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No. of Blows per sq. ft

No. of Blows per sq. ft

No. of Blows per sq. ft

Hi All,

What parameters you can find if you know the blows per sq. ft?  How do I find Undrained Cohesion(c) with the help of No. of Blows per sq. ft?

Thank you.

RE: No. of Blows per sq. ft

you can find all of them when you call the geotech

RE: No. of Blows per sq. ft

The number of "blows" is typically related to the Standard Penetration Test (SPT).  The parameter is not "blows per square foot", but just "blows". It refers to the number of blows required to drive a split barrel sampler a total of 12 inches, in 2-six-inch increments.

RE: No. of Blows per sq. ft

I've never heard of "blows per sq. ft."  I've heard of number of "blows" when it comes to borings for SPT, and also for deep dynamic compaction requirements.

Maybe a geotech would know better.

RE: No. of Blows per sq. ft


You need to get someone involved who knows what they are doing.

There is much more involved than just a simple conversion.

RE: No. of Blows per sq. ft

Is this a plate test you are refering to? You would normally use plate tests to determine CBR


RE: No. of Blows per sq. ft

Also sounds to me like a plate load test - used to determine the compressibility and sometimes the bearing capacity of soil.

I stand to be corrected but to find undrained shear strength of cohesive soils you can use the triaxial compression test.


RE: No. of Blows per sq. ft

If sach4u1 is getting the number of blows from a drilling log they could be blows recorded from a number of different methods (bs1377 part 9 gives details of CPT, SPT and DP tests). There are significant limitations on the intrepretation as outlined in Clayton, CRI, The standard penetration test: Methods and use CIRIA 143 LONDON 1995.Correlation between dynamic probing results and soil properties are being developedby Butcher et et al and Tonks et al in the UK.

RE: No. of Blows per sq. ft

There are significant limitations in almost everything we do - which is why we keep trying to plug first order data into 5th order theoretical equations and keep wondering why it doesn't fit that well . . .  it is obvious that the original poster (who, by the way, doesn't seem that interested in the responses) it talking about the SPT test.  And yes, there are correlations - very rough unless a specific firm has correlated the N value to a particular location's soil types - that can estimate the undrained shear strength.  Always with a grain or two of salt. While the rough correlations do depend to a degree on the degree of plasticity of the clayey material - this is found in most text books 0-2 (up to 12.5 kPa); 2-4 (12.5 to 25 kPa); 4-8 (25 to 50 kPa); 8-15 (50 to 100 kPa); 15-30 (100 to 200 kPa); > 30 (> 200 kPa).  Approximate but perhaps that is all that's required for a green light / red light situation.

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