IBC Site Class Average Blow Count N Value 1

[cap4000]

Does anyone know the formula for averaging of the N value per the IBC Site Class Definition. I have the following boring log. Down at 2ft N=20, to 4ft N=25, to 6ft N=32, to 8ft N=42, hit solid rock at 9feet N=70. Any tips will be greatly appreciated.

 

[cap4000]

My average blow count is 95.4 over 100 feet per IBC. Can some one verify that number. Thanks.

 

[Ron]

No. Your average blow count can't be determined over a 100 foot depth since your boring was terminated at less than 10 feet. Since you don't have sufficient data to do such an average, the code instructs you to use Site Class D.

 

[cap4000]

Ron,

I have less than 10 feet of soil to bedrock. The Site Class is B and could be an A with additional shear wave testing . I have 91 feet of bedrock. Site Class D I don't think so.

 

[Ron]

You have N=70 in rock? Why not refusal? Must be some pretty ratty rock.

I agree you could go up in site class, but you have to prove it as N values won't do it above C.

 

[BigH]

Likely the "70" is 70 for X inches where X is some small number like 1 inch or so. Many confuse 70/1 as an N of 70 - it is not. Many company's stop the test at 50 blows/inch or 50 blows/6 inches - calling it refusal - but still it goes on the log and can be confusing.