SPT Value Correction – The observed SPT values as measured in the field are not reliable due to multiple factors and therefore need to be corrected. Two types of corrections which are usually applied for this task are: dilatancy and overburden pressure corrections.
Dilatancy correction – The dilatancy correction can only be applied when the test is conducted in fine or silty saturated sand when recorded blow count is greater than 15. The correction factor as suggested by Terzaghi and Peck (1967) is as follows.
Where, No = Observed N-Value
Nc‘= Corrected N – Value
In no case, the corrected values shall be greater than twice the observed values.
Overburden correction – With the increase in confining pressure in sands N-Values are increased considerably. As a result values of relative density other than actual values are obtained and therefore, the observed values are to be corrected. The N-Values at shallow depths are under-estimated and at greater depths are over-estimated. Various methods for observed SPT value corrections are available. They are briefly summarized in the followings. The correction on SPT value for overburden pressure as suggested by Peck et al (1974) is:
Where, Nc‘ = N-Value corrected for dilatancy
Po‘ = Effective overburden pressure
CN = Correction factor
When, NC = NC‘, PO‘ = 100 kN/m2
When, NC < NC‘, PO‘ > 100 kN/m2
The above analysis suggests that the corrected N-Value is infinite for the test conducted at ground surface which is absurd. The observed and corrected values are equal when PO‘ = 100 kN/m2 or at a depth of about 5 m below the ground surface. Above this depth, the corrected values are greater than the observed values and below a depth of about 5 m the corrected values are smaller than the observed Values.
Gibbs and Holtz – The Correction suggested by them is:
In no case the corrected values shall be greater than twice the observed values.
Bazara:- The correction suggested by Bazara is:
In this case also the corrected values shall not be greater than twice the observed values.