Code Language for Caisson Pile Rock Socket

[KevinChez]

From NYC building code 2014

1810.7.3 Rock socket design.
The depth of the rock socket in Class 1c rock or better shall be
sufficient to develop the full load bearing capacity of the caisson
pile based upon the sum of the allowable bearing pressure on the bottom of the socket in
accordance with Table 1804.1 plus an allowable bond stress of 200 psi on the sides of the socket.
The depth of the socket in Class 1c rock or better below the bottom of the pipe shall not be less than 3 feet (914 mm)of the outside diameter of the pipe.


I am not following "shall not be less than 3 feet of the outside diameter of the pipe"


Can someone please expand on this language?

Also there appears to be an exception in the NYC code that allows for the factor of safety to be dropped for caisson piles socketed into class 1c rock or better. I have this situation but am cautious to design with no factor of safety.

Thank you.

 

[Settingsun]

Maybe a typo. "3 feet OR the OD of the pipe" is typical sort of language in these situations to cover the range from small to large pipes.

 

[KevinChez]

thx steveh49

 

[BUGGAR]

"I have this situation but am cautious to design with no factor of safety."
Check with your Geotech, but there is generally no way the factor of safety is eliminated. Rock socketed caissons are common in NYC (the Hudson River Treatment Plant is supported on some 2400 of 'em) and their design is mostly based on experience of what works.
These things are expensive and there is usually a test program to confirm their capacity. Testing also justifies a lower safety factor.

Regarding "safety factors": When they found that an earthquake fault (Spanish Bight) extended below the new aircraft carrier wharves here in San Diego, The Navy Geotechs just scratched their heads and all they could do was reduce the safety factor. Luckily, the aircraft carriers have hydraulic foundations that isolate them.