I've never relied on RQD as an indicator of strength since it is highly dependent on the quality of your core driller. Two different core drillers will yield two different RQDs within the same weak rock profile. Not as much of a concern if the rock is very strong.
Also, RQD is a measurement of fracturing in a rock core. If there are fractures, how can you assume a tensile strength value? A sample with a 100% RQD could have fractures every 4 inches throughout the entire run or it could be a soild piece. Without seeing the samples or a good written sample description, how can you tell what you have from the RQD value, other than the rock is or may be fractured?
I have never seen RQD vs Tension Values. The Brazilian Test ASTM 3967 & 3936 are routine rock tensile tests. ASCE Publication # 16 "Rock Foundations" I highly recommend you get. As far fractures every 4 inches exactly thru-out a sample its unlikely. With an RQD of 75 to 90 the fracture spacing is wide, above 90 very wide. As a general range of tensile values: Granite is 2000psi and Schist is 300psi What kind of rock are you dealing with??
Of course it's unlikely (but possible) to have all fractures at 4 inches. My point was that, with fractures at 4 inches, the RQD is 100 but the rock is still highly fractured and should not be able to resist tensile stress. An RQD of 100 does not necessarily mean you have solid rock. 100 also does not always mean that the fractures a widely spaced.
true,
well, i am taking a different approach for my problem. can anyone suggest where i can find the phi and tensile strength for various types of rocks.
Any decent rock mechanics book will have a table of typical rock parameters for different types of rock, along with all of the appropiate disclaimers about using typical values for specific designs...
In Rock mechanics and engineering, by Charles Jeager, Cambridge University Press, 1972 (I know, that's a long time ago and things may have changed), page 119 says, "There is no known method for the direct measurement of tensile strength of rock in situ. An indirect method is to trace the so-called intrinsic curve (using circles of Mohr) in the area near the point sigma = 0. The tensile strength is expected to be about the same magnitude as the rock cohesion, but there is no proof that this applies for rock masses."
May I suggest "Rock Slope Engineering", 3rd Revised Edition by Evert Hoek and John W. Bray. This 1973 volume is an oldie but a goodie. If there are more recent versions I am not aware of them. Refer to chapter 5, "Shear Strength of Rock" for an in depth discussion of the nature and variability of rock considering fracture spacing, RQD, joint roughness, joint filling, intact unconfined compressive strength, and other factors. Enjoy!
