DCP Correlations

[nagatalluri]

I am wondering if there are any correlations for DCP penetration index (mm/blow) to Undrained shear strength and friction angle in Granular soils. I checked ASTM D6951 and it only offers correlation with CBR value but cannot find anything relating to undrained shear strength for clays or friction angle for granular soils. Your help is appreciated

 

[EireChch]

There are many correlations for friction angle or to SPT N but none that is industry accepted and hey vary depending on what your DCP spec is i.e. the hammer weight, drop height, cone tip angle etc. There are many types.

My approach is to assess whether the soil is loose, medium dense, or dense and then give a conservative friction angle based on its density.

SPT to Cu correlations are well established and used, Stroud for example. Some people have issues with them but I use them routinely, and found them to be reasonable for the CLAY I work with. We always have UU tests so Cu from SPT is supplemental information.

I would not use a DCP to Cu correlation.

 

[GeoEnvGuy]

As EireChch mentioned there are many correlations from SPT N values. You would need to provide more details on which DCP type you are using to find an appropriate correlation. As for the undrained strength of granular soils the typical practice is to assume all drained strengths or Phi values in granular soils. This is because the undrained strength is typically higher than the undrained strength. Where the industry struggles is recognizing loose and very loose granular deposits can liquefy and a friction angle of 25 to 28 does not adequately represent the peak undrained strength or communicate the hazards of liquefied strength.

 

[nagatalluri]

Thanks for the reply. I am planning to use ASTM D6951, 17.6lb DCP for testing the subgrade. Is there a correlation as such I can use for correlating DCP resistance to friction angle? US Army has a correlation to convert DCP resistance to CBR value, however I cannot find one that can be used to get friction angle. The soils are mostly mixed soils so if there is a correlation I can use to calculate undrained shear strength that would be useful

 

[Settingsun]

Thoughts on these correlations? I've never used them for design, and never come across anything else. If DCP is all that's available, I've required geotech confirmation on site.

 

[BigH]

It is interesting to know what is meant by DCP. This looks like the TTRL DCP. There is also the Mackintosh and the Canadian Pentest (SPT energy on 60deg cone attached to A-rods). So that there is no confusion, below is the DCP according to Look. We found that the Pentest was quite useful for profiling -