Drilling and CPT

[pdsb5uc]

I am a fresh-out-of-college geotechnical engineer and I'll be going on my first drilling and CPT job soon. I'll appreciate any in-the-field tips about drilling and CPT. From some previous borings we are expecting compacted fill overlying SF young bay-mud. Any tips about procedures/split spoon sampler/California modified sampler etc., and any other tips will be more than welcome.

Thanks!

 

[coneboy]

A few general tips based on my field experience;

Possibly, the best approach to maxiumize your budget would be to conduct any SPT tests in the upper compacted fill and concentrate your cone tests on the underlying soft Bay silts/clays. As you will see on your job, the electric cone is far faster, more accurate and more econimical than SPT testing. This is especially true for deep testing.

During the electronic cone tests I would recommend that you conduct a few dissipation tests in order to get a understanding of the soil permeability and Cv values. The CPT staff or your senior Eng staff will be able to help you determine a good depth for dissipation testing. The dissipation tests do not need to be carried all the way to 100% dissipation as there are some very nice correlation charts that use a 50% value. This wil save you time and money.

You can also compare the interpreted Cv values from the dissipation test with those from your lab testing. Most dissipation tests will also tell you the water table depth. (if needed)

Vane testing would also be a really good idea so you can get a handle on the site specific NKt value. Most CPT contractors own a vane testing unit.

If the Bay muds are soft/v.soft then expect some to lots of sample disturbance if you use shelby tubes. A better approach in obtaining undisturbed samples would involve the use of Piston sampler. (they are more costly though)

Has the SPT hammer been energy calibrated? If not then your SPT blow counts should be viewed with caution. The SPT blow count values can be calculated from the CPT data quite easily.

There are many publications available that discuss dissipation testing, site specific cone factors (NKt), sources of error in SPT testing and calculation methods for determing Cv values. The guide by Robertson, Lunne and Powell is pretty good.

Finally, I would recommend that the drillers/CPT staff bring some casing (H.stem + BQ) which might be needed if you can not advance the electric cone through the fill. Better to have brought it than have to send the drillers back to there shop to get it.

Best of luck in your investigation.
Coneboy

 

[coneboy]

A few general tips based on my field experience;

Possibly, the best approach to maxiumize your budget would be to conduct any SPT tests in the upper compacted fill and concentrate your cone tests on the underlying soft Bay silts/clays. As you will see on your job, the electric cone is far faster, more accurate and more econimical than SPT testing. This is especially true for deep testing.

During the electronic cone tests I would recommend that you conduct a few dissipation tests in order to get a understanding of the soil permeability and Cv values. The CPT staff or your senior Eng staff will be able to help you determine a good depth for dissipation testing. The dissipation tests do not need to be carried all the way to 100% dissipation as there are some very nice correlation charts that use a 50% value. This wil save you time and money.

You can also compare the interpreted Cv values from the dissipation test with those from your lab testing. Most dissipation tests will also tell you the water table depth. (if needed)

Vane testing would also be a really good idea so you can get a handle on the site specific NKt value. Most CPT contractors own a vane testing unit.

If the Bay muds are soft/v.soft then expect some to lots of sample disturbance if you use shelby tubes. A better approach in obtaining undisturbed samples would involve the use of Piston sampler. (they are more costly though)

Has the SPT hammer been energy calibrated? If not then your SPT blow counts should be viewed with caution. The SPT blow count values can be calculated from the CPT data quite easily.

There are many publications available that discuss dissipation testing, site specific cone factors (NKt), sources of error in SPT testing and calculation methods for determing Cv values. The guide by Robertson, Lunne and Powell is pretty good.

Finally, I would recommend that the drillers/CPT staff bring some casing (H.stem + BQ) which might be needed if you can not advance the electric cone through the fill. Better to have brought it than have to send the drillers back to there shop to get it.

Best of luck in your investigation.
Coneboy

 

[pdsb5uc]

Thanks a lot for the info coneboy. That was helpful

pds5uc