66% Shelby tube recovery

[enginerding]

I am looking at a soil report that I need to use to design a foundation. Maybe I should first mention that about 75% of the text of the report discusses what was not done and what is not covered, and some information on local tort law.

In the text of the report, there are three locations that are listed as having c less than or equal to 2000 psf. When I look at the boring log, these locations all have penetrometer readings of 4.5 TSF, just like all the other tested depths. When I called the soil engineer to ask about the discrepancy, I was told that there was only 66% recovery in the shelby tube at these locations. So they provided a design value of 66% of the measured value. They have correlated c with 1/3 the penetrometer reading. So 4.5 TSF / 3 * 0.66 = 1 TSF = 2000 psf.

Two questions:
1. Is it typical to reduce the cohesion by the % recovery? I have not seen this before
2. If the soil is "loosely compacted" as the soil engineer suggested on the phone, does compression by the shelby tube actually increase the measured penetrometer resistance of the soil as the engineer suggests?

Thanks!

 

[molerat2210]

Percent recovery of the tube sample has no relationship to the shear strength. The samples for shear strength tests are cut from the undisturbed portion of the tube sample. A typical sample for unconfined compressive strength of a cohesive soil is 6 inches.

I am not sure why the geotechnical engineer is obsessed with legal issues. It is not clear from you post how the penetrometer readings were obtained. If these are pocket penetrometer measurements are from tube or spoon samples then that is high strength material. A pocket penetrometer value is not used directly to determine allowable bearing pressure. It can be used with various bearing capacity evaluations (Terzaghi, etc.).

 

[jgailla]

I suppose this engineer may have been sued several times if he's multiplying the estimated strength by the percent recovery. That doesn't even make any sense.

 

[GeoPaveTraffic]

Never seen anyone reduce measured shear strengths by percent recovery in Shelby tubes. Sounds like the soils engineer needs some help.

Mike Lambert

 

[Patgeotech]

Must say that I would like to know where this 'soil engineer' studied as we have some over here that can also predict soil strength without the need for geotech investigation - sounds like they went to the same educational facility. The question why only 66% recovery is a bit of a concern (loose/soft soil conditions?). Never heard of strength being reduced due to poor tube recovery. I would be questioning why only 66% recovered - problems in the ground?

 

[GeoPaveTraffic]

66% recovery doesn't bother me much. I've such recoveries in soft soil and in very stiff clays, could be either.

Mike Lambert