Opinions requested for use or misuse of "corrected" SPT values.

[oldestguy]

I've been reviewing some reports in the last few years with SPT values on the logs given as "corrected". With many older tables out there based on past experience, I question anyone making any changes to the SPT values coming out from this crude test in the field. I view them as wishful thinking. Your comments?

 

[Ron]

OG...the fact that it is a crude test is one reason that corrections are often applied. Schmertman did extensive work on corrections to the SPT for energy input and soil characterization. His method is commonly used.

 

[BigH]

Agree Ron - but the charts developed and put in the books by Peck Hanson and Thornburn and Terzaghi and Peck didn't correct to energy differences between the cat-head/rope method or the safety hammer method, etc. As I posited in the other recent post - the charts for allowable bearing pressure vs N value - I'd use the values coming out in the soils report - uncorrected (and it is crude) - Bowles, for what it's worth) basically doubled the older charts. For seismic/liquefaction, as the charts have been developed based on a standardized energy - the corrected values should be used . . . but does this include the depth/OB pressure corrections? - never ever did that. In my opinion, corrected or non-corrected - depending on the type of analysis one is using.

 

[oldestguy]

A situation comes to mind from way back. Hough had a business on the side from his Cornell post. Between him and another former Corps of Engineer employee they developed a spec for SPT. Not sure of the background, but their spec called for the 140 lb weight to be pin guided into A rods. However, the impact area of the pin guided weight had a cylindrical cavity about 3 inches deep and about 4 inches diameter into which was a hardwood cushion block. They then tested it against the 140 pound weight sliding down on a pipe, of about 3" OD, onto a ring mounted on the A rods. In general the pin guided weight with the cushion block would result in about half the blows of a comparable pipe guided. All were used with cathaad and one wrap of the rope. When I was at Wisconsin DOT we adopted that. Of course less dangerous methods came in later. With this confusion and past tables still in use along with different methods, I pose the original title. Where is this going?

 

[Ron]

BigH....excellent points. I agree that each has to be looked at and engineering judgment applied....whether a correction is warranted or not depends on the application.

I'll add one more caveat....Schmertman's work was done completely in coastal plains soils, so that limits its applicability.

 

[emmgjld]

As oldestguy, BigH & Ron discuss, the method of actually accomplishing the SPT and the soils involve can dictate what kind of correction(s) needs to be applied.
I abhor the geotech report that makes these correction(s) on the Drill Log, with NO description of what has been considered. I desire (& have reported) the basic field values, with information on the Logs and Legend describing the test & methods. The body of the report should deal with the corrections.

 

[bdbd]

Oldestguy, In Turkey, we have %45 energy ratio, so we correct that to 60%. Also there are factors that we use in every project. After presentation of soil logs, we put tables in every report that presents SPT corrections.
N1_60 = N * Ce * Cn * Cb * Cs * Cr* Cbf

Some of them are 1. Ce: Energy correction factor. Cn: Overburden correction factor (we use it only for granular materials.) Cb: Drilling diameter orrection Cs: Tube type correction Cr: Rod length correction Cbf: Impact rate correction.

In order to use correlations, of course, we check for what type of SPT value that they have been used. But before 1970s energy ratio was not defined. So, correlations before that date, like Terzaghi Peck and Thornburn was based on approximately 55% energy ratio.