( Nphi+1) but i guess this holds true only for good RQD quality rock and with rigid rocks like limestone in my case . Another approach consists of calculating S/B with B the width of the strip footing and S the distance between successive discontinuities and then inject them in a formula
