Do any of you have references for or know of any correlations between unconfined compression strength and shear wave velocity? Specifically for hard rock?
Not familiar with one, but a priori, I would not expect it to be a real tight correlation, because I expect that the discontinuities would have a substantial effect on SWV, whereas unconfined is really just the strength of the best sticks of core that hold together enough to test.
In the interest of full disclosure, however, I should tell you that my knowledge of rock mechanics literature ends about 1983, when I finished my MS thesis "Undrained Behavior of Rock Joints Under Dynamic Loading."
What type of test are you using to generate/measure the shear waves? Shear wave velocity can be used to calculate the small strain stiffness (Gmax) of a soil or rock which can then be related to it's shear modulus. Following link deals with its use in measuring ground improvement work but gives a bit more background on the theory...
As dgillette says this could be misleading depending on how fractured/jointed the rock is as failure is often due to weakest plane rather than in-situ strength.
I have seismic shear wave velocity from Remi for a bridge site I am studying. I am trying to come up with an appropriate value of unconfined compression strength of the granodiorite noted at the site to backcheck my design against the other data we got from the investigation.
The rock was noted at the bottom of several hollow stem auger boring that were conducted. The shear wave velocity was 5600, and 5900 ft/s. This seems somewhat low for most of the granitics I have seen before, but it is high enough to make me think it will still act in as a fairly hard rock would. I am specifically thinking about end bearing for piles, and bearing capcity of spread footings.
I have asked our geologist to give me an estimation of what he thinks RQD would be from his experience, but we do not have actual core to get it from, or I would have just goten the unconfined from that.
I have tried to use the path that Rockmynci was suggesting to correlate with the elastic modulus and back into the strenght that way, however the units are not right.
If you use Go = P*Vs^2 and substitue into the E = 2*G*[((2+nu)*(1+nu))/nu] for transformation between shear modulus and elatic modulus you will get units of lb/(ft*sec^2). For that matter you would get those some units for Go.
Did I miss my units along the way somewhere?
Yuck. Why the didn't we ever go to the metric system?
My cheat sheet shows 5000 ft/s as typical for shale, siltstone, soft limestone, and schist, and maybe even some denser tills. 5000 ft/s -> G ~800,000 psi.
Are you analyzing the ReMi data yourself or is someone else? What is your equipment setup (phone freq, takeout dist, recording length and sample rate)? What depths are you dealing with? The book BigHarvey mentions is good place to get lots of data. I have profiled deep tunnel alignments and correlated to RQD with "good" results (as geophysical data goes) and may be able to help. Gut feeling says the velocities are a bit too low.
I had expected the velocities to be much higher than what i got. I had a local geophysist do the work for us. The first time he analyised it he only had 2,000 fps, there was to much interference. He redid it with some newer software and got the better result. The rock is either a coarse grained granodiorite, or it could be a tonalite from what my geologist researched. He only got samples from right at the contact with the hollow stem.
How well can you work up RQD from the shear velocity? That would help me some if it is possible? I was kind of assuming a moderate amount of jointing and weathering from the low velocity number, but the rock itself from the contact looked decent. So it was hard for me to judge.
