chisel Blow count vs rock strength in Pile foundation

[vinothsekar]

Hi,

We are performing insitu pile drilling using winch machine (chisel weight is 3.5ton) in weathered rock strata. Is there any correlation/reference to find out rock strength vs number of chisel blow counts.

Thank you.

Vinothkumar S
Geotechnical Engineer

 

[oldestguy]

Sometimes we judge rock hardness by progress of drilling the rock. However, it is anybody's guess what your "chisel" does. Maybe take some rock cores and crush them and in time you might have some correlation.

 

[vinothsekar]

Hi

Please find the enclosed attachment link to find out the correlation of weathered rock strength.

Vinothkumar S
Geotechnical Engineer

 

[oldestguy]

Your quoted reference does have a possible valid relationship for end bearing of piles. As with any of these relationships, the more experience you have with them the more reliance might be placed on the method. I'd tie that together with actual load testing before placing much dependency on the method. Seeing how crude any rock drilling method is with varying tool efficiencies, etc. I'd only give it a rough OK. Use a big safety factor.