Soil consistency in granular material (FDOT) and Field SPT-N correction 3

[pelelo]

Engineers,


Something that i noticed few days ago, the Florida Department of Transportation (FDOT) Soils and Foundations Handbook 2016 (see attached) presents soil consistencies based on Automatic hammer that are somewhat different to the ones that we have seen in typical geotechnical books based on the field SPT-N value.


I was wondering if you stick to this FDOT reference for your projects in Florida area, or if you base the consistencies of the soils based on the tradditional geotechnical engineering references?


On other note, just to make sure, the corrected SPT-N value [N1(60)], i guess it is not a common practice to use it for consistency purposes.

 

[GeoEnvGuy]

I do not work in the Florida area and the safety hammer consistency table is what I have used with a specified equipment, hammer weight and drop height on the log. I have always seen logs consistency based on field N value. I think the fdot is just trying to update field classification based on the increased energy ratio between safety hammer and automatic. Probably due to results from a larger study completed.

In my opinion I am not in favour of just having consistency cross referenced from a table of N values. I think making the effort to gauge the penetration resistance in the field provides more value, especially in gravel deposits.

 

[Ron]

Schmertman did a study on this back in the 70's. Check his paper on it. He was a professor at the University of Florida.

 

[fattdad]

Unless stated otherwise, consistency and density tables were originally related to N60 values. If somebody in the field is using an automatic hammer, you'd first normalize the automatic values to N60 and then get the density or consistency.

We'd require this step in developing logs for our transportation program.

f-d

ípapß gordo ainÆt no madre flaca!

 

[vinothsekar]

Hi,

I have enclosed document of SPT correction factors for your kind reference.

Vinothkumar S
Geotechnical Engineer

 

[BigH]

The subject was "consistency" of granular soils . . . I've never used a consistency term for granular soils . . . has thing changed?

 

[Ron]

BigH....agree. Terminology we would use for clays. The FDOT Manual Table that the OP refers to is titled "Table 1- Relative Density or Consistency" and shows relative density for granular materials and consistency for clays. I think the OP might have thought from the title that the terms were interchangeable.