RQD and Point Load for Rock

[SLaryea]

Good day colleagues,

I’ve just received some lab results on rock that don’t seem to make sense.

It’s on siltstones and sandstones

Excellent RQDs throughout but the point loads are less than 1Mpa.

Am I right to question these results or are they plausible?

 

[TigerGuy]

RQD and load capacity can be independent values. I'm not sure I understand the question.

Did they provide photos of the cores?

 

[EireChch]

I work this rocks in the Middle East and it’s very common to have PLI less than 1MPa.

Have you no local experience to get an idea of typical rock strength

Also good RQD doesn’t necessarily mean high strength.

And if you have good RQD then just use UCS

 

[SLaryea]

Thanks both.

Unfortunately I don’t have local knowledge to confirm typical strengths but I’m looking it up.

I’ve always been of the view that RQDs relate in some way to rock strength so with the logs showing 90% RQD, I was surprised at the low PLT.

I don’t have UCS for the samples but I’ve tried to use correlations but there are that many of them, I question the accuracy of the approach.

Of what use then is the RQD for design purposes

 

[TigerGuy]

RQD is an indicator of how solid or unfractured the rock mass is. If you are drilling in a boulder field, you have no RQD, even if you have long pieces of core retrieved.

If you have an RQD of 0.25, but a strength of 100 MPa, you need to really look at how fractured and deteriorated the rock actually is.

 

[EireChch]

You need to find a k factor that is appropriate for your local. In literature k is often presented as 16-24 for most rocks however this isnt true.

We have thousands of UCS and PLTs at the same depth in rock strata. k is typically between 4-6, so a lot less than 16.

Our rocks are extremely week to weak, design UCS of 1MPa is common.

And TG brings up a valid point when assessing rock strength. UCS testing is on a discrete intact piece of rock. The discontinuities dictate the performance of the rock mass as a whole.

You need to be considering the Hoek Brown GSI method to properly assess your rock. Note, its only applicable to jointed rock mass so if RQD is near 100 then your UCS test results are applicable to your whole rock mass.