I'm sorry, but your question does not make sense.   Can you re-phrase it?

Hello~
I am first here to post something~
My question is as follows~

Is there any way to convert 50 blows per 30cm into N blows per 30cm ?

For example, 50/15 means 50 blows were needed to penetrate  15cm.
What I want is that in the case of blow count which exceeds 30 blows like above example..how can I convert it to blow count needed to penetrate 30cm...

50/15 ---> ?blows/30cm
50/1  ---> ?blows/30cm

Sometimes it is needed to and wish to convert them to N-blow count per 30 cm....

Pleasre let me know the way to convert(if exists) and reference which concerns about it~

Thank u

Your question is still blur,
Anyway N-value gives data for soil index properties based on Standard penetration test, which is made under well-defined conditions such as 140-pound hammer dropping 30 inches in free fall. If don't have some maps or equations for establishing a relationship between your two tests blows. You could do the equivalence between the energies delivered to samplers. Of course you need to know all the parameters for both test.

best regards,
kope

If your question is how you convert 50 blows/3" into an equivalent N-value, the answer is that you have hit hard or dense material that created a spoon refusal condition, and you do not need to convert the blows into an equivalent N-value.  The N-value exceeds 100.  The accuracy of the SPT is severely diminished for very weak soils (N less than 2) and for very hard materials (N greater than 100).  If you need to know the strength of the dense material, you are better off coring it, or using a pressuremeter.

For the SPT, the N-value should be used following the standard test (as mentioned by "Kope"). You just have to report the number of blows per 3 consecutive 6 in. for the SPT values. As for the interpretation, N value is defined as the total number of blows for the 2nd and 3rd 6in. penetration (ie. the last 12in. of the 18 in. penetration). Some people carry out counts for 4 x 6 in. interval (for the purpose of getting full sample in the spoon), but the N value is still the addition between the 2nd and 3rd 6in. penetration.   

There are also other considerations as "KAM" indicated. If there is a stone at the tip of the SPT spoon, number can jump quickly. It may not be representative value.  So people have to be careful about this. Also, the spoon is assumed to start at the bottom of an open hole, and clear from fallen debris (disturbed materials or cuttings). If not, theoritically, it is not done correctly (although, since the first 6in count is not used, it may probably be OK). I used to check the depth of the tip of the spoon , and make sure it is the same depth measured previously during advancement of the hole (drilling/augering, etc).

Hope that this helps you along.

Thank u for all guys advice~
All ur coments are really helpful for me~

I asked u about the way of converting N value to equavelent N value...
U know SPT N value has some relationship with P-wave
(nd there are also some published emperical relationship)
And elastic modulus from PMT and dynamic modulus by Downhole test has also relationship.
If i can colerate  site-specificcorelation with N-value, Dynamic elastic modulus , and static elastic modulus~
that can produce Elastic static modulus with N-value at each point where SPT onducted(That is a reason why i asked u guys about Equivalent SPT N-value)
That can be helpful for Pile design~

How's my concept ?
Is there any idea about my statement ? leave a msg for me ^^

Regards converting N-value from shear wave velocity, does anybody has the relationship between N-value and Vs (shear wave velocity). Not only for sand, because I have those ones.

      
Imai(1969) Vs=92.1*N^0.329

I have a chart also but don't know how to attact it on this post ~

thanks a lot Chunil, this one was very helpful for me!
you can send me the chart at kope@go.ro

I would be very careful using correlations between N-values and geotechnical parameters unless you have measured the efficiency of the hammer used to drive the spoon (and the operator if using a manual rope-cathead system). The test is highly variable.

Actually your right, the ER delivered by a particular SPT setup depends primarily on the type of hammer and anvil in the drilling system and on the method of hammer release. An energy ratio, ERm, of 60% has generally been accepted as the reference value. But basically, approximate correction factors (ERm/60%) to modify the SPT results to a 60% energy ratio for various types of hammers and anvils are known. I'll calculate the standard deviation from the field data and I'll take only the lower bound values. Then I'll iterate to find the curve fitting parameters using various formulas (like the one given by Chunil).

Hi Chunil
Regarding relating N values to a modulus of elasticity value I have found some information in the text Foundation Analysis and Design by J. E. Bowles.

While many equations are presented the onus is on the user to establish which equation is the most representative given the soils in that specific locality.

It should be noted that Kam has already addressed the advantages/disadvantages of N values and therefore sound engineering judgement is required if you intend to venture in the direction of using E values strictly based on blow counts.

Finally Kam (May 11) mentions that 50/3" can be considered as N being more than 100.  I usually treat blow counts as read that is 50/3" is 50/3".  If we had continued driving the spoon chances are we could have achieved 60/7" or 55/12" none of which is N=100

Dear Chunil,

I understand your words "Sometimes it is needed to and wish to convert them to N-blow count per 30 cm...." in some of cases as SPT in weathered rock.

SPT tests can be carried out in weathered rock with maximum number of blows restricted to 100. This is because the equipment can get damaged if number of blows are exceeding 100. The actual penetration (in mm) after 100 blows is measured and then extrapolate for 300 mm penetration. This extrapolated 'N' values can be used to assess the shear strength of weathered rock using Cole and Stroud, 1977 correlationship. This is the standard practice followed in many countries.

Dear Chunil,

Like everybody I have also met cases when N value is too high and the test is interrupted and the penetration is recorded, as you mention like 50/12cm etc. Unfortunately I don't know a way to treat it. Of course there are mathematical ways to extrapolate but as with everything in statistics and relations one must be sure that the laws that govern the pfysical phenomenon remain the same.

I often thought that if for example (imaginary case) one hit a limestone or a granite with SPT sampler he would obviously get absolute refusal in both cases. Never-the-less strength, deformability and stress wave velocities are tottaly dofferent in granite and limestone. It seems to me that there is a limit to the pfysical properties that can be measured by SPT. The actuall mechanism changes and as mentioned by Georam and KAM there is a different material formed due to its nature and compaction that SPT cannot function as it needs shear and/or redistribution of grains to occur.

It is a pitty not to be able to use such in situ measured characteristics in cases of good soil but simple tests have their limits. If you permit me I would follow the indications from the specialists above and limit any correlations within the frame that actually coresponds to the nature of the test.

Thank you for the usefull question that permit me to read a lot of useful ideas and experience from the tips above.
Regards
Dimitrios