Interpreting SPT and N values 3

[ijlm]

I have boring logs on a project which give SPT values and a classification chart for the N values. Some of the SPT values have two numbers, eg. 50/3. I understand this to mean it takes 50 blows for the split spoon to penetrate 3" - is this correct? (I am not a geotechnical engineer!) I also presume that this is very hard material (Yes?) but why would the blow count not continue until the foot penetration was achieved and report the resulting N value (blow vounts per foot)

 

[StephenA]

If the result is 50 blows for 3", then there is a good chance that you will damage the spoon after 12".

In any case the spoon was not designed for such hard material

StephenA

 

[DRC1]

Refusal is often assumed to be 100 blows per 6 inches.
Over driving will damage the ring at the front of the spoon.
If the ring is damaged, it can alter the rest of the samoling.

 

[BigH]

ijlm - yes, it is hard material - likely you are either on a bedrock (we usually get 50blows/2inches in Toronto in the hard weathered shale) or, perhaps, you are driving in dense material but your spoon is on a boulder or cobble. No sense to go over 100 blows per sample location, and, if it is in the order of 20 to 30 blows for an inch or two, that is enough. What else are you going to prove by continued driving? Squat, I would believe.