Shaft Coring Method

[escrowe]

OK experts.

What is the preferred method for cutting a vertical full depth core from an existing drilled shaft foundation?

(cue Syncopated Clock)

 

[msucog]

stick a drill rig over it and go to town coring. i don't know what you're looking for but i'd try to use ndt methods to check it out first then fall back on coring.

 

[escrowe]

NDT suggested a problem. Contractor disagrees...

 

[BigH]

Coring is about the "only" method of cutting down a shaft. Be careful on setting up - I've cored some shafts way too many years ago - slight deviations of the drill and you'll end up drilling down a rebar - we even got a core barrel stuck once - drillers spent 2 days and a crane to try to recover - their prob! I'd at least double tube core.

 

[escrowe]

OK, we managed to miss the rebar.
We collected 4, 5-ft long cores using a 2-inch id double wall core barrel.
The resulting cores look just awful. Numerous horizontal separations, discoloration, and I can abrade the surface with my fingernail.
Makes me think 'bad tremie job.'
Any thoughts?

 

[hokie66]

Bad bored pile, period, whether placed by tremie or not.

 

[dirtguy4]

Well before we throw the baby out with the bath water......check the compressive strength of the cores to confirm. If bad, then we have a problem......were they all bad....or just a few......so do we core them all or is it just a select few........if it is just a few, possibly installing a couple of piles with a pile cap may solve the problem (talk to structural) If they are all bad....well it just sucks to be a contractor some days.

By the way.......who was in charge of the pile supervision during the project........was a geotech firm hired to perform pile monitoring......if not........hmmmm....now there was a bad move......and if there was.....well the supervising firm better had tonnes of paperwork covering themselves.

Remember......engineers do on a dime.......what fools do on a dollar.

 

[escrowe]

We had four fragments (from a total cored length of 20 feet) that actually met the 2:1 height/diameter ratio for compressive strength testing.

That said, the resulting breaks ranged from 5000 to 7500 psi.

The remaining core sections were fractured horizontally at an interval of 2 inches to 1/4 inch...

FYI, the top ten feet of the shaft had failed NDT and had to be rebuilt in September '08. The shaft repair was about 3 months old when we cored it.

And dirtguy4, the contractor 'overlooked' the inspection requirement, because it was a repair job(!) :0

Could be third time's the charm.

 

[BigH]

Seems you had some inexperienced drillers doing the coring? Can't believe that you couldn't get better results (on recovery of pieces with L/D>2) using double tube core barrel - even a single tube. . . . Just a thought....