SPT test - Depth Limit

Luiz Ferreira · Feb 1, 2022

Hello all,

I am new to this forum, but ive searched about this subject but didn`t find a topic.

My question i think is fairly simple, which is the depth limit we can do SPT`s and obtain reliable results?

There is an unwritten rule, at least where i work, that beyond 150ft (~45m) we shouldn`t do SPT tests because is not reliable.

But is there any reference, article about it? I can`t find any.

Thanks in advance

TigerGuy · Feb 1, 2022

There are several correction factors that are to be used for calculating your N60 value. Of course, they have to be used correctly by everybody to calculate the N60 value correctly.

Generally, the N60 is reduced by a factor, until you get to a depth greater than 10 meters (30 ft).

EireChch · Feb 2, 2022

We have done SPTs to 100m, requested by clients in Oil and Gas. I do have some concerns also as you have to question how much energy is actually making it to the cone/split spoon at 100m.

I have learned to live with it by using correlations to phi etc based on N1-60.

In dense sands were were typically getting raw Ns of 150+ when extrapolated, which are maybe 30 when corrected to N160 (guessing here, so done quote me.

From Mayne (?), phi values are 40 deg plus....

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SPT test - Depth Limit

Luiz Ferreira

Geotechnical

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TigerGuy

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EireChch

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