NQ vs HQ rock cores 2

[Silty]

Hi everyone, what are limitations of NQ coring for geotechnical investigations? The structure is a light weight industrial structure which will be on piles penetrating to the rock and the client want to use NQ coring. Is there any limitations in type of information or tests that can be done on smaller NQ cores versus HQ core. Thanks

 

[EireChch]

I have looked in to this briefly before. Generally the bigger the core barrel the better your recovery, which means better samples for tests. Although on the flip side, Hoek and Brown show that UCS decreases with increasing core diameter.

Based on International Society of Rock Mechanics (ISRM) Suggested Methods (ISRM 2007), the diameter of the core specimen used for uniaxial compression testing should not be less than 54 mm, while it is 47 mm for the American Society for Testing and Materials (ASTM; ASTM 1994; ASTM 2000).

NQ = 47mm dia so still acceptable to run UCS test. Note sure on any other limitations.


We typically use PQ which is 85mm dia core. Sometimes we have to change to NQ to run pressure meter tests. Have never been questioned on the core size possibly leading to lower UCS. I think clients might be more concerned with quality of rock recovered that a slight loss of strength as fractures, discontinuities have likely a bigger effect.

 

[Silty]

Thanks for the detailed response. The difference in lab requirement is interesting. It appears that for poor rocks it’s better to get larger core but if the rock quality is good, NQ may be enough. One article was saying RQD should be based on a diameter more than 54 mm, but I see many practitioners get RQD from NQ size. Thanks again

 

[miningman]

There is another obscure consideration which I have personal experience with. There isnt a huge cost differential between the cost per foot on different hole diameters.. When coring in atrocious ground conditions, if ope starts with a larger diameter, when the drillers start to encounter major difficulties they then have the option to drop down a diameter size and sucessfully complete the hole.