Depth of test for piling 2

[DiscipleofScience]

Hi

Electrical Engineer here, I have question regarding borehole sampling.

What depth should borehole be tested to for piling? The expected depth of the pile?

 

[oldestguy]

A general question with differing answers. For uniform soil, the engineer calling the shots should be on site or in communication so an estimated pile length can be made as the hole is advanced. Go that deep and maybe 10 to 20 feet deeper to check for weak lower layers.
For a site where hard material may be encountered, such as rock, some sort of rule-of-thumb might be stop after x many feet 0f 50 blow or better material by the standard penetration test. A lot depends on the geology of the site. I'd not try to do it without a geotech engineer involved.

 

[DiscipleofScience]

Hi, thanks for reply..

There was a failure with rig, looking at results it got to clay before failing at 1.5m (I mean the rig broke down!)

Believe the pile will be double that.

 

[oldestguy]

Having been in the business with rigs, etc. I know what can happen. Sometimes knowing the geology you can wing it.

 

[DiscipleofScience]

Yes the engineer is making call that deeper testing is not required (so no revisit required)

Does the investigation need to be in same spot as the intended piling location?

 

[oldestguy]

If possible, yes, but not always possible, as with bridge foundations. Depends on the geology.

 

[cvg]

depth depends a lot on the expected loading for engineering analysis, however the second reason for borings is to give the contractor information on what to expect when he installs the piles. if the boring is not deep enough or in a different location, the contractor assumes more risk and may charge more to cover that risk.

 

[Ron]

At some point you have to trust that the geotech knows what he/she is doing. Familiarity with the local geology coupled with experience on other area projects typically allows them to make such assessments. Not uncommon.

 

[BigH]

Let me relate a little story if I may. I worked half a year or so in New Jersey for a Geotech firm that did a lot of work at the time in Atlantic City. Also quite a bit along the Jersey coast. One of the issues we had with the later was paludal deposits that were then covered with sand fill, settled as surcharge loading and then constructed upon. The firm “assumed” that the paludal deposits had sand layers in between and took them for faster drainage under the surcharge loading at "regular spacings". I suggested to them to use a DCP in the investigations (Canadian Pentest – 140 pound hammer dropping 30 inches) on a 60deg cone tip. The sacrificial part of the cone was truncated that fit over a tip that stayed on the A rod – but still when driving it was a cone. I figured that this would show up the locations of the sand layers, if present. “No. We do not need to do that!” . . . okay.

So I went back to Canada and a number of years later, the owner called me up and said – “Hey, do you remember that cone you were telling me about years ago?” – He wanted to get some of the cones like that – so I ended up arranging a shipment for him. Why did he need them? He pushed the pile load capacities in Atlantic City (typically Franki expanded base piles)– but confirmed with load tests – to values much larger than had been used before. Atlantic City was sand, sand, sand!. But, at one site, they did the load test and the pile punched downwards far more than typical. Investigations revealed that a soft clay deposit (unheard of at that time) existed about 1.5 m below the pile tip. He wanted the cone, when the tip location was reached, to drive down below the tip to see if there were other clayey deposits. Yes, it is always good to consider going deeper than the pile tip . . . and even when the geology is well known (20 odd jobs prior). God made sure that there are always surprises.
For what it is worth.