RQD in conglomerates 2

[pelelo]

Hello engineers,

We are getting involved in a project which includes drilling few 15 m deep borings on an area surrounded by conglomerates, most likely in the top 20 mts.

The specifications state to record the recovery and RQD whenever rotational drilling is performed (e.g coring).

I understand that RQD is a way to measure the quality of a rock core extracted from an specific point of a rock mass.

However, I personally think that from an engineering point of view REC and RQD values in conglomerates (between 2.5 and 24 inches in size) are meaningless. They are just conglomerates instead of rocks.

I understand the requirement of the specifications. Sometimes specifications are grabbed from other projects and engineers don't realize that maybe there are conditions from other projects that do not apply to the current one.

Please let me know your thoughts.

Thanks.

 

[TigerGuy]

You are correct. RQD only applies to intact rock. I have had techs report to me their "apparent RQD" when drilling in boulders, just to have an idea on how large the boulders are, but it is a meaningless number outside that.

 

[EireChch]

Most conglomerates I have worked with are rock, formed of boulder, gravels etc and cemented together. They are usually of very weak 1-5MPa. I dont treat them any different to any other rock?

There is potential for fractures to form in a core when drilling if you have a large gravel, cobble in your core. It will likely fail along that edge of the gravel. However this is a drilling induced fracture which should be easily identified and excluded from the RQD calc.