CPT pwp dissapation rate test

[geodan]

I posted this in Foundation Engineering, but I'll ask here also.

During CPT testing on a current project, the cone was stopped and the pore water pressure was allowed to drop. This was done for several intervals within the clay layer (prone to settlement) in question.

Results were given on a plot with pwp on the y axis and time on the x axis. The resulting drop in pwp over time resembles the consolidation plot (e vs square root time) for a single load.

I am wondering if anyone has any information on how to use this data from the cpt? Can I get a coefficient of consolidation from it? Or a time to 50% consolidated? (which would equal 50% pwp dissapated) Coefficient of permeability? Are there any empirical equations for this data?

The site is a retail project. There is a 50foot thick layer of the clay in question. We have several consolidation tests as well, but I am looking to verify the Cv from the lab test results (which indicates a very fast rate of settlement).

Thanks

 

[dgillette]

Try this:

http://www.greggdrilling.com/Resources/CPT Apps. Pages/applicationsguide2.htm

Robertson is one of the best-known CPT researchers. Apparently left U of Alberta recently and moved to California. I might too, just to get away from Alberta winters.

Regards,
DRG

 

[coneboy]

One method of using the pore pressure dissipation data is to use a ch correlation graph. The graph we have used in the past shows how the t50 value from your CPT dissipation test correlates to a ch value from lab testing. The chart is titled "Average lab ch values and CPTU test results". I found this chart on page 79 of the book titled "Cone Penetration Testing in Geotechnical practice". The authors are Lunne, Powell and Robertson.

You will need to estimate the ground water table depth and then determine the t50 value from the dissipation data. It will also be necessary to estimate the rigidity index (Ir).

I have found that most of the dissipation data that we have reviewed generally agrees with the chart.

If you don't have this chart maybe the CPT contractor that performed the test has this chart and/or book.

I applaude your wisdom in gathering dissipation data in soft fine grained layers.

Coneboy