SPT overburden correction for offshore borings

[tpk72]

General questions on the overburden correction (Cn) for SPT.

1) For offshore drilling, the weight of the water column should be included in the effective overburden pressure calculated, correct? This isn't deep drilling, we are looking at 20 - 30 feet of water above lakebed materials.

2) In general practice, I'm curious how many geotechs go through the trouble or correcting each blow count before developing the soil profile/ using correlations. I do this, but I find many others don't.

Thanks for any feedback!

 

[GeoEnvGuy]

1. Yes, total stress has water then soil, pore water pressure is the total depth of water, effective subtracts the two.
2. The effort of correcting SPT's depends on the purpose, for my liquefaction assessments yes all correction factors are accounted for in detail, for a foundation design typical values are used and overburden correction based on the CFEM as I am in Canada.

 

[EireChch]

The height of the water column above it has nothing to do with it. Is based on effective unit weight which the same no matter how much water is above the soil element.